Surveillance Summaries / Vol. 60 / No. 12 September 23, 2011 

Surveillance for Waterborne Disease Outbreaks and Other 

Health Events Associated with Recreational Water — 

United States, 2007–2008 

and

Surveillance for Waterborne Disease Outbreaks Associated with 

Drinking Water — United States, 2007–2008

Morbidity and Mortality Weekly Report

U.S. Department of Health and Human Services

Centers for Disease Control and Prevention
Surveillance Summaries

The MMWR series of publications is published by the Office of Surveillance, Epidemiology, and Laboratory Services, Centers for Disease Control and Prevention (CDC), 

U.S. Department of Health and Human Services, Atlanta, GA 30333.

Suggested Citation: Centers for Disease Control and Prevention. [Title]. MMWR 2011;60(No. RR-#):[inclusive page numbers].

Centers for Disease Control and Prevention

Thomas R. Frieden, MD, MPH, Director

Harold W. Jaffe, MD, MA, Associate Director for Science

James W. Stephens, PhD, Director, Office of Science Quality

Stephen B. Thacker, MD, MSc, Deputy Director for Surveillance, Epidemiology, and Laboratory Services

Stephanie Zaza, MD, MPH, Director, Epidemiology and Analysis Program Office

MMWR Editorial and Production Staff

Ronald L. Moolenaar, MD, MPH, Editor, MMWR Series

Christine G. Casey, MD, Deputy Editor, MMWR Series

Teresa F. Rutledge, Managing Editor, MMWR Series

David C. Johnson, Lead Technical Writer-Editor

Jeffrey D. Sokolow, MA, Project Editor

Martha F. Boyd, Lead Visual Information Specialist

Julia C. Martinroe, Stephen R. Spriggs, Terraye M. Starr

Visual Information Specialists

Quang M. Doan, MBA, Phyllis H. King

Information Technology Specialists

MMWR Editorial Board

William L. Roper, MD, MPH, Chapel Hill, NC, Chairman

Virginia A. Caine, MD, Indianapolis, IN

Jonathan E. Fielding, MD, MPH, MBA, Los Angeles, CA

David W. Fleming, MD, Seattle, WA

William E. Halperin, MD, DrPH, MPH, Newark, NJ

King K. Holmes, MD, PhD, Seattle, WA

Deborah Holtzman, PhD, Atlanta, GA

John K. Iglehart, Bethesda, MD

Dennis G. Maki, MD, Madison, WI

Patricia Quinlisk, MD, MPH, Des Moines, IA

Patrick L. Remington, MD, MPH, Madison, WI

Barbara K. Rimer, DrPH, Chapel Hill, NC

John V. Rullan, MD, MPH, San Juan, PR

William Schaffner, MD, Nashville, TN

Anne Schuchat, MD, Atlanta, GA

Dixie E. Snider, MD, MPH, Atlanta, GA

John W. Ward, MD, Atlanta, GA

CONTENTS

Surveillance for Waterborne Disease Outbreaks and Other Health 

Events Associated with Recreational Water — United States, 

2007–2008

Introduction ............................................................................................................2

Background .............................................................................................................2

Methods....................................................................................................................3

Results.......................................................................................................................5

Discussion............................................................................................................. 16

Conclusion............................................................................................................ 28

Acknowledgments............................................................................................. 29

References............................................................................................................. 30

Appendix A........................................................................................................... 33

Appendix B ........................................................................................................... 36

Surveillance for Waterborne Disease Outbreaks Associated with 

Drinking Water — United States, 2007–2008

Introduction ......................................................................................................... 39

Methods................................................................................................................. 41

Results.................................................................................................................... 44

Discussion............................................................................................................. 54

Conclusion............................................................................................................ 62

Acknowledgments............................................................................................. 65

References............................................................................................................. 65

Appendix A........................................................................................................... 69

Appendix B ........................................................................................................... 73
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 1

Surveillance for Waterborne Disease Outbreaks and Other 

Health Events Associated with Recreational Water — 

United States, 2007–2008

Michele C. Hlavsa, MPH1

Virginia A. Roberts, MSPH1

Ayana R. Anderson, MPH2

Vincent R. Hill, PhD1

Amy M. Kahler, MS1

Maureen Orr, MS2

Laurel E. Garrison, MPH3

Lauri A. Hicks, DO3

Anna Newton, MPH1,4

Elizabeth D. Hilborn, DVM5

Timothy J. Wade, PhD5

Michael J. Beach, PhD1

Jonathan S. Yoder, MSW, MPH1

1Division of Foodborne, Waterborne, and Environmental Diseases, National Center for Emerging and Zoonotic Infectious Diseases, CDC

2Division of Health Studies, Agency for Toxic Substances and Disease Registry

3Division of Bacterial Diseases, National Center for Immunization and Respiratory Diseases, CDC 4Atlanta Research and Education Foundation, Decatur, Georgia

5U.S. Environmental Protection Agency, Research Triangle Park, North Carolina

Abstract

Problem/Condition: Since 1978, CDC, the U.S. Environmental Protection Agency, and the Council of State and Territorial 

Epidemiologists have collaborated on the Waterborne Disease and Outbreak Surveillance System (WBDOSS) for collecting and 

reporting data on waterborne disease outbreaks associated with recreational water. This surveillance system is the primary source 

of data concerning the scope and health effects of waterborne disease outbreaks in the United States. In addition, data are collected 

on other select recreational water–associated health events, including pool chemical–associated health events and single cases of 

Vibrio wound infection and primary amebic meningoencephalitis (PAM).

Reporting Period: Data presented summarize recreational water–associated outbreaks and other health events that occurred 

during January 2007–December 2008. Previously unreported data on outbreaks that have occurred since 1978 also are presented.

Description of the System: The WBDOSS database includes data on outbreaks associated with recreational water, drinking 

water, water not intended for drinking (excluding recreational water), and water use of unknown intent. Public health agencies 

in the states, the District of Columbia, U.S. territories, and Freely Associated States are primarily responsible for detecting and 

investigating waterborne disease outbreaks and voluntarily reporting them to CDC using a standard form. Only data on outbreaks 

associated with recreational water are summarized in this report. Data on other recreational water–associated health events reported 

to CDC, the Agency for Toxic Substances and Disease Registry (ATSDR), and the U.S. Consumer Product Safety Commission 

(CPSC) also are summarized.

Results: A total of 134 recreational water–associated outbreaks were reported by 38 states and Puerto Rico for 2007–2008. These 

outbreaks resulted in at least 13,966 cases. The median outbreak size was 11 cases (range: 2–5,697 cases). A total of 116 (86.6%) 

outbreaks were associated with treated recreational water (e.g., pools and interactive fountains) and resulted in 13,480 (96.5%) 

cases. Of the 134 outbreaks, 81 (60.4%) were outbreaks of acute gastrointestinal illness (AGI); 24 (17.9%) were outbreaks of 

dermatologic illnesses, conditions, or symptoms; and 17 (12.7%) were outbreaks of acute respiratory illness. Outbreaks of AGI 

resulted in 12,477 (89.3%) cases. 

The etiology was laboratory-confirmed for 105 (78.4%) of 

the 134 outbreaks. Of the 105 outbreaks with a laboratoryconfirmed etiology, 68 (64.8%) were caused by parasites, 22 

(21.0%) by bacteria, five (4.8%) by viruses, nine (8.6%) by 

chemicals or toxins, and one (1.0%) by multiple etiology 

types. Cryptosporidium was confirmed as the etiologic agent 

Corresponding author: Michele C. Hlavsa, MPH, Division of 

Foodborne, Waterborne, and Environmental Diseases, National Center 

for Emerging Zoonotic and Infectious Diseases, CDC, 1600 Clifton 

Road, N.E., MS C-9, Atlanta, GA 30333. Telephone: 404-639-1700; 

E-mail: healthywater@cdc.gov. 
Surveillance Summaries

2 MMWR / September 23, 2011 / Vol. 60 / No. 12

of 60 (44.8%) of 134 outbreaks, resulting in 12,154 (87.0%) cases; 58 (96.7%) of these outbreaks, resulting in a total of 12,137 

(99.9%) cases, were associated with treated recreational water. A total of 32 pool chemical–associated health events that occurred 

in a public or residential setting were reported to WBDOSS by Maryland and Michigan. These events resulted in 48 cases of 

illness or injury; 26 (81.3%) events could be attributed at least partially to chemical handling errors (e.g., mixing incompatible 

chemicals). ATSDR’s Hazardous Substance Emergency Events Surveillance System received 92 reports of hazardous substance 

events that occurred at aquatic facilities. More than half of these events (55 [59.8%]) involved injured persons; the most frequently 

reported primary contributing factor was human error. Estimates based on CPSC’s National Electronic Injury Surveillance System 

(NEISS) data indicate that 4,574 (95% confidence interval [CI]: 2,703–6,446) emergency department (ED) visits attributable 

to pool chemical–associated injuries occurred in 2008; the most frequent diagnosis was poisoning (1,784 ED visits [95% CI: 

585–2,984]). NEISS data indicate that pool chemical–associated health events occur frequently in residential settings. A total of 

236 Vibrio wound infections were reported to be associated with recreational water exposure; 36 (48.6%) of the 74 hospitalized 

vibriosis patients and six (66.7%) of the nine vibriosis patients who died had V. vulnificus infections. Eight fatal cases of PAM 

occurred after exposure to warm untreated freshwater.

Interpretations: The 134 recreational water–associated outbreaks reported for 2007–2008 represent a substantial increase over 

the 78 outbreaks reported for 2005–2006 and the largest number of outbreaks ever reported to WBDOSS for a 2-year period. 

Outbreaks, especially the largest ones, were most frequently associated with treated recreational water and characterized by AGI. 

Cryptosporidium remains the leading etiologic agent. Pool chemical–associated health events occur frequently but are preventable. 

Data on other select recreational water–associated health events further elucidate the epidemiology of U.S. waterborne disease by 

highlighting less frequently implicated types of recreational water (e.g., oceans) and detected types of recreational water–associated 

illness (i.e., not AGI).

Public Health Actions: CDC uses waterborne disease outbreak surveillance data to 1) identify the types of etiologic agents, 

recreational water venues, and settings associated with waterborne disease outbreaks; 2) evaluate the adequacy of regulations and 

public awareness activities to promote healthy and safe swimming; and 3) establish public health priorities to improve prevention 

efforts, guidelines, and regulations at the local, state, and federal levels.

Introduction

During 1920–1970, data on waterborne disease outbreaks 

in the United States were collected by multiple researchers and 

federal agencies (1). Since 1971, CDC, the U.S. Environmental 

Protection Agency (EPA), and the Council of State and 

Territorial Epidemiologists (CSTE) have collaborated on 

the Waterborne Disease and Outbreak Surveillance System 

(WBDOSS), which tracks outbreaks associated with drinking 

water and other water exposures (2–12). Data on outbreaks 

associated with recreational water have been collected by 

WBDOSS* since 1978 (13–15). Data on 2007–2008 outbreaks 

associated with drinking water are presented separately (16).

Since 1978, surveillance activities have expanded to include 

multiple types of outbreaks and other health events associated 

with recreational water. Recreational water–associated outbreaks 

of Pontiac fever (PF) were first included in the 1981 surveillance 

summary (17); recreational water–associated outbreaks of 

Legionnaires’ disease (LD) were added in the 2001–2002 

surveillance summary; however, no LD outbreaks were reported 

for those years (15). Data on single cases of recreational 

water–associated vibriosis were first added to the 2003–2004 

surveillance summary (14). Pool chemical–associated health 

events reported to CDC were first included in the 2005–2006 

surveillance summary (13). Agency for Toxic Substances and 

Disease Registry (ATSDR) and U.S. Consumer Product Safety 

Commission (CPSC) data on pool chemical–associated health 

events have been added to this 2007–2008 surveillance summary. 

The data provided in this report represent only a portion of 

the burden of illness and injury associated with recreational 

water. They do not include endemic waterborne disease cases 

(i.e., sporadic cases not known to be associated with an outbreak), all types of other recreational water–associated health 

events, nor estimates of the number of unrecognized and unreported waterborne disease outbreaks or other health events.

Background

Regulation of Recreational Water Quality

In the United States, state and local governments establish 

and enforce regulations for protecting recreational water from 

naturally occurring and human-made contaminants. No federal 

regulatory agency has authority over treated recreational water 

(e.g., pools and interactive fountains), and no minimum federal 

design, construction, operation, disinfection, or filtration 

standards exist, except the Virginia Graeme Baker Pool and *Available at http://www.cdc.gov/healthywater/statistics/wbdoss/index.html.
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 3

Spa Safety Act (15 U.S.C. §§ 8001 et seq.), which aims to 

prevent entrapment. Swimming pool codes are developed and 

enforced by individual state and local public health agencies, 

resulting in substantial variation across the country in terms 

of regulation, compliance, and enforcement. 

EPA sets water quality guidelines for natural, untreated 

recreational water (e.g., lakes, rivers, and oceans). In 1986, EPA 

developed recommended bacterial water quality criteria for coastal 

recreational waters (18) and, in 2004, established federal standards 

for those states and territories that had not yet adopted water 

quality criteria that met or exceeded the 1986 criteria (19). For 

freshwater, full-body contact beaches (e.g., lakes and rivers), EPA 

recommends that the monthly geometric mean water quality 

indicator concentration be <33 CFU/100mL for enterococci or 

<126 CFU/100mL for Escherichia coli. For marine water, full-body 

contact beaches, EPA recommends that the monthly geometric 

mean water quality indicator concentration be <35 CFU/100mL for 

enterococci. However, state and local jurisdictions have discretionary 

authority to adopt specific criteria for a designated use, determine 

the extent and frequency of monitoring and testing, and choose 

which interventions should be implemented when state limits are 

exceeded (e.g., posting signs to alert visitors to water contamination 

or closing the beach to swimmers). EPA provides grants to eligible 

coastal and Great Lakes states to help them implement programs 

to monitor water quality at the beach and to notify the public 

when problems are detected. Beach Watch, EPA’s Action Plan for 

Beaches and Recreational Waters, was published in 1999 as part 

of the Clean Water Action Plan. The mission of Beach Watch is 

to assist state, tribal, and local authorities in strengthening and 

extending existing programs to protect users of fresh and marine 

recreational waters. Congress enacted the BEACH Act of 2000 

(33 U.S.C. §§ 1346 and 1375a), which directed EPA to update its 

guidelines for recreational water use on the basis of improved water 

quality indicators and testing. To this end, since 2002, EPA has 

been collaborating with CDC on the National Epidemiologic and 

Environmental Assessment of Recreational (NEEAR) Water Study. 

Methods

Data Sources

Public health agencies in the states, the District of Columbia 

(DC), U.S. territories, and the Freely Associated States (FAS)† 

have the primary responsibility for detecting and investigating 

waterborne disease outbreaks, which they report voluntarily to 

CDC using a standard form (CDC 52.12).§ The form solicits 

data on characteristics of outbreaks (e.g., number of cases, time, 

and location), results from epidemiologic and environmental 

investigations, and results from clinical specimen and water 

sample testing. CDC annually requests reports of outbreaks 

from persons designated as waterborne disease coordinators 

and obtains additional information regarding epidemiologic 

investigations, water quality, and water treatment to supplement 

submitted outbreak reports as needed. Data on all of the 

outbreaks in this summary were collected through paper-based 

reporting. Numeric and text data are abstracted from outbreak 

report forms and supporting documents and analyzed by using 

SAS 9.2 (SAS Institute, Inc. Cary, North Carolina). 

National reporting includes only data on waterborne disease 

outbreaks. Data on pool chemical–associated health events that 

occurred in a public or residential setting and on single cases of 

primary amebic meningoencephalitis (PAM) caused by Naegleria 

fowleri also are solicited from coordinators.

To ensure completeness of legionellosis outbreak data, CDC 

reviewed and compared data from WBDOSS with data from 

the Travel-Associated Legionellosis in the United States System. 

In addition, legionellosis outbreak data were abstracted from 

historic Epidemic Intelligence Service outbreak investigation 

reports and peer-reviewed publications and entered into the 

WBDOSS database.

Other data presented in this surveillance summary have been 

reported to ATSDR’s Hazardous Substance Emergency Events 

Surveillance (HSEES) System,¶ CPSC’s National Electronic 

Injury Surveillance System (NEISS), and CDC’s Cholera 

and Other Vibrio Illness Surveillance (COVIS) System. For 

this report, the HSEES database was searched for data on 

hazardous substance events that occurred at aquatic facilities 

during 2007–2008. To identify such events, comment and 

synopsis variables were queried for the terms “pool,” “Jacuzzi,” 

“spa,” “hot tub,” “whirlpool,” “chlorine,” “fountain,” and “water 

park” (20). Although “chlorine” was the only chemical term 

queried, other chemicals were not excluded from the analysis. 

Only events that occurred on-site at aquatic facilities, regardless 

of setting (e.g., hotel/motel or waterpark), were included in 

the analysis. Events that were caused by pool chemicals but 

did not occur on-site at aquatic facilities (e.g., at a warehouse) 

were excluded. NEISS captures data on emergency department 

(ED) visits for injuries associated with consumer products, 

specifically diagnosis and patient demographic data. The 

methodology for analysis of NEISS data on pool chemical–

associated health events and the findings of a detailed analysis 

of 2007 NEISS data are presented elsewhere (21); this report 

includes a summary of 2008 NEISS data. Data on cases of 

¶Participating states: Colorado, Florida, Iowa, Louisiana, Michigan, Minnesota, 

North Carolina, New Jersey, New York, Oregon, Texas, Utah, Washington, and 

Wisconsin. 

†Comprising the Republic of the Marshall Islands, the Federated States of 

Micronesia, and the Republic of Palau; formerly part of the U.S.-administered 

Trust Territory of the Pacific Islands. 

§Available at http://www.cdc.gov/healthywater/statistics/wbdoss/nors/forms_

archive.html. 
Surveillance Summaries

4 MMWR / September 23, 2011 / Vol. 60 / No. 12

vibriosis are reported to CDC by use of a COVIS report 

form.** Vibriosis was added to the list of nationally notifiable 

diseases in January 2007 and is reportable in 50 states. As 

described previously, this report summarizes COVIS System 

data on cases with a reported recreational water exposure but 

no reported seafood consumption (14). Cases reported to 

have occurred in another jurisdiction were reassigned to the 

jurisdiction of exposure for analysis.

Definitions

Waterborne Disease Outbreak

Two criteria must be met for a health event to be defined as a 

waterborne disease outbreak associated with recreational water: 1) 

two or more persons must be linked epidemiologically by time, 

location of exposure to water, and illness characteristics, and 2) 

the epidemiologic evidence must implicate recreational water or 

volatilization of water-associated compounds into the air surrounding the water as the probable source of illness. Outbreak 

reports with limited or no environmental data might be included 

in WBDOSS, but outbreak reports that lack epidemiologic data 

linking the outbreak to water are excluded. Two outbreak reports 

that were received are not included in this summary. After 

swimming in April 2008, a total of 13 members of a swim team 

in Florida developed AGI caused by norovirus. However, the 

epidemiologic evidence implicated a restaurant in which they 

had shared a meal. An outbreak of cryptosporidiosis in Maine 

in July 2007 among four persons who swam in a residential 

pool was not included because other common exposures could 

not be ruled out.

Recreational water venues include but are not limited to 

pools, interactive fountains, spas (or hot tubs), waterslides, and 

fresh or marine bodies of water. For this report, outbreaks are 

categorized by association with treated or untreated recreational 

water and by location of the water exposure (i.e., not on state 

of residence of the ill persons). Waterborne disease outbreaks 

occurring on cruise ships are not reported to WBDOSS; 

CDC’s Vessel Sanitation Program tracks outbreaks of acute 

gastrointestinal illness (AGI) occurring on cruise ships (22).

Other Recreational Water–Associated Health 

Events

Other recreational water–associated health events that do not 

meet the definition of an outbreak associated with recreational 

water are included in this summary because of their implications 

to the health and safety of the swimming public and aquatics 

staff. Some pool chemical–associated health events reported 

to WBDOSS do not meet the outbreak definition (i.e., one 

case identified or not associated with recreational water). The 

HSEES System and NEISS do not focus on water exposure, and 

whether pool chemical–associated health events reported to these 

systems involved recreational water exposure is not always clear. 

Consequently, some pool chemical–associated health events 

reported to WBDOSS, the HSEES System, and NEISS are 

analyzed and presented separately from outbreak data.

Additional terms used in this report are defined elsewhere 

(Appendix A).

Strength-of-Evidence Classification for 

Waterborne Disease Outbreaks

In this report, all outbreaks reported to WBDOSS for 

2007–2008 have been classified according to the strength of 1) 

epidemiologic and clinical laboratory data and 2) environmental 

data implicating water as the vehicle of transmission (Table 1). 

The classification (i.e., Classes I–IV) of outbreak investigations in 

this report is based on the epidemiologic and environmental data 

reported to WBDOSS. These classes were first delineated in the 

1989–1990 surveillance report (10) and have since been updated.

Outbreaks and subsequent investigations occur under 

different circumstances, and not all outbreaks can be 

investigated rigorously. Classifications of II, III, or IV do 

not necessarily imply that the investigation was inadequate 

or incomplete because multiple factors (e.g., timeliness of 

outbreak detection) contribute to the ability to collect optimal 

epidemiologic, clinical laboratory, and environmental data.

Changes in 2007–2008 Surveillance 

Summary

Strength-of-Evidence Classification

Molecular epidemiology is being used increasingly to 

understand pathogen transmission patterns, detect outbreaks, 

and identify important risk factors and outbreak sources. 

The criteria used to determine the strength-of-evidence 

classifications have been revised to reflect the increasing 

use of molecular characterization of pathogens identified 

in clinical specimens and environmental samples collected 

during outbreak investigations. Molecular data that link 

multiple persons who had an identical water exposure now are 

considered adequate epidemiologic data to support a Class I 

or Class II assignment; previously, epidemiologic study data 

were required to receive a strength-of-evidence classification 

of I or II. Molecular data that link at least one person to the 

implicated water exposure now are considered adequate water 

quality data to support a Class I or Class III assignment. In this ** Available at http://www.cdc.gov/nationalsurveillance/PDFs/CDC5279_

COVISvibriosis.pdf.
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 5

report, the previously used categories “epidemiologic data” and 

“water quality data” have been renamed “epidemiologic and 

clinical laboratory data” and “environmental data,” respectively.

Number of Cases

For this surveillance summary, case counts were based on the 

estimated number of total cases if sufficient supporting evidence 

was provided. For example, this might include applying the 

attack rate found during a cohort study to the entire population 

exposed to contaminated water to estimate the total number 

of ill persons associated with an outbreak. If no “estimated ill” 

number was provided, the actual number of reported cases 

(e.g., laboratory-confirmed and probable cases as reported by 

the coordinator) was used. CDC requests that coordinators 

report only cases with primary exposure to water, so secondary 

cases (e.g., those resulting from person-to-person transmission 

among household members) are not included in case counts of 

waterborne disease outbreaks in WBDOSS.

Results

Waterborne Disease Outbreaks

A total of 134 outbreaks associated with recreational water 

(84 in 2007 and 50 in 2008) were reported to CDC by 38 

states and Puerto Rico (Tables 2–5; Figure 1). These 134 

outbreaks resulted in at least 13,966 cases of illness (Table 6); 

the median number of cases associated with an outbreak was 

11 (range: 2–5,697 cases). Minnesota reported 12 outbreaks, 

Florida reported 11, and New York reported eight. Short 

narratives on select outbreaks are provided in Appendix B.

Treated recreational water venues were associated with 116 

(86.6%) outbreaks (Figure 2), resulting in 13,480 (96.5%) 

cases; the median number of cases in these outbreaks was 10.5 

(range: 2–5,697 persons) (Tables 2 and 3). Of the 116 outbreaks, 

50 (43.1%) were associated with exposures in settings in which 

recreational water was not the focus of activities (i.e., apartment 

complex, assisted living facility, child care center, farm, hotel/

motel, membership club, school, or zoo).

Untreated venues were associated with the remaining 18 

(13.4%) outbreaks (Figure 2), resulting in 486 (3.5%) cases; the 

median number of cases in these outbreaks was 13 (range: 2–200) 

(Tables 4 and 5). Of these 18 outbreaks, 15 (83.3%) were 

associated with fresh water and three (16.7%) with marine water. 

The 134 outbreaks occurred in every calendar month 

(Figure 3); 83 (61.9%) started during June, July, or August. 

The route of entry implicated for each outbreak was ingestion 

for 81 outbreaks (60.4%), contact for 25 (18.7%), inhalation 

for 18 (13.4%), combined routes for six (4.5%), and unknown 

for four (3.0%) (Figure 2).

Illness and Etiologies

Of the 134 outbreaks, 81 (60.4%) were of AGI; 24 (17.9%) 

of dermatologic illnesses, conditions, or symptoms; 17 (12.7%) 

of acute respiratory illness (ARI); one (0.7%) of ear-related 

illnesses, conditions, or symptoms; and one (0.7%) of other 

TABLE 1. CDC strength-of-evidence classification of investigations of waterborne disease outbreaks — United States 

Class Epidemiologic and clinical laboratory data Environmental data

I Provided and adequate Provided and adequate

Epidemiologic data provided about exposed and unexposed persons, 

with relative risk or odds ratio ≥2 or p-value <0.05; or

Laboratory data or historic information (e.g., history of a chlorinator or acid 

feed pump malfunction, no detectable free-chlorine residual, or a 

breakdown in circulation system); or

Molecular characterization of pathogens linked multiple persons who had 

a single identical exposure

Molecular characteristics of pathogens isolated from water and at least one 

clinical specimen were identical

II Provided and adequate Not provided or inadequate

Epidemiologic data provided about exposed and unexposed persons, 

with relative risk or odds ratio ≥2 or p-value <0.05; or

E.g., laboratory testing of water not conducted and no historic information 

available

Molecular characterization of pathogens linked multiple persons who had 

a single identical exposure

III Provided but limited Provided and adequate

Epidemiologic data provided that did not meet the criteria for Class I or II 

or claim made that ill persons had no exposures in common, besides 

water, but no data provided

Laboratory data or historic information (e.g., history of a chlorinator or acid 

feed pump malfunction, no detectable free-chlorine residual, or a 

breakdown in circulation system); or

Molecular characteristics of pathogens isolated from water and at least one 

clinical specimen were identical

IV Provided but limited Not provided or inadequate

Epidemiologic data provided that did not meet the criteria for Class I or II 

or claim made that ill persons had no exposures in common, besides 

water, but no data provided

E.g., laboratory testing of water not conducted and no historic information 

available
Surveillance Summaries

6 MMWR / September 23, 2011 / Vol. 60 / No. 12

See table footnotes on page 7.

TABLE 2. Waterborne disease outbreaks associated with treated recreational water (n = 74), by state/jurisdiction — Waterborne Disease and 

Outbreak Surveillance System, United States, 2007

State/

Jurisdiction Month Class* Etiology

Predominant 

illness†

No. of cases§ 

(n = 10,749) Venue Setting

Alabama Aug IV Cryptosporidium AGI 5 Fill-and-drain pool Child care center

California Apr IV Escherichia coli O157:H7 AGI 11 Temporary waterslide Private residence

California Jun IV Norovirus genogroup II AGI 6 Interactive fountain Community/Municipality

California Sep III Cryptosporidium AGI 2 Pool Aquatic facility

Florida Jun IV Cryptosporidium AGI 25 Interactive fountain Community/municipal park

Florida Jul IV Unidentified¶ AGI 6 Interactive fountain Zoo

Florida Aug IV Cryptosporidium AGI 8 Pool Neighborhood/Subdivision

Florida Aug III Chlorine gas ARI 13 Pool Apartment complex

Florida Sep IV Cryptosporidium parvum AGI 8 Wading pool Waterpark

Georgia Apr IV Unidentified Skin 2 Spa Unknown

Georgia Aug IV Cryptosporidium AGI 6 Pool, temporary 

waterslide

Neighborhood/Subdivision, 

private residence

Idaho May II Norovirus genogroup II AGI 50 Pool, wading pool Community/Municipality

Idaho Jun III E. coli O157:H7 AGI 31 Interactive fountain Community/Municipality

Idaho Jul I Cryptosporidium hominis AGI 2,000 Interactive fountain Community/Municipality

Idaho Aug II C. hominis AGI 32 Interactive fountain Community/Municipality

Illinois Aug IV Cryptosporidium AGI 6 Wading pool Membership club

Illinois Aug IV Cryptosporidium AGI 4 Pool Community/Municipality

Indiana May IV Unidentified ARI, AGI 9 Pool, spa Private residence

Iowa Jan IV Unidentified Skin 4 Spa Hotel/Motel

Iowa Jan I Unidentified** Skin 20 Pool, spa Hotel/Motel

Iowa Feb IV Unidentified** Skin 10 Spa Hotel/Motel

Iowa Jun I Cryptosporidium AGI 238 Pool, wading pool Community, membership club

Iowa Aug IV Cryptosporidium AGI 34 Pool Community/Municipality

Kansas Mar IV Unidentified Skin 5 Pool Community/Municipality

Kansas Jun IV Cryptosporidium AGI 79 Pool, interactive 

fountains

Waterpark, community, private 

residence, hotel/motel

Kansas Jul IV Cryptosporidium AGI 43 Pools Community/Municipality

Kentucky May IV Cryptosporidium AGI 131 Pools Community/Municipality

Maryland Aug III Chlorine gas ARI 2 Pool Membership club

Massachusetts Aug IV Giardia intestinalis AGI 10 Pool Membership club

Michigan Nov IV Unidentified Skin 2 Pool Hotel/Motel

Michigan Dec III Unidentified Skin 2 Pool Hotel/Motel

Minnesota May IV Unidentified†† Skin, ARI, Eye 4 Pool Hotel/Motel

Minnesota Jun IV Unidentified†† Skin 7 Wading pool Community/Municipality

Minnesota Jul II C. parvum AGI 20 Pool Membership club

Minnesota Sep I C. hominis AGI 58 Pools Waterpark

Minnesota Nov III Pseudomonas aeruginosa Skin 5 Spa Hotel/Motel

Minnesota Nov I P. aeruginosa Ear 6 Pool, spa Hotel/Motel

Minnesota Nov II C. hominis AGI 31 Pool Membership club

Minnesota Dec III Unidentified†† Skin 3 Spa Hotel/Motel

Mississippi Aug III Cryptosporidium AGI 11 Pool Neighborhood/Subdivision

Missouri Jun IV Cryptosporidium AGI 15 Wading pool Community/Municipality

Nebraska Jul IV Cryptosporidium AGI 2 Pool Community/Municipality

Nebraska Jul IV Cryptosporidium AGI 3 Pool Waterpark

Nebraska Jul IV Cryptosporidium AGI 3 Pool Community/Municipality

Nebraska Aug IV Cryptosporidium AGI 6 Pool Hotel/Motel

Nebraska Sep IV Cryptosporidium AGI 2 Pool Private residence

New Hampshire Feb IV Unidentified** Skin 4 Spa Hotel/Motel

New York Jan IV Unidentified Skin 25 Pool Waterpark

New York Jul IV Unidentified†† ARI 8 Spa Membership club

Ohio Jan I Chloramines, endotoxins ARI, Eye 665 Pool, interactive 

fountain, spa

Waterpark

Ohio Mar IV Unidentified** Skin 2 Spa Membership club

Ohio Mar II Unidentified Skin 31 Pool Hotel/Motel

Ohio Oct III Legionella pneumophila 

serogroup 1

ARI 2 Pool, spa Hotel/Motel

Oklahoma Jul I C. hominis AGI 93 Pool Community§§

Oklahoma Jul I C. parvum AGI 17 Pool State park

Pennsylvania Apr III Cryptosporidium AGI 76 Pool Membership club

Pennsylvania Jun IV C. parvum AGI 730 Pool Community/Municipality

Pennsylvania Jun IV G. intestinalis AGI 3 Fill-and-drain pool Private residence
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 7

illness (Table 6; Figure 2). The remaining 10 (7.5%) outbreaks 

were of combined illness types, nine of which included ARI. 

Outbreaks of AGI accounted for 12,477 (89.3%) of the total 

outbreak-related cases; 64 (79.0%) of the 81 AGI outbreaks 

started during June, July, or August (Figures 3 and 4).

The etiologic agent was confirmed for 105 (78.4%) outbreaks 

(Table 7; Figure 2). Of the outbreaks with a confirmed etiology, 

68 (64.8%) were caused by parasites; 22 (21.0%) by bacteria; 

five (4.8%) by viruses; nine (8.6%) by chemicals or toxins; 

and one (1.0%) by multiple etiology types. Outbreaks caused 

by parasites accounted for almost 50 times more cases than 

those caused by bacteria (12,492 and 254, respectively). Of 

the 90 outbreaks associated with treated recreational water 

and caused by an identified etiologic agent: 62 (68.4%) were 

caused by parasites, 18 (20.0%) by bacteria, eight (8.9%) by 

chemicals or toxins, and two (2.2%) by viruses (Table 7). Of 

the 15 outbreaks associated with untreated recreational water 

and caused by an identified etiologic agent, six (40.0%) were 

caused by parasites, four (26.7%) by bacteria, three (20.0%) 

by viruses, one (6.7%) by multiple pathogen types, and one 

(6.7%) by a toxin.

Parasites

All 62 outbreaks associated with treated recreational water 

and caused by parasites were outbreaks of AGI. A total of 

58 (93.5%) outbreaks were caused by Cryptosporidium, 

resulting in 12,137 cases. Some of the cryptosporidiosis 

outbreaks were communitywide outbreaks. A communitywide 

cryptosporidiosis outbreak typically starts as a focal outbreak 

associated with one recreational water venue and evolves into 

an outbreak associated with multiple recreational water venues 

or other settings (e.g., child care centers). As the outbreak 

progresses, recreational water exposure might decrease in its 

importance as a risk factor while secondary transmission (i.e., 

contact with an infected person) might become increasingly 

important. The statewide cryptosporidiosis outbreak in Utah 

in 2007 is an example of a communitywide cryptosporidiosis 

outbreak. Of the remaining four outbreaks of parasitic 

disease associated with treated recreational water, three were 

caused by Giardia intestinalis, and one was caused by both 

Cryptosporidium and Giardia. A July 2007 outbreak associated 

with a Florida interactive fountain is suspected, on the basis 

of common exposures to the implicated interactive fountain 

of persons with primary cases and diagnostic laboratory 

TABLE 2. (Continued) Waterborne disease outbreaks associated with treated recreational water (n = 74), by state/jurisdiction — Waterborne 

Disease and Outbreak Surveillance System, United States, 2007

State/

Jurisdiction Month Class* Etiology

Predominant 

illness†

No. of cases§ 

(n = 10,749) Venue Setting

Pennsylvania Jul III Cryptosporidium AGI 51 Pool Camp/Cabin

Pennsylvania Jul IV C. parvum AGI 39 Pool Membership club

Puerto Rico Aug II C. hominis AGI 107 Interactive fountain Waterpark

Tennessee May IV L. pneumophila serogroup 1 ARI 2 Spa Hotel/Motel

Tennessee Jul IV Shigella AGI 17 Pool, temporary 

waterslide

Camp/Cabin

Tennessee Jul IV Cryptosporidium AGI 18 Pool Community/Municipality

Tennessee Jul IV Cryptosporidium AGI 16 Splash pad Waterpark

Tennessee Aug IV C. hominis AGI 24 Pool Private residence

Utah May I Cryptosporidium AGI 5,697 Pools Community/Municipality

Virginia Jul IV G. intestinalis AGI 6 Pool Community/Municipality

Washington Aug IV Cryptosporidium AGI 14 Pool Membership club

West Virginia Apr II Unidentified** Skin 15 Wading pool Community/Municipality

West Virginia Nov IV Unidentified Skin 7 Spa Camp/Cabin

Wisconsin Aug IV Cryptosporidium AGI 35 Pool Community/Municipality

Wisconsin Aug IV Cryptosporidium AGI 38 Pool Aquatic facility

Wyoming Jul IV Cryptosporidium AGI 27 Pool Community/Municipality

Abbreviations: AGI = acute gastrointestinal illness; ARI = acute respiratory illness; Ear = illnesses, conditions, or symptoms related to the ears; Eye = illnesses, conditions, or symptoms related to the eyes; Skin = illnesses, conditions, or symptoms related to the skin.

* On the basis of epidemiologic and clinical laboratory data, and environmental data (see Table 1) provided to CDC. 

† The category of illness reported by ≥50% of ill respondents.

§ No deaths were reported in cases associated with outbreaks reported during 2007.

¶ Etiology unidentified: Cryptosporidium suspected on the basis of clinical diagnoses of secondary cases.

** Etiology unidentified: P. aeruginosa suspected on the basis of clinical syndrome and setting.

†† Etiology unidentified: contamination from excess chlorine levels, pool disinfection by-products (e.g., chloramines), or altered pool chemistry suspected.

§§The implicated pool was located at a membership club but functioned as a community pool.
Surveillance Summaries

8 MMWR / September 23, 2011 / Vol. 60 / No. 12

TABLE 3. Waterborne disease outbreaks associated with treated recreational water (n = 42), by state — Waterborne Disease and Outbreak 

Surveillance System, United States, 2008

State Month Class* Etiology

Predominant 

illness†

No. of cases§

(n = 2,731) Venue Setting

Alabama May IV Legionella pneumophila

serogroup 1

ARI 3 Spa Hotel/Motel

Arizona Jun I Cryptosporidium hominis AGI 57 Pool Community/Municipality

Arizona Jul III C. hominis AGI 9 Interactive fountain Waterpark

Arizona Jul I Cryptosporidium AGI 13 Pool Community/Municipality

California Jun IV Cryptosporidium AGI 11 Pool Membership club

California Aug IV Cryptosporidium AGI 5 Pool, interactive fountain Community/municipal park

Florida Mar I Legionella ARI 5 Spa Hotel/Motel

Florida Jun III L. pneumophila serogroup 1 ARI 3 Spa Membership club

Florida Aug IV Cryptosporidium parvum AGI 13 Pool Membership club

Idaho Jul IV Cryptosporidium AGI 2 Pool Community/Municipality

Illinois Jan III Chlorine ARI, Skin, Eye 20¶ Wading pool Waterpark

Illinois May I L. pneumophila serogroup 1 ARI 3 Pool, spa Hotel/Motel

Iowa Aug IV Cryptosporidium AGI 24 Pool Community/Municipality

Iowa Nov III Unidentified AGI 10 Pool Hotel/Motel

Kansas Jun IV Cryptosporidium AGI 6 Pool Community/Municipality

Kansas Oct II C. hominis AGI 22 Pool Hotel/Motel

Kentucky Jul III Hydrochloric acid ARI, AGI 5 Pool Waterpark

Maryland Jul IV Shigella sonnei AGI 12 Temporary waterslide, 

dunk tank

Farm

Massachusetts Mar IV Unidentified** Skin 50 Pool Waterpark

Michigan Apr IV Unidentified** ARI, AGI 4 Pool Hotel/Motel

Michigan May IV Unidentified** ARI, AGI 5 Pool Hotel/Motel

Michigan Aug IV Cryptosporidium AGI 3 Pool Neighborhood/Subdivision

Michigan Aug IV Unidentified** ARI 2 Pool Waterpark

Minnesota Apr II C. parvum AGI 12 Pool Membership club

Minnesota Aug II C. hominis AGI 12 Pools Waterpark

Minnesota Sep II C. hominis, Giardia 

intestinalis

AGI 19 Pools Membership club

New Jersey Jan IV Legionella ARI 98 Spa Community

New Mexico Jul III C. hominis AGI 89 Pool Community/Municipality

New York Apr I Pseudomonas aeruginosa Skin 23 Spa Hotel/Motel

New York Jun III Chlorine gas ARI, Eye 6 Pool Membership club

New York Jul IV Cryptosporidium AGI 3 Pool Community/Municipality

New York Aug III Pool chemical†† ARI 19 Wave pool Waterpark

New York Aug III Chlorine ARI 2 Pool School

New York Aug III Unidentified ** ARI, Eye 29 Pool School

Ohio Jul III L. pneumophila serogroup 1 ARI 2 Spa Hotel/Motel

Ohio Jul IV Cryptosporidium AGI 8 Pool Community/Municipality

Oklahoma Aug II Cryptosporidium AGI 44 Pool Camp

South Carolina Jan III L. pneumophila serogroup 1 ARI 2 Spa Hotel/Motel

Texas Jun I C. hominis AGI 2,050 Lake, pool, interactive 

fountain

Community/Municipality

Virginia May IV Unidentified§§ Skin 6 Pool, spa Hotel/Motel

Virginia Nov III L. pneumophila serogroup 1 ARI 2 Spa Assisted living facility

Wisconsin Feb II P. aeruginosa Skin 18 Pool, spa Hotel/Motel

Abbreviations: AGI = acute gastrointestinal illness; ARI = acute respiratory illness; Eye = illnesses, conditions, or symptoms related to the eyes; Skin = illnesses, 

conditions, or symptoms related to the skin.

* On the basis of epidemiologic and clinical laboratory data, and environmental data (see Table 1) provided to CDC.

† The category of illness reported by ≥50% of ill respondents.

§ No deaths were reported in cases associated with outbreaks reported during 2008.

¶ The case count reflects the number of persons who were hospitalized; no data were available on the total number of persons who were ill. An unknown number 

of persons experienced chemical burns, eye irritation, or gastrointestinal symptoms. 

** Etiology unidentified: contamination from excess chlorine levels, pool disinfection by-products (e.g., chloramines), or altered pool chemistry suspected.

†† One or more chemicals in the water caused illness; however, it was unclear which chemical(s) led to illness.

§§ Etiology unidentified: P. aeruginosa suspected on the basis of clinical syndrome and setting.
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 9

results of persons with secondary cases, to have been caused 

by Cryptosporidium.

Six outbreaks associated with untreated recreational water 

were caused by parasites including two by Cryptosporidium, 

resulting in 17 cases. Four additional outbreaks of cercarial 

dermatitis (sometimes called “swimmer’s itch”) were caused by 

avian schistosomes; three of these outbreaks were associated 

with fresh water and one with marine water.

Bacteria

A total of 18 outbreaks were associated with treated 

recreational water and caused by bacteria. Four were 

outbreaks of AGI. An April 2007 outbreak, caused by 

Escherichia coli O157:H7 and linked epidemiologically to a 

temporary inflatable waterslide at a California home, resulted 

in 11 persons becoming ill. The waterslide was not designed 

for disinfection or filtration equipment. In June 2007, another 

outbreak caused by E. coli O157:H7 resulted in 31 cases and 

was linked epidemiologically to an interactive fountain at an 

Idaho waterpark. An environmental health investigation of 

the implicated, unregulated recreational water venue noted 

free chlorine levels <0.5 mg/L (or parts per million [ppm]). A 

shigellosis outbreak in July 2007 was associated with a pool 

or temporary inflatable waterslide at a Tennessee camp and 

resulted in 17 cases. An outbreak in Maryland in July 2008 

caused by Shigella sonnei that resulted in illness in 12 persons was 

linked epidemiologically to a waterslide or dunk tank at a farm. 

Municipal water with no additional disinfection or filtration 

was used for the recreational water activities. Four outbreaks 

caused by Pseudomonas were each linked epidemiologically to 

hotel/motel pools or spas and resulted in 52 cases of skin- or 

ear-related illnesses, conditions, or symptoms. Ten outbreaks 

TABLE 4. Waterborne disease outbreaks associated with untreated recreational water (n = 10), by state — Waterborne Disease and Outbreak 

Surveillance System, United States, 2007

State Month Class* Etiology

Predominant† 

illness

No. of cases§

(n = 321) Venue Setting

California May IV Plesiomonas 

shigelloides

AGI 2 River/Stream Public outdoor area

Colorado Jun IV Shigella flexneri

type I

AGI 2 Lake/Reservoir Public outdoor area

Colorado Jun III Schistosomes Skin 57 Lake/Reservoir Community/municipal park

Colorado Aug IV Schistosomes Skin 200 Lake/Reservoir Public outdoor area

Florida Jul III Karenia brevis ARI 15 Ocean Public outdoor area

Florida Aug IV Unidentified AGI, Skin 2 Ocean Community/municipal park

New Mexico Jun III Schistosomes Skin 12 Lake/Reservoir Public outdoor area

North Dakota Jul IV Cryptosporidium AGI 10 Lake/Reservoir Camp/cabin

Tennessee Aug II Unidentified AGI 14 River/Stream Public outdoor area

Washington Sep IV Cryptosporidium AGI 7 Lake/Reservoir State park

Abbreviations: AGI = acute gastrointestinal illness; ARI = acute respiratory illness; Skin = illnesses, conditions, or symptoms related to the skin.

* On the basis of epidemiologic and clinical laboratory data, and environmental data (see Table 1) provided to CDC.

† The category of illness reported by ≥50% of ill respondents.

§ No deaths were reported in cases associated with outbreaks reported during 2007.

TABLE 5. Waterborne disease outbreaks associated with untreated recreational water (n = eight), by state — Waterborne Disease and Outbreak 

Surveillance System, United States, 2008

State Month Class* Etiology

Predominant 

illness†

No. of cases§

(n = 165) Venue Setting

California Jun III Schistosomes Skin 31 Ocean State park

Connecticut Jul I Norovirus genogroup I AGI 16 Lake/Reservoir Unknown

Florida Mar IV Shigella AGI 2 Spring County park

Illinois Jul II Escherichia coli O157:H7 AGI 3 Lake/Reservoir Camp/Cabin

Minnesota Jul I Norovirus AGI 26 Lake/Reservoir Camp/Cabin

Ohio Jul II S. sonnei, norovirus genogroup I, 

Yersinia enterolytica

AGI 54 Lake/Reservoir¶ Waterpark

Vermont Jul IV Unidentified** Other 10 Lake/Reservoir Camp/Cabin

Wisconsin Jul IV Norovirus genogroup I AGI 23 Lake/Reservoir Camp/Cabin

Abbreviations: AGI = acute gastrointestinal illness; Skin = illnesses, conditions, or symptoms related to skin; Other = undefined illnesses, conditions, or symptoms.

* On the basis of epidemiologic and clinical laboratory data, and environmental data (see Table 1) provided to CDC.

† The category of illness reported by ≥50% of ill respondents.

§ No deaths were reported in cases associated with outbreaks reported during 2008.

¶ A lake with a sand bottom in a freshwater waterpark setting. Some modifications had been made to the lake to support swimming by patrons of the waterpark.

** Etiology unidentified: reported symptoms were consistent with leptospirosis, but clinical specimens tested negative for Leptospira infection.
Surveillance Summaries

10 MMWR / September 23, 2011 / Vol. 60 / No. 12

caused by Legionella were linked epidemiologically to pools or 

spas and resulted in 122 persons developing legionellosis (i.e., 

LD or PF). The largest legionellosis outbreak resulted in 98 

cases of PF and was linked epidemiologically to a New Jersey 

pay-on-entry spa. Exposures leading to seven of the outbreaks 

occurred in a hotel/motel setting.

Four outbreaks of AGI were caused by bacteria and associated 

with untreated water. These included two outbreaks caused by 

Shigella, an outbreak caused by Plesiomonas shigelloides, and an 

outbreak caused by E. coli O157:H7.

Six additional outbreaks, resulting in a total of 57 cases, had 

unidentified etiologies but were suspected, on the basis of the 

clinical symptoms and common exposures to implicated pools 

or spas, to have been caused by Pseudomonas; exposures leading 

to four of these outbreaks occurred in a hotel/motel setting. An 

additional outbreak of an unidentified etiology was suspected, 

on the basis of the clinical symptoms and common exposure 

to a pond, to have been caused by Leptospira.

Viruses

Five outbreaks of AGI had a viral etiology and resulted in 

121 cases. Norovirus was identified as the etiologic agent 

in each of the outbreaks; two were associated with treated 

water and three with lakes. The two treated venue–associated 

outbreaks were caused by norovirus genogroup II. One of 

them occurred in June 2007 and was linked epidemiologically 

to a California interactive fountain; an environmental health 

investigation noted inadequacies in facility design. Design 

plans were not submitted to public health officials for review 

before construction. Two of the three outbreaks associated with 

lakes were caused by norovirus genogroup I.

Chemicals/Toxins

Nine outbreaks associated with chemicals or toxins resulted 

in a total of 747 cases. The largest outbreak occurred during 

January–March 2007 and was associated with an indoor Ohio 

waterpark. Swimmers and employees at the waterpark experienced 

respiratory and eye irritation. An environmental health investigation 

revealed elevated trichloramine and endotoxin levels (23,24). Three 

outbreaks were caused by exposure to toxic chlorine gas at pools at 

one apartment complex and two membership clubs. Environmental 

health investigations of two of these outbreaks noted that the 

circulation pump shut down while the chlorine and muriatic (i.e., 

≥4

3 

2 

1 

0 

FIGURE 1. Number of waterborne disease outbreaks associated with 

recreational water (n = 134), by state/jurisdiction — Waterborne Disease 

and Outbreak Surveillance System, United States, 2007–2008*

* Note: These numbers are largely dependent on surveillance and reporting 

activities in individual states/jurisdictions and do not necessarily indicate the 

true incidence of waterborne disease outbreaks.

TABLE 6. Number of waterborne disease outbreaks associated with recreational water (n = 134), by predominant illness* and type of water 

exposure — Waterborne Disease and Outbreak Surveillance System, United States, 2007–2008

Type of exposure

Predominant illness

Treated Untreated Total†

Outbreaks Cases Outbreaks Cases Outbreaks (%) Cases (%)

AGI 70 12,318 11 159 81(60.4) 12,477 (89.3)

ARI 16 168 1 15 17 (12.7) 183 (1.3)

AGI and ARI 4 23 0 0 4 (3.0) 23 (0.2)

Ear 1 6 0 0 1 (0.7) 6 (0.0)

Eye and ARI 3 700 0 0 3 (2.2) 700 (5.0)

Skin 20 241 4 300 24 (17.9) 541 (3.9)

Skin and AGI 0 0 1 2 1 (0.7) 2 (0.0)

Skin, ARI, and Eye 2 24 0 0 2 (1.5) 24 (0.2)

Other 0 0 1 10 1 (0.7) 10 (0.0)

Total (%) 116 (86.6) 13,480 (96.5) 18 (13.4) 486 (3.5) 134 (100.0) 13,966 (100.0)

Abbreviations: AGI = acute gastrointestinal illness; ARI = acute respiratory illness; Ear = illnesses, conditions, or symptoms related to the ears; Eye = illnesses, conditions, 

or symptoms related to the eyes; Skin = illnesses, conditions, or symptoms related to skin; Other = undefined illnesses, conditions, or symptoms.

* The category of illness reported by ≥50% of ill respondents.

† Percentages do not add up to 100.0% due to rounding.
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 11

hydrochloric) acid feed pumps continued to run, allowing the 

chlorine and acid to mix within the circulation system plumbing 

without dilution, thus generating toxic chlorine gas. When the 

circulation pumps were restarted, the gas was released into the pools 

and caused respiratory distress in swimmers. Two outbreaks were 

attributed to release of excess chlorine caused by issues related to 

the automatic chemical controllers and poolside water testing. In 

addition, one outbreak was caused by the release of excess muriatic 

acid during filter backwash. Another outbreak was caused by 

one or more pool chemicals; however, which chemical(s) caused 

the outbreak was unclear. Both outbreaks occurred at outdoor 

waterparks. The remaining outbreak was linked epidemiologically to 

untreated water and was caused by exposure to brevetoxins released 

by algae, specifically Karenia brevis (25).

Nine additional outbreaks, resulting in 112 cases, of 

unidentified etiologies were suspected, on the basis of clinical 

symptoms and common exposures to implicated pools or spas, to 

have been caused by excessive levels of chlorine, pool disinfection 

by-products, or altered pool chemistry. Exposures leading to 

four of these outbreaks occurred in a hotel/motel setting. Two 

outbreaks, resulting in 79 cases, were suspected to have been 

caused by chloramines and were associated with an indoor 

Massachusetts waterpark and an indoor New York school pool.

Previously Unreported Outbreaks

Data on five previously unreported recreational water–

associated outbreaks were received (Table 8). Four children 

at a residential school became ill with AGI after using a 

Skin 17.9%

Type of exposure (n = 134)

Treated water 86.6%

Untreated water 

13.4%

ARI 12.7%

Ear 0.7%

Other§ Multiple 7.5% 0.7%

AGI 60.4%

Predominant illness (n = 134)*†

Etiology (n = 134)† Route of entry (n = 134)

Parasitic 50.7%

Viral 3.7% Multiple 0.7%

Chemical/toxin 6.7%

Unidentied 21.6%

Bacterial: Legionella 7.5%

Bacterial: 

Non-Legionella 9.0%

Ingestion 60.4%

Contact 18.7%

Inhalation13.4%

Combined 4.5% Unknown§

 3.0%

FIGURE 2. Recreational water–associated outbreaks, by type of exposure, predominant illness,* etiology, and route of entry — Waterborne 

Disease and Outbreak Surveillance System, United States, 2007–2008

Abbreviations: AGI = acute gastrointestinal illness; ARI = acute respiratory illness; Ear = illnesses, conditions, or symptoms related to the ears; Skin = illnesses, conditions, or symptoms related to the skin; Multiple = a combination of predominant illnesses; Other = undefined illnesses, conditions, or symptoms.

* The category of illness reported by ≥50% of ill respondents. 

† Percentages do not add up to 100.0% due to rounding. 

§ Etiology was not identified for one outbreak. Reported symptoms were consistent with leptospirosis but clinical specimens tested negative for Leptospira infection. 

Route of transmission was unclear.
Surveillance Summaries

12 MMWR / September 23, 2011 / Vol. 60 / No. 12

fill-and-drain pool. All four tested positive for Campylobacter. 

Published and unpublished data on four legionellosis outbreaks 

that were associated with recreational water and occurred 

during 1993–1999 were added to the WBDOSS database 

(26,27). Three of the legionellosis outbreaks resulted in four 

deaths; these deaths are included in the respective outbreak 

case counts. Data on these five outbreaks are summarized but 

not included in the analysis for this report.

Pool Chemical–Associated Health Events

Maryland and Michigan reported 32 pool chemical–

associated health events that did not meet the definition of 

an outbreak associated with recreational water and occurred 

during 2007–2008 (Table 9). These health events resulted in 48 

cases of illness or injury; the median number of cases associated 

with an event was one (range: 1–15 cases). No deaths were 

reported. Nineteen (59.3%) of the events resulted in ARI, six 

(18.8%) in ocular symptoms, three (9.4%) in AGI, and four 

(12.5%) in ARI and other illness. Almost half (15 [46.9%]) of 

the events occurred at a private residence. Six (18.8%) occurred 

at a public setting and 11 (34.4%) at an unknown setting. 

All six events affecting a single child aged ≤13 years occurred 

at private residences. Of the 32 events, 26 (81.3%) were the 

result of pool chemical–exposures that occurred in June, July, 

or August. Twenty-six (81.3%) events could be attributed 

at least in part to chemical handling errors, such as mixing 

incompatible chemicals (e.g., acid and chlorine or different 

chlorines) and not using personal protective equipment (PPE).

Hazardous Substances Emergency Events 

Surveillance Associated With Aquatic Facilities

For 2007–2008, a total of 12 state health departments††

reported 92 hazardous substance events involving aquatic facilities, 

55 (59.8%) of which involved injured persons. The 92 events 

led to 663 persons being evacuated, 231 being injured (median 

number of persons injured in an event with injuries: one; range: 

1–44), and 111 persons (84 [75.7%] of whom were injured) 

being decontaminated (Table 10). No deaths were reported.The 

majority of injured persons (132 [57.1%]) were members of 

the general public; 50 (21.6%) were students. Over half of the 

injured persons (140 [60.6%]) had injuries/symptoms requiring 

hospital treatment but not admission. Although injured persons 

primarily reported respiratory irritation or gastrointestinal injuries/

symptoms, they also reported suffering from a wide range of 

injuries/symptoms (Table 11). A total of 33 (35.9%) of the overall 

events and 22 (40.0%) of the events with injured persons occurred 

in residential areas (Table 12).

FIGURE 3. Number of waterborne disease outbreaks associated with recreational water (n = 134), by predominant illness* and month — 

Waterborne Disease and Outbreak Surveillance System, United States, 2007–2008

Abbreviations: AGI = acute gastrointestinal illness; ARI = acute respiratory illness; Ear = illnesses, conditions, or symptoms related to the ears; Skin = illnesses, conditions, or symptoms related to the skin; Multiple = a combination of predominant illnesses; Other = undefined illnesses, conditions, or symptoms.

* The category of illness reported by ≥50% of ill respondents.

Other

Multiple

Ear

ARI

Skin

AGI

Month

No. of outbreaks

35

30

25

20

15

10

5

0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

†† Florida, Iowa, Louisiana, Michigan, Minnesota, North Carolina, New Jersey, 

New York, Oregon, Texas, Utah, and Wisconsin.
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 13

A primary contributing factor was reported for 88 (95.7%) 

of events overall and 52 (94.5%) of events with injured persons 

(Figure 5). Among this subset of events, human error was the 

leading primary contributing factor for events overall and those 

with injured persons (51 [58.0%] and 36 [69.2%], respectively). 

Equipment failure was also frequently reported to be the primary 

contributing factor of events overall and those with injured 

persons (31 [35.2%] and 16 [30.8%], respectively).

National Electronic Injury Surveillance System

During 1999–2008, the median estimated number of annual 

ED visits for pool chemical–associated injuries was 4,120 

(range: 3,315–5,216) (Figure 6). In 2008, an estimated 4,574 

persons (1.5 per 100,000 population [95% confidence interval 

(CI): 0.9–2.1]) visited an ED for pool chemical–associated 

injuries (Table 13). The most common injury diagnoses were 

poisoning, which includes ingestion of pool chemicals as well 

as inhalation of their vapors, fumes, or gases (1,784 [95% 

CI: 585–2,984]) and dermatitis/conjunctivitis (1,452 [95% 

CI: 936–1,969]); more than half of the injuries occurred at 

a residence (2,870; [95% CI: 1,363–4,377]). Almost three 

quarters of the injuries occurred during June, July, or August. 

No deaths were documented.

FIGURE 4. Recreational water–associated outbreaks of acute gastrointestinal illness, by type of exposure and etiology — Waterborne Disease 

and Outbreak Surveillance System, United States, 2007–2008

* Percentages do not add up to 100.0% due to rounding.

Cryptosporidium spp. 

74.1%

Type of exposure (n = 81)

Unidentied 3.7%

Shigella sonnei, norovirus GI, 

Yersinia enterolytica 1.2%

 

Etiology (n = 81)*

Giardia intestinalis 3.7%

Untreated water 13.6%

Treated water 86.4%

S. sonnei, norovirus GI, 

Y. enterolytica 9.1%

P. shigelloides

9.1%

Norovirus 27.3%

 

Escherichia coli O157:H7 3.7%

Etiology: untreated water (n = 11)* Etiology: treated water (n = 70)*

Unidentied 9.1% 

Cryptosporidium spp. 18.2%

E. coli O157:H7

9.1%

Shigella spp. 18.2%

Shigella spp. 4.9%

Norovirus 6.2%

Plesiomonas shigelloides 1.2%

Cryptosporidium spp., Giardia intestinalis 1.2%

Cryptosporidium spp. 

82.9%

Cryptosporidium sp., 

G. intestinalis 1.4%

Norovirus 2.9%

G. intestinalis 4.3%

Shigella spp. 2.9%

Unidentied 2.9%

E. coli O157:H7 2.9%
Surveillance Summaries

14 MMWR / September 23, 2011 / Vol. 60 / No. 12

TABLE 7. Number of waterborne disease outbreaks associated with recreational water (n = 134), by etiology and type of water exposure — 

Waterborne Disease and Outbreak Surveillance System, United States, 2007–2008

Etiology

Type of exposure

Treated Untreated Total

Outbreaks Cases Outbreaks Cases Outbreaks (%)* Cases (%)*

Bacterium 18 245 4 9 22 (16.4) 254 (1.8)

Esherichia coli O157:H7 2 42 1 3 3 45

Legionella spp. 10 122 0 0 10 122

Plesiomonas shigelloides  0 0 1 2 1 2

Pseudomonas aeruginosa 4 52 0 0 4 52

Shigella spp. 2 29 2 4 4 33

Parasite 62 12,175 6 317 68 (50.7) 12,492 (89.4)

Schistosomes 0 0 4 300 4 300

Cryptosporidium spp. 58 12,137 2 17 60 12,154

Cryptosporidium hominis, Giardia intestinalis 1 19 0 0 1 19

G. intestinalis 3 19 0 0 3 19

Virus 2 56 3 65 5 (3.7) 121 (0.9)

Norovirus 2 56 3 65 5 121

Chemical/Toxin 8 732 1 15 9 (6.7) 747 (5.3)

Chloramines and endotoxins 1 665 0 0 1 665

Chlorine 2 22 0 0 2 22

Chlorine gas 3 21 0 0 3 21

Hydrochloric acid 1 5 0 0 1 5

Karenia brevis 0 0 1 15 1 15

Pool chemical(s)† 1 19 0 0 1 19

Multiple§ 0 0 1 54 1 (0.7) 54 (0.4)

S. sonnei, norovirus genogroup I, Yersinia enterolytica 0 0 1 54 1 54

Unidentified 26 272 3 26 29 (21.6) 298 (2.1)

Suspected chemical exposure¶ 7 33 0 0 7 33

Suspected chloramines 2 79 0 0 2 79

Suspected Cryptosporidium 1 6 0 0 1 6

Suspected P. aeruginosa 6 57 0 0 6 57

Unidentified 10 97 3 26 13 123

Total (%) 116 (86.6) 13,480 (96.5) 18 (13.4) 486 (3.5) 134 (100.0) 13,966 (100.0)

* Percentages do not add up to 100.0% due to rounding.

† One or more chemicals in the water caused illness; which chemical(s) caused the outbreak was unclear.

§ Outbreaks with multiple etiologies are defined as outbreaks in which more than one type of etiologic agent is identified in clinical specimens from affected persons, 

and each etiologic agent is found in ≥5% of positive clinical specimens (e.g., the outbreak caused by Shigella sonnei and Yersinia enterolytica [bacteria] and norovirus 

genogroup I [virus], in which each agent was identified in ≥5% of stool specimens).

¶ Etiology unidentified: for nine outbreaks, the etiologies were suspected on the basis of clinical symptoms to be excessive levels of chlorine, pool disinfection byproducts, or altered pool chemistry.

TABLE 8. Previously unreported outbreaks associated with recreational water (n = five), by state — Waterborne Disease and Outbreak Surveillance 

System, United States, 1993–2005*

State Month Year Class† Etiology

Predominant 

illness§

No. of cases [deaths] 

(n = 65 [4])¶ Venue Setting

Florida Apr 1993 IV Legionella pneumophila ARI 3 [1] Spa Hotel/Motel

Georgia Apr 1999 I L. pneumophila serogroup 6 ARI 24 —** Spa Hotel/Motel

Iowa Jan 1995 II L. pneumophila serogroup 1 ARI 11 [1] Spa Hotel/Motel

Maryland Jun 2005 III Campylobacter AGI 4 — Fill-and-drain pool Residential school

Virginia Sep 1996 I L. pneumophila serogroup 1 ARI 23 [2] Spa Store

Abbreviations: AGI = acute gastrointestinal illness; ARI = acute respiratory illness.

* Previously unreported legionellosis outbreaks included in this table were identified through a review of the published literature on Legionella as well as reports 

from past CDC-led investigations occurring during 1971–2000. Data presented are based on review and interpretation of available outbreak investigation reports 

and published articles.

† On the basis of epidemiologic and clinical laboratory data, and environmental data (see Table 1) provided to CDC. 

§ The category of illness reported by ≥50% of ill respondents.

¶ Deaths are included in the overall case count.

** No deaths were reported.
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 15

TABLE 9. Number of pool chemical–associated health events (n = 32), by state — Hazardous Substance Emergency Events Surveillance System, 

Maryland and Michigan, 2007–2008*

State Month Year Etiology Predominant illness† No. of cases (n = 48) Venue Setting

Maryland May 2008 Chlorine gas§ ARI 1 Pool Apartment/Condominium

Michigan May 2007 Chlorine¶ ARI 1 Pool Waterpark

Michigan Jun 2007 Chlorine gas§ ARI 1 Pool Private residence

Michigan Jun 2007 Chlorine¶ ARI 1 Pool Unknown

Michigan Jun 2007 Unidentified** ARI 1 Pool Private residence

Michigan Jun 2007 Unidentified** ARI 1 Pool Private residence

Michigan Jul 2007 Chlorine¶ ARI, AGI 1 Pool Unknown

Michigan Jul 2007 Sodium hypochlorite Eye 1 Pool Neighborhood/Subdivision

Michigan Jul 2007 Chlorine gas§ ARI 1 Pool Unknown

Michigan Aug 2007 Chlorine gas§ ARI, AGI 1 Pool Unknown

Michigan Aug 2007 Chlorine¶ ARI 1 Pool Waterpark

Michigan Nov 2007 Algaecide AGI 1 Pool Private residence

Michigan Mar 2008 Unidentified** ARI 15 Pool Hotel/Motel

Michigan Jun 2008 Chlorine¶ ARI 1 Pool Private residence

Michigan Jun 2008 Unidentified†† ARI 1 Pool Private residence

Michigan Jun 2008 Chlorine powder ARI 1 Pool Private residence

Michigan Jun 2008 Unidentified†† Eye 1 Pool Private residence

Michigan Jun 2008 Chlorine gas AGI 1 Pool Private residence

Michigan Jun 2008 Chlorine powder ARI 1 Pool Private residence

Michigan Jun 2008 Chlorine¶ ARI 1 Pool Private residence

Michigan Jun 2008 Chlorine¶ Eye 1 Pool Community/Municipility

Michigan Jun 2008 Chlorine¶ ARI 1 Pool Private residence

Michigan Jun 2008 Unidentified†† AGI 1 Pool Unknown

Michigan Jun 2008 Unidentified†† ARI, Eye 2 Pool Private residence

Michigan Jul 2008 Chlorine liquid Eye 1 Pool Unknown

Michigan Jul 2008 Chlorine powder ARI 2 Pool Private residence

Michigan Jul 2008 Chlorine liquid ARI 1 Pool Unknown

Michigan Jul 2008 Chlorine¶ ARI, AGI 1 Pool Unknown

Michigan Aug 2008 Chlorine¶ ARI 1 Pool Unknown

Michigan Aug 2008 Chlorine gas§ ARI 1 Pool Unknown

Michigan Sep 2008 Sodium hypochlorite Eye 1 Pool Private residence

Michigan Nov 2008 Unidentified†† Eye 1 Pool Unknown

Abbreviations: AGI = acute gastrointestinal illness; ARI = acute respiratory illness; Eye = illnesses, conditions, or symptoms related to the eyes; Skin = illnesses, conditions, or symptoms related to the skin.

* Reports submitted by individual states were based on data from injury surveillance activities within those states. Events reported to the Hazardous Substances 

Emergency Events Surveillance (HSEES) system were omitted from this list and are discussed in aggregate with HSEES data (see Table 10).

† The category of illness reported by ≥50% of ill respondents.

§ Chlorine gas was generated after inappropriate chemical mixing (e.g., liquid chlorine bleach and acid).

¶ Unknown if chlorine was in a gaseous, liquid, or powder form when exposure occurred.

** Etiology unidentified: chlorine gas suspected on basis of event summary and symptoms.

†† Etiology unidentified: one or more person(s) were handling pool chemical(s) used to “shock” swimming pools.

TABLE 10. Number of persons injured in pool chemical–associated health events (n = 55), by treatment category — Hazardous Substance 

Emergency Events Surveillance System, 12 states,* 2007–2008

Injured person 

type

No. of 

injured 

persons

No. of persons 

decontaminated

Predominant 

symptoms†

No. treated

On scene 

(first aid)

At hospital 

(not admitted)

At hospital 

(admitted)

At hospital 

(not treated)

By private physician 

within 24 hrs

Injury reported 

by an official§

General public¶ 132** 26 Respiratory 

irritation

20 63 28 1 4 15

Student†† 50 42 Respiratory 

irritation

1 48 1 0 0 0

Employee§§ 42 16 Respiratory 

irritation

2 22 3 0 14 1

Responder¶¶ 7 0 Gastrointestinal 0 7 0 0 0 0

Total 231** 84 23 140 32 1 18 16

* Florida, Iowa, Louisiana, Michigan, Minnesota, North Carolina, New Jersey, New York, Oregon, Texas, Utah, and Wisconsin. 

† At least 50% of persons had the symptom.

§ Injuries reported within 24 hours of event by officials (e.g., fire department, emergency medical technician, police, or poison control center).

¶ Includes persons who were neither employees of a company when the event occurred nor either responders or students.

** Data on public health consequences were missing for one injured person.

†† Includes a child or an adult who was a student when the event occurred at a school (includes elementary, middle, high, or vocational school or colleges).

§§ Includes injured persons who worked at the company where the event occurred, including owners.

¶¶ A person whose job is to bring the chemical release under control, provide medical assistance to injured persons, or conduct crowd control.
Surveillance Summaries

16 MMWR / September 23, 2011 / Vol. 60 / No. 12

Vibriosis

During 2007–2008, a total of 236 individual vibriosis cases 

associated with recreational water exposure were reported by 

25 states, representing 20.6% (236/1,148) of the total number 

of vibriosis cases reported for these years (28,29). Of the 236 

patients, 74 (31.4%) were hospitalized, and nine (3.8%) 

patients died (Table 14). The most frequently isolated Vibrio

species was Vibrio alginolyticus, which was isolated from clinical 

specimens from 106 (44.9%) patients overall, 11 (14.9%) 

of those hospitalized, and two (22.2%) of those who died. 

V. vulnificus was isolated from clinical specimens from 48 

(20.3%) patients overall, 36 (48.6%) of those hospitalized, and 

six (66.7%) of those who died. Recreational water–associated 

Vibrio infections occurred most commonly during summer 

months (Figure 7).

Nearly all vibriosis patients reported that they were exposed 

to recreational water in coastal states (Figure 8). The most 

frequently reported exposure location was the Atlantic coastal 

states (86 [36.4%); followed by the Gulf Coast states, which 

include Florida (81 [34.3%]); Pacific coastal states (59 [25.0%]); 

and noncoastal states (10 [4.2%]) (Table 15). Florida, Hawaii, 

California, and Texas had the highest number of reported 

exposures in their jurisdictions (31, 29, 27, and 27, respectively).

Primary Amebic Meningoencephalitis

Eight individual fatal cases of PAM caused by Naegleria 

fowleri were reported for 2007–2008 (Table 16). Illness 

occurred after exposure to warm untreated freshwater in 

Florida (three cases), Texas (two cases), Arizona (one case), 

California (one case), and Oklahoma (one case). The median 

age at death was 13 years (range: 9–22 years); seven (87.5%) 

patients were male.

Discussion

Overview of Outbreaks

A total of 134 recreational water–associated outbreaks were 

reported to CDC for 2007–2008. They occurred year-round, 

but almost two thirds of them started during the traditional 

summer swim season (Memorial Day through Labor Day). 

The 2007–2008 outbreak count represents a 71.8% increase 

over that for 2005–2006 (n = 78) and the largest number of 

outbreaks ever reported for a 2-year period (Figure 9). The 

number of drinking water–associated outbreaks reported to 

TABLE 11. Number and percentage of injuries/symptoms attributed 

to pool chemical–associated health events, by injury/symptom type — 

Waterborne Disease Outbreak Surveillance System, 12 states,* 

2007–2008

Type No.† (%)§

Respiratory irritation 187 (56.8)

Gastrointestinal 59 (17.9)

Eye irritation 32 (9.7)

Shortness of breath 20 (6.1)

Dizziness/central nervous system 12 (3.6)

Headache 9 (2.7)

Skin irritation 6 (1.8)

Burn 2 (0.6)

Heart problem 1 (0.3)

Other 1 (0.3)

Total 329 (100.0)

* Florida, Iowa, Louisiana, Michigan, Minnesota, North Carolina, New Jersey, 

New York, Oregon, Texas, Utah, and Wisconsin. 

† The total number of injuries/symptoms is higher than the total number of victims 

because injured persons could report more than one injury/symptom.

§ Percentages do not add up to 100.0% due to rounding.

TABLE 12. Number of pool chemical–associated health events (n = 91), by substance category and setting — Hazardous Substance Emergency 

Events Surveillance System, 12 states,* 2007–2008†

Substance category

All events Events including injured persons

Residential§ Nonresidential¶ Both** Residential Nonresidential Both

Chlorine 19 15 22 12 11 10

Acids 6 3 3 6 2 1

Other inorganic substances†† 3 2 1 0 0 0

Mixture across chemical category§§ 3 5 4 3 5 2

Pesticides 1 1 0 0 0 0

Oxy/Organics 0 0 1 0 0 1

Volatile organic compounds 0 0 1 0 0 0

Other¶¶ 1 0 0 1 0 0

Total 33 26 32 22 18 14

* Florida, Iowa, Louisiana, Michigan, Minnesota, North Carolina, New Jersey, New York, Oregon, Texas, Utah, and Wisconsin. 

† Data on setting were missing for one event involving one injured person.

§ Includes land that predominantly consists of housing.

¶ Includes commercial, recreational, and industrial areas.

** Includes events that occurred in both residential and nonresidential areas.

†† Includes all inorganics except for acids, bases, ammonia, and chlorine.

§§ Includes substances from different categories that were mixed or formed from a reaction.

¶¶ Includes substances that do not belong to one of the existing categories.
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 17

CDC concurrently increased 80%, from 20 for 2005–2006 

to 36 for 2007–2008 (16). Overall, the number of recreational water–associated outbreaks reported annually has 

increased substantially since reporting to CDC began in 1978. 

Possible contributing factors to the increased recreational 

water–associated outbreak reporting include but are not 

limited to 1) changes in detection, investigation, and reporting of waterborne disease outbreaks and 2) the emergence of 

Cryptosporidium.

Dedicating human resources to waterborne disease detection, 

investigation, and reporting could be contributing to the 

observed increase. In late 2006, CDC and its state, DC, 

territorial, and FAS partners began developing a waterborne 

disease network and identified points of contact for waterrelated issues in each reporting jurisdiction, and CDC hired 

a permanent full-time surveillance coordinator dedicated to 

running WBDOSS. Having staff dedicated to water-related 

complaints and inquiries has been reported to be key in 

optimizing waterborne disease surveillance (30). This network 

allows CDC and its partners to share outbreak investigation tools 

(e.g., water testing, Cryptosporidium subtyping, questionnaires, 

and press releases) and lessons learned (e.g., via webinars and 

newsletters). In addition, CDC, EPA, and state partners have 

been collaborating to strengthen waterborne disease outbreak 

detection, investigation, response, and reporting under the 

umbrella of the Environmental Health Specialists Network 

(EHS-Net) Water program, which provides funding for 

waterborne disease staff and projects in state public health 

agencies. One EHS-Net Water project that involved conducting 

a retrospective review of state-specific waterborne disease 

outbreak data at the states and at CDC identified outbreaks that 

had not been reported previously to CDC (13).

The increasing number of outbreaks of AGI is driving the 

overall increase in outbreak reporting (Figures 9 and 10). 

Since the first reported U.S. recreational water–associated 

cryptosporidiosis outbreak was identified in 1988 (31), 

Cryptosporidium has emerged as the single most important 

etiologic agent of recreational water–associated outbreaks 

(Figure 11). In 2007–2008, of 81 outbreaks of AGI, 60 (74.0%) 

were caused by Cryptosporidium; all but two were associated 

FIGURE 5. Distribution of primary contributing factors reported for pool chemical–associated health events — Hazardous Substance Emergency 

Events Surveillance System, 12 states,* 2007–2008

* Florida, Iowa, Louisiana, Michigan, Minnesota, North Carolina, New Jersey, New York, Oregon, Texas, Utah, and Wisconsin.

Equipment failure 30.8%

Primary contributing 

factor for all events

(n = 88)

Primary contributing factor for 

events with injured persons 

(n = 52)

Human error

58.0% Equipment failure 

35.2% Human error 69.2%

Intentional/illegal 

act 5.7%

Other 1.1%

FIGURE 6. Number of estimated emergency department (ED) visits 

for injuries attributed to pool chemicals (n = 40,803), by year — 

National Electronic Injury Surveillance System (NEISS), United States, 

1999–2008

Source: NEISS. Estimates Query Builder. Bethesda, Maryland: U.S. Consumer 

Product Safety Commission. 1999–2008. Code 938, swimming pool chemicals. 

Available at http://cpsc.gov/cgibin/NEISSQuery/home.aspx.

* 95% confidence interval.

7,500

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

Year

6,000

4,500

3,000

1,500

0

No. of estimated ED visits

*
Surveillance Summaries

18 MMWR / September 23, 2011 / Vol. 60 / No. 12

TABLE 13. Estimated number, percentage, and rate of pool chemical–associated injuries treated in hospital emergency departments — National 

Electronic Injury Surveillance System (NEISS), United States, 2008*†

Characteristic No.

Weighted 

estimate§¶ 95% CI %** Annual rate†† 95% CI

Injury diagnosis

Poisoning§§ 38 1,784 585–2,984 39.0 —¶¶ —

Dermatitis/conjunctivitis 38 1,452 936–1,969 31.7 0.5 0.3–0.6

Other (e.g., chemical burns) 35 1,338 725–1,951 29.3 0.4 0.2–0.6

Affected body part

All parts of the body (>50% of body) *** 48 2,181 862–3,500 47.7 — —

Eyeball 39 1,452 838–2,066 31.7 0.5 0.3–0.7

Other (e.g., hand) or not recorded (n = three) 24 942 447–1,436 20.6 — —

Patient disposition

Treated and released, or examined and released without treatment 101 4,214 2,903–5,526 92.1 1.4 1.0–1.8

Other††† 10 360 0–731 7.9 — —

Incident location

Residence 60 2,870 1,363–4,377 62.7 — —

Public location (e.g., school or place of recreation or sports) 16 352 71–632 7.7 — —

Unknown 35 1,352 580–2,124 29.6 — —

Patient age (yrs)

0–17 55 2,051 1,168–2,934 44.8 2.8 1.6–3.9

18–45 35 1,698 1,012–2,384 37.1 1.4 0.9–2.0

≥46 21 825 90–1,561 18.0 — —

Patient sex

Male 60 2,403 1,313–3,493 52.5 1.6 0.9–2.3

Female 51 2,171 1,374–2,969 47.5 1.4 0.9–1.9

Patient race/ethnicity

White 60 3,039 1,713–4,364 66.4 — —

Other 14 365 76–655 8.0 — —

Unknown 37 1,170 404–1,937 25.6 — —

Total 111 4,574 2,703–6,446 100.0 1.5 0.9–2.1

Abbreviation: CI = confidence interval. 

* Source: NEISS. Estimates Query Builder. Bethesda, Maryland: U.S. Consumer Product Safety Commission. 1999–2008. Code 938, swimming pool chemicals. Available 

at http://www.cpsc.gov/cgibin/NEISSQuery/home.aspx.

† For 2007 NEISS summary data, refer to CDC. Pool chemical–associated health events in public and residential settings—United States, 1983–2007. MMWR, 2009; 

58:489–93. Available at http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5818a1.htm. 

§ Each injured person was weighted based on the inverse probability of the hospital being selected, and the weights were summed to produce national estimates.

¶ Categorical weighted counts might not add up to 4,574 due to rounding.

** Categorical percentages might not add up to 100.0% due to rounding.

†† Rates per 100,000 population were calculated using U.S. Census Bureau population estimates accessed September 10, 2010; 95% confidence intervals were calculated by using SAS survey procedures that accounted for the sample weights and complex sampling design. 

§§ Poisoning includes ingestion and inhalation of vapors, fumes, or gases.

¶¶ If the sample count was <20, weighted count was <1,200, or the coefficient of variation >30%, the estimate was considered unstable and rates were not reported. 

Rates by location and race are not reported because of the high percentage of unreported location and race. 

*** For a poisoning injury diagnosis, NEISS requires that affected body part be coded as “all parts of the body (more than 50% of body).”

††† Treated and transferred to another hospital (n = three), treated and admitted for hospitalization (within same facility) (n = four), or left without being seen/left 

against medical advice (n = three).

TABLE 14. Number of vibriosis cases associated with recreational water (n = 236), by species, hospitalization, death, and year — Cholera and Other 

Vibrio Illness Surveillance System, United States, 2007–2008

Species

Year

2007 2008 Total

Cases Hospitalized Deaths Cases Hospitalized Deaths Cases Hospitalized Deaths

Vibrio alginolyticus 57 3 1 49 8 1 106 11 2

V. cholerae non-O1, non-O139 5 1 0 6 1 1 11 2 1

V. damsela 2 0 0 0 0 0 2 0 0

V. fluvialis 1 0 0 2 0 0 3 0 0

V. parahaemolyticus 29 11 0 22 9 0 51 20 0

V. vulnificus 24 21 3 24 15 3 48 36 6

Vibrio spp. 6 1 0 7 3 0 13 4 0

Multiple 0 0 0 2 1 0 2 1 0

Total (%) 124 37 (29.8%) 4 (3.2%) 112 37 (33.0%) 5 (4.5%) 236 74 (31.4%) 9 (3.8%)
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 19

with treated recreational water venues. The percentage of 

outbreaks of AGI caused by Cryptosporidium and associated 

with treated recreational water was 82.9% (Figure 4) for both 

2007–2008 (58/70) and 2005–2006 (29/35) (13). In contrast, 

it was 68.2% (15/22), 50.0% (9/18), and 55.6% (10/18) 

in 1999–2000, 2001–2002, and 2003–2004, respectively 

(5,14,15). Its predominance as an etiologic agent, particularly 

among outbreaks associated with treated recreational water 

venues, is related to its chlorine tolerance, which allows it to 

survive in properly chlorinated recreational water >10 days 

(32). Since 2004, the number of cases of cryptosporidiosis 

reported to CDC annually has more than tripled (33). 

Although the reasons for increased cryptosporidiosis case and 

outbreak reporting are not understood completely, treated 

recreational water venues continue to play a key role in 

Cryptosporidium transmission.

Treated Recreational Water Venues and 

Pathogens

In treated recreational water venues, chlorine and bromine 

are the primary barriers to pathogen transmission. Maximizing 

disinfectant effectiveness requires maintaining appropriate 

pH levels and circulating the water through a filter to remove 

particulates that deplete disinfectants. Chlorine- and brominesusceptible pathogens (e.g., Shigella, norovirus, and Giardia) 

caused 23 (25.6%) of 90 treated recreational water–associated 

outbreaks of known etiology and indicate lapses in operation. 

Such lapses also are identified frequently on routine inspection. 

An analysis of data from >121,000 pool inspections conducted 

across the United States in 2008 indicated that 10.7% of 

inspections identified disinfectant level violations, and 12.1% of 

inspections resulted in immediate closure because of violations 

that threatened public health or safety (34). Implementation of 

proper pool operation practices (e.g., adequate disinfection) has 

been demonstrated to stop transmission of chlorine-susceptible 

pathogens effectively and quickly (35), which underscores the 

need for improved operator training.

Molecular epidemiology data from investigations of the 

2007–2008 outbreaks caused by norovirus suggested a possible 

association between recreational water type and genogroup. 

This led to a review of 19 reports of outbreaks (1990–2006) 

confirmed to be caused by norovirus, a chlorine-susceptible 

pathogen (36). The investigation of one (20.0%) of five treated 

recreational water–associated outbreaks included molecular 

typing of isolates, which belonged to genogroup II. Investigations 

of three (21.4%) of 14 untreated recreational water–associated 

outbreaks included molecular typing of isolates. Two of the 

outbreaks were caused by norovirus genogroup I and one by 

norovirus genogroup II. Data on further characterization by 

genotyping were not available. Only genogroup II was isolated 

from implicated treated recreational water venues; however, the 

limited number of outbreaks with genogroup data precludes 

further interpretation of these data and underscores the need for 

molecular typing to elucidate the epidemiology of waterborne 

disease outbreaks caused by norovirus (37).

In contrast to chlorine-susceptible pathogens, Cryptosporidium

requires extended contact time for inactivation at typically 

required or recommended free chlorine levels (1–3 mg/L). 

Thus, Cryptosporidium transmission can occur even in 

well-maintained treated recreational water venues. During 

1999–2008, the parasite caused 74.4% (122/164) of treated 

recreational water–associated outbreaks of AGI (Figure 10). 

During 2007–2008, treated recreational water–associated 

outbreaks of cryptosporidiosis accounted for 86.9% 

(12,137/13,966) of cases overall; the communitywide 

cryptosporidiosis outbreak in Utah in 2007 alone accounted for 

40.8% (5,697/13,966). Such communitywide cryptosporidiosis 

outbreaks can occur for the following reasons: Cryptosporidium

can survive in properly chlorinated recreational water >10 days 

(32); it has a protracted incubation period (approximately 7 

days) (38), which prolongs the amount of time that elapses 

between infection and epidemiologic implication of the 

outbreak source; and swimmers who continue to swim while 

ill can introduce the parasite to multiple recreational water 

venues. By the time reported cryptosporidiosis cases are linked 

epidemiologically to a particular venue, transmission already 

might be occurring at another venue in the community 

despite public health efforts to control the outbreak. An 

investigation of a communitywide cryptosporidiosis outbreak 

in Utah indicated that 20% of case-patients swam while ill 

FIGURE 7. Number of vibriosis cases associated with recreational water 

exposure (n = 236), by species and month — Cholera and Other Vibrio

Illness Surveillance System, United States, 2007–2008

* Includes V. cholerae (non-O1, non-O139) (n = 11), V. damsela (n = two), V. fluvialis 

(n = three), unidentified Vibrio co-infection (n = two), and Vibrio species not 

identified (n = 13).

0

10

20

30

40

50

60

No. of cases

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

Other species* 

Vibrio parahaemolyticus

V. alginolyticus

V. vulnicus 
Surveillance Summaries

20 MMWR / September 23, 2011 / Vol. 60 / No. 12

with diarrhea and identified approximately 450 potentially 

contaminated recreational water venues (39). 

Modifying swimmer behavior is a critical component of 

reducing recreational water–associated outbreaks, particularly 

those caused by chlorine-tolerant Cryptosporidium. Fecal-oral 

transmission associated with recreational water can occur 

when swimmers ingest contaminated water. The water can 

be contaminated by pathogens that cause AGI when a person 

has a fecal incident in the water or fecal material washes off of 

a swimmer’s body. Diarrhea-exclusion policies for all patrons 

should be established, implemented, and enforced at all 

recreational water venues and particularly for young children 

and visitors from high-risk settings (e.g., child care centers), 

which have diarrhea-exclusion policies but might not always 

enforce them (40). In addition, policies for restricting staff who 

are ill with diarrhea from entering the water, similar to those 

restricting ill foodhandlers from food preparation, should be 

established, implemented, and enforced (41).

For water contamination to be minimized, good swimmer 

hygiene is imperative. Swimmers should wash with soap, 

especially the perianal area, before entering the water; washing 

young children thoroughly with soap and water before they 

enter the water is particularly important (42). An adequate 

number of clean, functioning, well-stocked (e.g., with soap) 

and easily accessible facilities with showers, toilets, and 

handwashing sinks located near the water might promote 

good swimmer hygiene. Taking frequent bathroom breaks, 

particularly for young children, and checking swim diapers 

every 30–60 minutes also might help minimize water contamination. Diaper-changing facilities with handwashing stations 

should be located close to the water to encourage hygienic 

diaper-changing and handwashing. Increased awareness of 

the risk for recreational water–associated illness, its potential 

severity, and the efficacy and the simplicity of the prevention 

steps (e.g., not swimming while ill with diarrhea and not swallowing water) (43,44) might make the public more likely to 

adopt healthy swimming behaviors.

The increasing number of reports of treated recreational water–

associated outbreaks of cryptosporidiosis and their potential to 

evolve into communitywide outbreaks also call for prevention 

measures beyond conventional chlorination and filtration (45). 

Ultraviolet (46–48) and ozone (49,50) disinfection systems can 

effectively inactivate Cryptosporidium and are available for use at 

treated recreational water venues. Remedial biocidal treatment 

(i.e., hyperchlorination: 20 mg/L free chlorine for 12.75 hours or 

the equivalent at water pH ≤7.5 and temperature at ≥77°

F [25°C] 

in the absence of stabilized chlorine [32] or 40 mg/L free chlorine 

for approximately 30 hours at water pH 6.5 and temperature 

at ≥77°

F [25°C] in the presence of stabilized chlorine [51]) is 

another potential risk-reduction option. Increased circulation 

flow rates and occupancy-dependent water replacement might 

also help reduce risk (45).

Finally, Cryptosporidium’s ability to cause communitywide 

outbreaks underscores the need for more rapid implementation 

of control measures once an increase in case reporting is noted 

rather than waiting for an outbreak investigation to implicate 

a specific source of transmission. A response plan should 

include 1) establishing a strong communication network with 

community partners likely to be affected by a cryptosporidiosis 

outbreak (e.g., operators of treated recreational water venues 

and child care centers), 2) setting a pre-outbreak disease action 

threshold (e.g., a two- to threefold increase in number of cases 

over baseline), and 3) rapid mobilization of community partners 

to implement intensified control measures (e.g., communitywide 

hyperchlorination of treated recreational water venues) once 

the threshold is exceeded (52). To address the concerns of 

communitywide cryptosporidiosis outbreaks, Salt Lake County, 

Utah, and Idaho each developed a cryptosporidiosis prevention 

campaign before the 2008 summer swim season. Both campaigns 

engaged community partners and educated the public about 

healthy swimming behaviors. For 2008, Utah reported no 

waterborne cryptosporidiosis outbreaks, and Idaho reported 

only one outbreak with two identified cases.

FIGURE 8. Number of vibriosis cases associated with recreational 

water exposure (n = 236) — Cholera and Other Vibrio Illness 

Surveillance System, United States, 2007–2008*

* These numbers are largely dependent on surveillance and reporting activities 

in individual states and do not necessarily indicate the true incidence of vibriosis cases.

≥10 

5–9 

2–4 

1 

0 
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 21

TABLE 15. Number of vibriosis cases (n = 236) and deaths (n = nine) associated with recreational water, by region/state and species — Cholera 

and Other Vibrio Illness Surveillance System, United States, 2007–2008

Region/State

Species

Vibrio alginolyticus V. parahaemolyticus V. vulnificus Total

Other/unknown 

species*

Cases Deaths Cases Deaths Cases Deaths Cases Deaths Cases Deaths

Atlantic Coast 33 1 23 0 19 1 11 1 86 3

Connecticut 1 0 7 0 0 0 0 0 8 0

Delaware 4 0 0 0 0 0 1 0 5 0

Georgia 2 0 0 0 1 0 2 0 5 0

Maine 1 0 0 0 0 0 0 0 1 0

Maryland 4 0 3 0 3 0 1 0 11 0

Massachusetts 1 0 2 0 0 0 0 0 3 0

North Carolina 6 0 2 0 5 0 3 0 16 0

New Jersey 2 0 2 0 2 1 1 0 7 1

New York 1 1 0 0 1 0 0 0 2 1

South Carolina 3 0 3 0 2 0 1 1 9 1

Virginia 8 0 4 0 5 0 2 0 19 0

Gulf Coast 22 0 22 0 27 5 10 0 81 5

Alabama 1 0 3 0 3 0 1 0 8 0

Florida 14 0 8 0 5 0 4 0 31 0

Louisiana 1 0 2 0 8 1 0 0 11 1

Mississippi 0 0 1 0 1 0 2 0 4 0

Texas 6 0 8 0 10 4 3 0 27 4

Noncoastal 5 1 1 0 0 0 4 0 10 1

Arizona 2 0 0 0 0 0 0 0 2 0

Illinois 1 1 0 0 0 0 0 0 1 1

Minnesota 0 0 1 0 0 0 3 0 4 0

Ohio 1 0 0 0 0 0 0 0 1 0

Vermont 1 0 0 0 0 0 0 0 1 0

Wisconsin 0 0 0 0 0 0 1 0 1 0

Pacific Coast 46 0 5 0 2 0 6 0 59 0

California 22 0 1 0 1 0 3 0 27 0

Hawaii 23 0 3 0 1 0 2 0 29 0

Washington 1 0 1 0 0 0 1 0 3 0

Total (%) 106 (44.9) 2 (22.2) 51 (21.6) 0 (0) 48 (20.3) 6 (66.7) 31 (13.1) 1 (11.1) 236 (100.0) 9 (3.8)

* Includes V. cholerae (non-O1, non-O139), V. damsela, V. fluvialis, unidentified Vibrio coinfection, and Vibrio unknown species.

TABLE 16. Single cases of primary amebic meningoencephalitis associated with untreated recreational water (n = eight), by state — Waterborne 

Disease and Outbreak Surveillance System, United States, 2007–2008

State Month Year Etiology

Predominant 

illness*

No. of cases [deaths] 

(n = 8 [8])† Venue Setting

Arizona Sep 2007 Naegleria fowleri Neuro 1 [1] Lake State park

California Jul 2008 N. fowleri Neuro 1 [1] Lake Community

Florida Jun 2007 N. fowleri Neuro 1 [1] Unknown Neighborhood/Subdivision

Florida Aug 2007 N. fowleri Neuro 1 [1] Lake Community/Municipality

Florida Aug 2007 N. fowleri Neuro 1 [1] Lake Community/Municipality§

Oklahoma Aug 2008 N. fowleri Neuro 1 [1] Lake Camp/Cabin 

Texas Aug 2007 N. fowleri Neuro 1 [1] Lake Public outdoor area¶

Texas Aug 2007 N. fowleri Neuro 1 [1] Lake Public outdoor area¶

Abbreviation: Neuro = neurologic illnesses, conditions, or symptoms (e.g., meningitis).

* The category of illness reported by ≥50% of ill respondents.

† Deaths are included in the overall case count.

§ Epidemiologic and environmental health investigation identified county lakes and a lake at a water sports park as potential exposure sites for this case.

¶ The same lake in Texas was implicated as the source of water exposure for both cases with exposure occurring at different parts of the lake weeks apart.
Surveillance Summaries

22 MMWR / September 23, 2011 / Vol. 60 / No. 12

Venue-Specific Challenges to Prevention of 

Pathogen Transmission in Treated Recreational 

Water Venues

Spas and interactive fountains present particular challenges 

to prevention of recreational water–associated illness. They 

typically hold smaller volumes of water than pools, resulting 

in a relatively high bather load to water volume ratio, and 

aeration of the water depletes disinfectants.

Spas (hot tubs). The higher water temperatures of spas make 

it difficult to maintain proper disinfectant levels. As a result, 

poorly maintained spas are ideal for amplification of naturally 

occurring thermophilic environmental contaminants (e.g., 

Pseudomonas aeruginosa and Legionella). Common recurring 

themes in outbreaks of P. aeruginosa and Legionella infection 

include 1) epidemiologically implicating both the pool and 

spa in a given setting; 2) exposure occurring in a hotel/

motel setting, in which spa operation is not a full-time job; 

3) exposure occurring in the context of a group event (e.g., 

a wedding or birthday party), which can lead to the rapid 

depletion of disinfectant levels; and 4) the group event taking 

place on a weekend, when trained staff might not be on duty.

Frequent co-location and use of both spas and pools can 

make it difficult epidemiologically to implicate one or the 

other. However, Pseudomonas and Legionella are most likely 

to multiply in the higher water temperatures in spas (53,54). 

Group events might lead multiple participants to use a given 

spa simultaneously, possibly leading to a substantial number of 

bathers, which can rapidly overwhelm its disinfection capacity 

and spur bacterial amplification. Group events also might 

facilitate detection of an outbreak. Conversely, the repeated 

identification of these outbreaks in the hotel/motel setting 

highlights the role of travel in pathogen transmission and the 

potentially decreased likelihood of outbreak detection when 

travelers disperse to their resident states or countries.

To prevent spa-associated outbreaks, operators must 

understand the factors that contribute to bacterial amplification 

and take steps to limit contamination and prevent infection, 

particularly by thermophilic pathogens. Proper chlorination 

or bromination are effective in killing Pseudomonas and other 

skin-infecting bacteria. A review of 18 outbreaks caused 

by Pseudomonas demonstrated that the implicated spas 

with recorded chlorine levels all had levels ≤0.5 mg/L (55). 

Maintaining free chlorine or bromine levels at 2–6 mg/L 

and pH levels in a range of 7.2–7.8 can minimize bacterial 

amplification and biofilm build-up, although pathogens might 

not be eliminated completely from the water. Pseudomonas and 

Legionella can persist in spa biofilms, even in the presence of 

adequate disinfectant, and proliferate rapidly if the disinfectant 

level drops (54,56). A review of data from >5,200 spa inspections 

conducted across the United States in 2002 revealed that 17.1% 

Abbreviations: AGI = acute gastrointestinal illness; ARI = acute respiratory illness; Skin = illnesses, conditions, or symptoms related to the skin; Other = includes 

bronchitis; illnesses, conditions, or symptoms related to the ears; illnesses, conditions, or symptoms related to the eyes; hepatitis; leptospirosis; meningitis; meningoencephalitis; and multiple predominant illnesses.

* Single cases of primary amebic meningoencephalitis are not included in this figure, which therefore is not comparable with figures included in summaries prior to 

the 2005–2006 report.

† The category of illness reported by ≥50% of ill respondents. 

§ All outbreaks of legionellosis (i.e., Legionnaires’ disease and Pontiac fever) are classified as ARI. 

FIGURE 9. Number of waterborne disease outbreaks associated with recreational water (n = 696),* by predominant illness† and year — 

Waterborne Disease and Outbreak Surveillance System, United States, 1978–2008

90

80

70

60

50

40

30

20

10

0

No. of outbreaks

Year

1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008

Other

ARI§

Skin

AGI
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 23

of inspections identified improper disinfectant levels, and 11.0% 

of inspections resulted in immediate closure because of violations 

that threatened public health or safety, again underscoring the 

need for improved operator training (57).

The greater frequency of outbreaks of Pseudomonas

and Legionella infections associated with hotel/motel spas 

underscores the importance of proper operation in these 

settings. Hotels and motels should consider having only trained 

employees operate and maintain pools and spas, particularly 

on weekends, when usage might increase, and enhance water 

quality monitoring when large groups or events are scheduled.

Interactive fountains (splash pads or water play areas).

Interactive fountains are particularly prone to contamination 

by fecal material, vomit, and particulates because of open 

access to young users, persons in street clothes and shoes, and 

animals. Contaminants can drain into the water reservoir 

and be recirculated (i.e., sprayed back on users), increasing 

the likelihood of ingestion of contaminated water. Because 

interactive fountains typically do not have standing water above 

ground, they might be inadvertently exempt from pool codes. 

The lack of public health oversight in some jurisdictions might 

increase the likelihood of improper design, maintenance, or 

operation of these venues. An investigation of a 2008 Texas 

cryptosporidiosis outbreak epidemiologically linked multiple 

laboratory-confirmed cases to interactive-fountain exposures. 

At the time, interactive fountains were not regulated under the 

Texas state pool code; consequently, emergency regulations for 

interactive fountains were enacted in 2009, and the 2010 pool 

code was amended to include interactive fountains (Tex. Health 

& S § 341.0645 and Tex. Health & S § 0695).

Fill-and-drain pools/temporary water slides. The use of 

tap water to fill temporary venues used by young children 

continues to be a public health challenge. Lack of additional 

disinfection and filtration has resulted in multiple outbreaks 

being associated with use in residential and public settings (58). 

The potential risk for infection associated with using temporary 

venues filled with tap water without additional disinfection and 

filtration should be considered before use in residential settings. 

To reduce the risk for pathogen transmission, in addition to 

exclusion of persons with AGI, these pools should be emptied 

and cleaned at least daily. CDC recommendations on cleaning 

fill-and-drain pools are available at http://www.cdc.gov/

healthywater/swimming/pools/inflatable-plastic-pools.html. 

On the basis of documented outbreaks such as the previously 

unreported campylobacteriosis outbreak associated with a 

fill-and-drain pool at a Maryland residential school (Table 8), 

these temporary venues should be eliminated from public use 

(e.g., at child care centers).

Treated Recreational Water Venues and 

Chloramines

Three outbreaks of ARI or eye irritation, two of which 

occurred in January, were caused or suspected to be caused by 

an accumulation of chloramines in the water and air of two 

indoor waterparks and an indoor school pool. Chloramines are 

disinfection by-products that result from chlorine oxidation 

of nitrogenous compounds (e.g., perspiration, saliva, urine, 

and body oils) commonly shed into the water by swimmers. 

Chloramines are generated in the water and can volatilize into 

the surrounding air. Swimmers’ water activities and features 

FIGURE 10. Number of outbreaks of acute gastrointestinal illness associated with recreational water (n = 341), by type of exposure and year — 

Waterborne Disease and Outbreak Surveillance System, United States, 1978–2008 No. of outbreaks

60

50

40

30

20

10

0

Year

1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008

Untreated

Treated
Surveillance Summaries

24 MMWR / September 23, 2011 / Vol. 60 / No. 12

that splash, spray, and aerate water disturb the water surface 

and promote the dispersion of chloramines into the atmosphere 

surrounding indoor treated recreational water venues (59,60). 

Air in these enclosed settings might be recycled more often 

in the winter to minimize heating costs, limiting the amount 

of fresh, cold air that is introduced and the amount of warm 

chloramine-polluted air that is exhausted. The resulting high 

levels of chloramines can cause ocular, respiratory tract, and 

mucous membrane irritation and also might be linked to 

asthma (61). As swimming in the United States evolves from 

a summertime into a year-round activity, indoor air-quality 

issues are likely to increase, and chloramines could become 

an increasingly important etiologic agent of treated recreational water–associated outbreaks. Two of the three outbreaks 

occurred at indoor waterparks. Such waterparks might be 

more prone to accumulation of chloramines in the air than 

traditional indoor recreational water venues with one or two 

pools because of larger bather loads (i.e., more nitrogenous 

compounds are introduced into the water); more features such 

as wave pools, fountains, and slides that splash, spray, and aerate 

the water; and increased cost of maintaining the air temperature of an increased number of enclosed cubic square feet. An 

investigation of an outbreak associated with the indoor Ohio 

waterpark revealed that the high placement of air supply and 

return ducts caused chloramines to accumulate in the air at the 

pool surface and deck level (23). After the ventilation system 

was reconstructed considerably, no new cases of respiratory 

and ocular symptoms were detected.

The variability in indoor air quality and shortage of laboratories that perform analyses for airborne chloramines impedes 

investigators’ ability to respond to reports of ocular and respiratory distress. The length of time required for testing makes 

it difficult to obtain rapid and quantitative measurements 

of contamination of the air surrounding implicated indoor 

FIGURE 11. Recreational water–associated outbreaks of acute gastrointestinal illness, by type of exposure and etiology — Waterborne Disease 

and Outbreak Surveillance System, United States, 1999–2008

* These include outbreaks caused by Salmonella, Campylobacter, Plesiomonas and outbreaks caused by multiple etiologies. 

† Percentages do not add up to 100.0% due to rounding. 

Cryptosporidium spp.

57.0%

Type of exposure (n = 228)

Treated water 71.9%

Untreated water 28.1% Escherichia coli 6.6%

Norovirus 9.6%

Shigella spp. 7.9%

Other* 4.4%

Unidentied 11.0%

Etiology (n = 228)

Etiology: untreated water (n = 64) Etiology: treated water (n = 164)†

E. coli 17.2%

Norovirus 20.3% Shigella spp. 15.6%

Cryptosporidium spp. 

12.5%

G. intestinalis 4.7%

Unidentied 21.9%

Other* 7.8%

Unidentied 6.7%

Norovirus 5.5%

Shigella spp. 4.9%

E. coli 2.4%

G. intestinalis 3.0%

Cryptosporidium spp. 

74.4%

Other* 3.0%

Giardia intestinalis 3.5%
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 25

treated recreational water venues. Using the water’s measured 

total chlorine (i.e., the sum of free and combined chlorine 

levels) and free chlorine levels to calculate the level of combined chlorine, of which chloramines are a subset, might be 

useful for evaluating indoor air quality. Levels that exceed test 

kit capacity should be remeasured by making dilutions using 

distilled water. These outbreaks underscore the need to train 

pool operators to routinely monitor combined chlorine levels 

to protect the health of patrons and staff and the need for public health authorities to include the calculation of the water’s 

combined chlorine level as a standard part of pool inspections.

Accumulation of chloramines in the air surrounding indoor 

treated recreational water can be prevented by improving water 

treatment, swimmer hygiene, and air ventilation. Studies 

suggest that installation of ultraviolet or ozone treatment 

devices in circulation systems can reduce chloramine levels in 

the water (62–64) and also can effectively inactivate pathogens, 

including chlorine-tolerant Cryptosporidium. Because the 

nitrogenous compounds introduced into the water by 

swimmers are precursors to chloramine formation, swimmers 

must be engaged in any chloramine-reduction plan. Such an 

effort should raise public awareness about the role of urine and 

sweat in creating the ocular and respiratory irritants at treated 

recreational water venues. Plans to improve air quality also 

can be aided by working with the public to improve swimmer 

hygiene. Aquatics staff should encourage swimmers to at least 

rinse before entering the water and facilitate frequent bathroom 

breaks, particularly for young children (e.g., instituting adultonly swim times or short closures for water quality testing). 

Showering for 17 seconds has been demonstrated to decrease 

by 35%–60% the load of sweat and other pollutants that wash 

off of swimmers’ bodies (65).

Pool Chemical–Associated Outbreaks and Health 

Events

Nine pool chemical–associated outbreaks and 32 additional 

pool chemical–associated health events were reported to 

WBDOSS by Florida, Illinois, Kentucky, Maryland, Michigan, 

and New York. Data on additional pool chemical–associated 

health events are captured by the HSEES System and NEISS. 

Pool chemical–associated health events not only have the 

potential to impact individual and public health adversely but 

also have a negative financial impact. Approximately 60% of 

the injured individuals in the HSEES cohort and 100% of 

those in the NEISS cohort sought treatment in hospital EDs. 

Pool chemical–associated health events also might require 

responses from multiple government agencies and officials 

(e.g., fire fighters, police officers, paramedics, and hazardous 

materials personnel). 

Analysis of HSEES data examined factors contributing to 

hazardous substance releases of pool chemicals. The most 

frequently reported factors contributing to HSEES events were 

human error and equipment failure. A review of the reports 

of 2007–2008 pool chemical–associated outbreaks and other 

health events reported to WBDOSS revealed common themes, 

including mixing of incompatible chemicals, improper handling 

of chemicals, and overreliance on automatic controllers. These 

findings indicate that pool chemical–associated health events are 

preventable through engineering, education, and enforcement. 

Examples of engineering-based prevention measures include 

electric interlocks and flow switch sensors, which shut down 

the chemical feed pumps when the circulation pump shuts 

down or during filter backwash and thus prevent concentrated 

chlorine and acid from mixing and generating toxic chlorine 

gas. Previous studies underscore that requiring pool operator 

training can reduce the number of water quality violations 

(66,67). Thus, future prevention efforts should require training 

for all public pool operators on preventive maintenance and 

how to read pool chemical labels and material safety data sheets, 

which include information on which chemicals are incompatible 

and proper chemical handling. Education efforts need to 

include operators in settings in which recreational water is not 

the primary focus (e.g., schools or hotels/motels). Given that 

pool chemical–associated health events frequently occur in the 

residential setting, messages about safe chemical handling (e.g., 

not handling chemicals in the presence of children) should also 

target residential pool owners.

Untreated Recreational Water Venues

The proportion of AGI outbreaks associated with untreated 

venues relative to treated venues decreased over the previous 

decade. However, the number of outbreaks associated with 

untreated venues has remained relatively constant. During 

2007–2008, a total of 11 outbreaks of AGI associated with 

untreated freshwater venues were reported; all of these were 

linked to a lake, river, stream, or spring. Untreated venues have 

a higher proportion of outbreaks caused by chlorine-susceptible 

pathogens compared with treated venues.

Studies have determined the utility of monitoring (18) and 

testing for fecal indicator bacteria to assess the risk for recreational 

water–associated gastrointestinal illness (68–70) in large bodies 

of water (e.g., the Great Lakes and oceans). The NEEAR study is 

being conducted to evaluate rapid water quality testing methods 

that can produce results in <2 hours and to correlate these 

indicators with health effects among beachgoers. Results from 

freshwater Great Lakes beaches have demonstrated an association 

between an increasing signal detected by a quantitative polymerase 

chain reaction–based test method for enterococci and human 

health effects (68,71). Children aged <10 years were at greater 
Surveillance Summaries

26 MMWR / September 23, 2011 / Vol. 60 / No. 12

risk for AGI following exposure to water with elevated levels 

of enterococci. The small inland water bodies associated with 

the outbreaks described in this report do not have consistent or 

identified external sources of contamination (e.g., sewage releases 

or overflows), suggesting that swimmers might be an important 

source of water contamination and pathogen transmission.

As with treated recreational water, human behavior plays 

a key role in the transmission of pathogens in untreated 

recreational water. Modification of swimmer behavior is 

critical in untreated water given the lack of disinfection and 

filtration barriers to pathogen transmission. Recommendations 

for swimmer hygiene in untreated recreational waters are the 

same as those discussed previously for treated recreational 

water. In addition, beach managers and swimmers should be 

aware that shallow, poorly circulated swimming areas, which 

are particularly desirable for young children learning toileting 

skills or wearing diapers, might pose a higher risk for exposure 

to swimmer-introduced pathogens compared with deeper, 

well-circulated swimming areas. Potential methods to improve 

circulation of water through beach areas should be explored to 

reduce the longevity of focal, swimmer-derived contamination 

and thus risk for pathogen transmission. In addition to 

improved swimmer hygiene, exposure to high bacteria levels 

also can be reduced by avoiding swimming after heavy rainfall 

at sites affected by runoff to reduce exposure to any increase 

in contaminants and by not swimming near storm drains or 

pipes that might release contamination into water bodies (72).

Chlorine was added routinely to both an Ohio lake associated 

with an outbreak of multiple infectious etiologies and a manmade lake associated with a 2008 Texas cryptosporidiosis 

outbreak. The Ohio beach manager reported adding chlorine to 

the water to improve water clarity, not to disinfect. Public health 

and environmental concerns about the addition of chlorine 

to untreated recreational water include swimmers believing 

mistakenly that the water is microbiologically safe.

Four outbreaks of confirmed and suspected cercarial dermatitis 

caused by avian schistosomes were reported during 2007–2008. 

Three were associated with fresh water and one with marine 

water. This self-limited disease is known to occur among 

persons exposed to lakes in which infected birds contaminate 

water inhabited by the intermediate host snail (73). The risk 

for acquiring cercarial dermatitis might be reduced by avoiding 

potentially contaminated lakes, avoiding shallow swimming 

areas in which infected snails reside, instituting a snail control 

program, and not attracting birds that transmit the schistosomes 

into swimming areas (e.g., not feeding them).

Marine Water

Three outbreaks were associated with exposure to contaminated 

marine recreational water. Since 1978, only six such outbreaks have 

been reported to WBDOSS. Although outbreaks associated with 

marine waters are reported infrequently, evidence from multiple 

sources demonstrates that contamination of marine waters is 

common and that swimming in marine waters is associated with 

increased risk for AGI (69,70). States and territories report water 

quality testing results and notification data for their coastal and 

Great Lakes recreational water to EPA. In 2009, one or more 

advisories or closure notices were issued for 1,642 (43.0%) of the 

3,819 monitored coastal beaches because monitoring results for 

bacteria exceeded state or EPA standards (74). This represents an 

increase over previous years that might be attributable to increased 

monitoring, increased precipitation during the summer swim season 

in some coastal states, or the inclusion of preemptive closings made 

because of rainfall. Multiple studies have linked these water quality 

indicators with increased risk for recreational water–associated 

illnesses (69,70,75) although such pathogen transmission also 

can occur when water quality indicators are within established 

limits (76). The reasons for the infrequency of reported marineassociated outbreaks might include the wide geographic dispersion 

of residences of beachgoers and the fact that illnesses might not be 

attributed to swimming in marine waters. Prospective epidemiologic 

studies indicate that these beaches can be associated with illness 

despite an absence of reported outbreaks (68,71,77).

Algae toxicity. Toxin- or chemical-associated outbreaks can 

occur naturally. One outbreak that was caused by exposure to 

brevetoxins released by Karenia brevis near the Florida coast in 

2007 resulted in at least 15 cases of illness (25). The outbreak 

investigation focused on dredging company workers, who reported 

symptoms such as throat and ocular irritation and coughing. The 

effects of this red-tide event on other populations are unknown. 

A study of the economic impact of Karenia brevis in one Florida 

county during 2001–2006 estimated the marginal costs of illness 

to be between $500,000 and $4 million dollars (78). Closure or 

restricted recreational use of marine beaches and freshwater lakes 

and ponds because of algal blooms might also result in concerns 

about lost revenue at the local or regional level (79). No U.S. 

federal regulations or official EPA guidelines specify allowable 

concentrations of toxins related to harmful algal blooms in water. 

Other Recreational Water–Associated 

Health Events

Vibriosis

The number of Vibrio infections associated with water 

exposures during 2007–2008 (n = 236) was higher than 

the number reported in 2005–2006 (n = 189) (28,29). The 

majority of deaths and hospitalizations were caused by V.

vulnificus infection, which occurs predominantly in the Gulf 

Coast. V. vulnificus wound infection can cause severe illness and 
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 27

sequelae, including septicemia, and require amputation. Disease 

is more common and severe among persons with preexisting 

wounds and chronic medical conditions (e.g., diabetes, heart 

disease, or liver disease) (80,81). In general, swimmers with open 

wounds or sores should refrain from swimming.

Primary Amebic Meningoencephalitis

Eight fatal cases of PAM caused by Naegleria fowleri were 

reported for 2007–2008 (Table 16). This rare disease is of 

public health importance because of the high (>99%) fatality 

rate associated with infection (82) and the public alarm it 

raises about the recreational use of freshwater. N. fowleri is a 

free-living ameba that proliferates in warm freshwater and hot 

springs. Disease occurs when the ameba coincidentally enters 

the nasal passages, travels to the olfactory lobe of the brain, 

and infects brain tissue. The eight cases all resulted from warm 

freshwater exposures in southern states during the summer.

The limited number of PAM cases makes it difficult to 

determine why certain persons become infected. As a result, 

the efficacy of existing risk-reduction strategies is uncertain. 

Lake water surveys conducted in southern states have 

frequently detected Naegleria (83–85), and PAM case reports 

demonstrate recent exposure to swimming in warm, freshwater 

lakes, rivers, or hot springs. Swimmers might reduce their risk 

for Naegleria infection by avoiding water-related activities in 

warm freshwater during periods of high water temperature 

and low water levels; holding the nose shut or using nose 

clips when taking part in water-related activities in bodies of 

warm freshwater; and refraining from digging in or stirring up 

sediment while swimming in shallow, warm, freshwater areas 

(86). CDC is collaborating with CSTE and individual states to 

improve case investigations by collecting exposure, symptom, 

treatment, and environmental data. Systematic collection of 

these data could help refine current risk-reduction measures 

and guidance.

Prevention

Recreational water–associated illness can be prevented through 

concerted efforts by public health professionals, recreational 

water venue operators, and the general public. Given the different 

stakeholders required to promote healthy and safe swimming, 

good communication among these groups is imperative.

Public health professionals regulate recreational water 

facilities and investigate outbreaks as a multidisciplinary team 

(e.g., including laboratorians, environmental health specialists, 

and epidemiologists within and among jurisdictions). They also 

function as an important source of information for operators 

and the swimming public. Public health professionals should

•	 have opportunities to maintain and build upon current

knowledge about recreational water–associated illness and 

operation,

•	 update and improve codes to stay current with changing

designs and needs demonstrated by findings of outbreak 

investigations,

•	 lead and collaborate with the aquatics industry to educate

the general public,

•	 useinspection and beachmonitoring data assurveillance data

to inform public health decision-making and program planning, and

•	 develop expertisein detecting and investigating recreational

water–associated outbreaks and other health events.

Various tools are available to operators of treated recreational 

water venues to protect the health and safety of swimmers 

and aquatics staff. The traditional paradigm of two barriers 

(disinfection and filtration) to pathogen transmission in treated 

recreational water needs to shift to include in-line (i.e., usually 

installed after filtration and before disinfection) secondary or 

supplemental treatment (e.g., ultraviolet treatment or ozonation). 

Ultraviolet and ozone treatment not only will increase the level 

of protection against chlorine-tolerant Cryptosporidium but also 

will break down chloramines. Because these systems depend on 

circulation, they alone will not eliminate outbreaks; a commitment 

to monitoring and maintaining water quality and educating the 

public (e.g., including healthy swimming messages in posters in 

bathrooms, on the back of ticket stubs, and in contracts for group 

events) also is critical. To maximize their ability to protect the 

health and safety of swimmers and aquatics staff, pool operators 

working in all public settings should complete training that 

includes such topics as disinfection, water chemistry, preventive 

maintenance, chemical safety, and illness and injury prevention. In 

response to the lack of protective barriers at untreated recreational 

water venues (e.g., lakes and oceans), beach managers and public 

health officials should implement water quality testing programs 

and educate swimmers about prevention measures specific to 

untreated recreational water venues (e.g., not swimming after 

heavy rainfall) and for illnesses unlikely to be prevented by current 

water quality guidelines (e.g., vibriosis, PAM, and otitis externa).

Swimmers are an important source of recreational water 

contamination (i.e., pathogens and nitrogenous compounds); 

thus, it is imperative to educate the public about healthy 

swimming behaviors. The general public also can participate in 

the healthy swimming effort by checking chlorine and pH levels 

before getting into treated recreational water, asking operators of 

treated recreational water venues about the latest inspection score, 

encouraging operators to take steps known to kill Cryptosporidium, 

educating other swimmers about healthy swimming, and reporting 

operational issues, if not corrected, to public health agencies.
Surveillance Summaries

28 MMWR / September 23, 2011 / Vol. 60 / No. 12

Limitations

The findings in this report are subject to at least four main 

limitations. First, differences in the numbers of outbreaks reported 

by different jurisdictions might be attributable to factors such 

as the variable requirements for notifiable diseases and variable 

public health capacity to detect, investigate, and report recreational 

water–associated outbreaks. Thus, the jurisdictions reporting 

outbreaks most frequently might not be the jurisdictions in which 

the outbreaks most frequently occur (87). Second, factors such 

as incubation period, size and location of the outbreak exposure, 

severity of illness, and geographic dispersion of ill swimmers also 

likely influence the detection, investigation, and reporting of 

recreational water–associated outbreaks. Larger outbreaks are more 

likely to be identified by public health authorities. In contrast, 

smaller outbreaks (e.g., those associated with residential pools and 

spas) might go undetected because fewer persons are ill, and they 

might attribute illness to other common exposures. In addition, 

outbreaks associated with venues that draw from a wide geographic 

range (e.g., large lakes and marine beaches or hotel/motel pools 

or spas) might be difficult to detect because potentially infected 

persons disperse widely from the site of exposure and, therefore, 

cases of illness might be less likely to be identified as part of an 

outbreak. Prospective epidemiology studies, such as EPA’s NEEAR 

Water Study (68), have revealed elevated rates of AGI in swimmers 

compared with nonswimmers at all beaches studied. Multiple other 

prospective studies of AGI associated with beach swimming have 

also indicated elevated rates of illness associated with swimming 

in lakes and oceans, though few outbreaks have been detected 

(69,76). Data on endemic recreational water–associated illnesses 

are not captured by WBDOSS, highlighting the need for more 

studies to estimate the magnitude of risk for illness for routine, 

nonoutbreak-associated exposures at recreational water venues. 

Third, the incidence of recreational water–associated outbreaks of 

cryptosporidiosis in 2007 might be overestimated in this report. 

The statewide outbreak in Utah and the individual outbreaks 

in neighboring Idaho and Wyoming might have been a single 

multistate outbreak. Of note, Colorado also detected a statewide 

increase in cryptosporidiosis cases in the summer of 2007 that was 

determined to be associated with multiple risk factors in multiple 

counties (88). In 2007, had Cryptosporidium isolates from each 

health event been available for subtyping, investigators might have 

been able to determine if they were linked to each other. Finally, 

few outbreak reports included findings from environmental 

health investigations of the implicated venue. It is unclear whether 

environmental health investigations were not conducted or the 

findings were not included in reports. Whereas epidemiologic 

investigations provide data that characterize outbreaks, data from 

environmental health investigations can be used to identify factors 

that contributed to the outbreak to help prevent future outbreaks.

This is the first surveillance summary to present pool chemical–

associated health event data from multiple surveillance systems, 

and the data provided in this report regarding such events are 

subject to at least six additional limitations. First, injuries, illness, 

or pool chemical spills might not be serious enough to require 

outside assistance and thus are not detected. Second, because 

communication channels might not be established between 

public health agencies and emergency responders, certain pool 

chemical–associated health events might not be reported. Such 

communication is important because the outbreaks and other 

health events caused by pool chemicals might be more likely 

to involve first responders and hazardous materials personnel 

than waterborne disease coordinators, who voluntarily report 

outbreaks to WBDOSS. This in part might be the reason why 

pool chemical–associated outbreaks and other health events were 

reported to WBDOSS by only six states. Third, water chemistry 

can change quickly and contaminated air can be ventilated rapidly, 

making it difficult to determine the etiology of a pool chemical–

associated health event. This might explain why the etiologies of 

nine outbreaks were not determined but are suspected to have 

been caused by excess chlorine, disinfection by-product levels, or 

altered pool chemistry. Fourth, the approximately 100 hospitals 

participating in NEISS and the 12 states participating in HSEES 

and reporting events might not be representative of the United 

States, and therefore the findings might not be generalizable. 

Fifth, the fact that “chlorine” was the only chemical term used to 

search the HSEES system database might have created a bias for 

not identifying events caused by other pool chemicals. Finally, no 

one surveillance system collects data on all events, but, rather, each 

collects data on a subset of the events. This leads to the possibility 

of not collecting data on events that do not meet the case definition 

of any of the systems but could also lead to duplicate reporting. 

For example, health event data reported to NEISS also might 

be reported to either WBDOSS or the HSEES System. NEISS 

location data are limited to type of setting (e.g., residence or school) 

and do not include information such as the state or county in which 

the event occurred; this precludes de-duplication of reports across 

all surveillance datasets.

Conclusion

CDC and ATSDR can lead the effort to prevent recreational 

water–associated outbreaks and other health events by 1) 

spearheading the efforts to set minimum national standards to 

protect the health and safety of swimmers and aquatics staff at 

treated recreational water venues; 2) providing data to advocate for 

waterborne disease prevention and control efforts; 3) continuing 

to provide web-based and other healthy swimming resources for 

public health partners, the public, and the aquatics industry; and 
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 29

4) coordinating efforts among agencies to prevent pool chemical–

associated health events. Multiple federal organizations provide 

assistance with investigations of recreational water–associated 

outbreaks and other health events (Box).

In 2005, at the request of CSTE, experts from local, state, 

and federal public health agencies and representatives from the 

aquatics industry met in Atlanta at a CDC-sponsored workshop 

on preventing recreational water–associated illnesses at treated 

recreational water venues. Setting uniform national aquatic standards was determined to be a key prevention and control measure.

Since 2007, the New York State Department of Health and 

CDC have spearheaded an all-stakeholder effort to develop a 

Model Aquatic Health Code (MAHC) (available at http://www.

cdc.gov/healthywater/swimming/pools/mahc). MAHC has two 

primary objectives: 1) to be a free, science-based health code that 

reduces risk for illness and injury at treated recreational water 

venues, and 2) to serve as a model available for voluntary adoption 

by state and local public health agencies to facilitate an evolution 

from disparate pool regulations across the country into a set of 

national uniform standards. The 12 MAHC modules cover such 

topics as disinfection and water quality, operator training, hygiene 

facilities, and ventilation and air quality; it is anticipated that the 

modules will be made publicly available in 2012.

To help support waterborne disease detection, investigation, 

and reporting, CDC should collect and report on data through 

research initiatives that underscore the importance of waterborne 

disease in the United States. Estimates of the magnitude and cost of 

waterborne disease and magnitude of recreational water exposure 

can be used to advocate for prevention and control measures. In 

addition, bolstering waterborne disease surveillance can promote 

prevention and control. For example, given that Cryptosporidium

is the predominant etiologic agent of recreational water–associated 

outbreaks and has the ability to cause communitywide outbreaks, 

CDC should systemically collect stool specimens and utilize 

molecular epidemiology tools to subtype isolates to help elucidate 

the epidemiology of cryptosporidiosis (89).

Since 2001, CDC has posted healthy swimming messages 

and resources on its Healthy Swimming website.§§ This 

communication tool allows CDC to share outbreak 

investigation resources and lessons learned with public health 

partners, educate the public about healthy swimming, and 

provide recommendations to the aquatics industry. The website 

highlights Recreational Water Illness and Injury Prevention 

Week, which is the week before Memorial Day and represents 

a health communications opportunity to educate the public 

about healthy swimming behaviors.

Finally, ATSDR and CDC collect data on pool chemical–

associated health events. The data suggest that such events are 

both common and preventable. The data also indicate the need 

for a coordinated effort to establish the magnitude of these health 

events, more completely characterize their epidemiology, refine 

prevention and spill response recommendations, and develop 

prevention tools. To optimize health communication to operators, 

residential pool owners, and pool chemical manufacturers, 

ATSDR/CDC messages should balance the needs to chemically 

disinfect to prevent and control pathogen transmission and to 

reduce the incidence pool chemical–associated health events.

In 2009, CDC transitioned to electronic reporting through 

the National Outbreak Reporting System (NORS)¶¶ for waterborne disease outbreaks occurring on or after January 1, 2009. 

As waterborne disease outbreak reporting to CDC transitions 

from a paper-based to an electronic system, the number 

of reported recreational water–associated outbreaks might 

increase. Efforts to improve the quality and completeness of 

WBDOSS data include activities such as NORS webinars for 

and e-mail updates to epidemiology, environmental health, 

and laboratory partners at the local, state, and federal levels; 

web-accessible training materials for public health staff responsible for voluntary reporting; and collaboration across CDC 

programs to streamline reporting, provide technical support, 

and develop new NORS reporting tools. One of the key 

improvements in NORS reporting is the inclusion of more 

standardized fields to report environmental health investigation 

data. All of these efforts aim to generate surveillance data that 

better inform the development and implementation of more 

effective waterborne disease prevention and control guidelines 

and standards such as the MAHC and ultimately reduce the 

burden of recreational water–associated illness and injury.

Acknowledgments

This report is based in part on data provided by state, territorial, local, 

and FAS waterborne disease coordinators; state, territorial, local, and FAS 

epidemiologists and environmental health personnel and on contributions 

by Sarah A. Collier, MPH, Lina I. Brou, MPH, Alexandra L. Shevach, 

Kelly A. Jackson, MPH, Ezra J. Barzilay, MD, Division of Foodborne, 

Waterborne, and Environmental Diseases, National Center for Emerging 

and Zoonotic Infectious Diseases; Rose A. Rudd, MSPH, Division of 

Unintentional Injury Prevention, National Center for Injury Prevention 

and Control; Aron J. Hall, DVM, Veronica P. Costantini, PhD, Division 

of Viral Diseases, National Center for Immunization and Respiratory 

Diseases; Lorraine C. Backer, PhD, Division of Environmental Hazards 

and Health Effects, National Center for Environmental Health; Charles 

Otto, MPA, Division of Emergency and Environmental Health Services, 

National Center for Environmental Health, CDC.

§§ Available at http://www.cdc.gov/healthywater/swimming.

¶¶ Available at http://www.cdc.gov/healthywater/statistics/wbdoss/nors/index.html.
Surveillance Summaries

30 MMWR / September 23, 2011 / Vol. 60 / No. 12

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BOX. Federal organizations that provide assistance with investigations of recreational water–associated outbreaks and other health events

State health departments and other reporting jurisdictions 

can request epidemiologic assistance and laboratory testing 

from CDC. CDC also can be consulted regarding engineering and environmental health aspects of recreational water 

treatment and collection of proper water samples to identify 

pathogenic viruses, bacteria, and parasites. 

Requests for assistance with waterborne disease outbreak 

investigations (e.g., epidemiologic assistance, water testing, diagnosis of free-living ameba infection, or molecular characterization of Cryptosporidium and Giardia) 

Waterborne Disease Prevention Branch

Division of Foodborne, Waterborne, and 

Environmental Diseases

National Center for Emerging and Zoonotic Infectious 

Diseases, CDC

Telephone: 404-639-1700

Email: healthywater@cdc.gov

Internet: http://www.cdc.gov/healthywater 

Requests for diagnostic testing for viral pathogens

Division of Viral Diseases

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Telephone: 800-232-4636

Requests for diagnostic testing for enteric bacterial 

pathogens

Enteric Diseases Laboratory Branch

Division of Foodborne, Waterborne, and 

Environmental Diseases

National Center for Emerging and Zoonotic Infectious 

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Telephone: 404-639-3334

Requests for information or diagnostic testing for parasites (except for Cryptosporidium, Giardia, or free-living 

amebas)

Division of Parasitic Diseases and Malaria

Center for Global Health, CDC

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Division of Bacterial Diseases

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Diseases, CDC

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Internet: http://www.cdc.gov/legionella

Resources for pool chemical–associated health events

ATSDR National Toxic Substances Incidents Program 

(NTSIP)

Internet: http://www.atsdr.cdc.gov/ntsip 

24 hours/7 days chemical spill hotlines

—To report chemical spills, telephone the National

Response Center (NRC) at 800-424-8802 or report via 

the Internet: www.nrc.uscg.mil 

—For medical advice, telephone the National Poison 

Center at 800-222-1222

—For public health assistance during a chemical emergency, telephone the CDC Emergency Operations 

Center at 770-488-7100.

Resources for harmful algal blooms

Health Studies Branch

Division of Environmental Hazards and Health Effects

National Center for Environmental Health, CDC

Telephone: 866-556-0544
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MMWR / September 23, 2011 / Vol. 60 / No. 12 31

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Appendix A

Glossary of Definitions

backwash Flow of water through filter element(s) or media in a reverse direction to dislodge and remove 

accumulated dirt, debris, or filter aid from the filter tank.

bather load The number of bathers using a swimming pool or spa at any one time. The maximum bather load is 

usually determined by a state or local pool code and is based on surface area and depth of the pool or spa.

biofilm Microbial cells that adhere to a moist or water-covered surface through a matrix of primarily 

polysaccharide materials in which they are encapsulated. Biofilms can grow on piping and 

surfaces of aquatic venues and can be very difficult to remove. They protect microbes from 

disinfectants (e.g., chlorine) in the water.

cercarial dermatitis Dermatitis caused by contact with or direct invasion through the skin or a break in the skin by 

the cercariae (larval stage) of certain species of schistosomes. The normal hosts of these species are 

birds and non-human mammals. Dermatitis is an allergic response to contact with cercariae and 

does not lead to parasitic infestation in humans and produces no long-term disease.

class Waterborne disease outbreaks are classified according to the strength of the epidemiologic and 

clinical laboratory data, and environmental data implicating recreational water as the source of 

the outbreak (see Table 1).

chloramines A group of disinfection by-products or weak disinfectants formed when free chlorine combines 

with nitrogen-containing compounds (e.g., urine or perspiration) in the water. Tri- and dichloramine can cause eye and respiratory (e.g., lung and throat) irritation and can accumulate 

in the water and air surrounding treated recreational water venues. In drinking water treatment, 

monochloramine is used for disinfection to reduce formation of disinfection by-products created 

when using chlorine as a disinfectant (see combined chlorine level). 

combined chlorine Chlorine that has combined with organic or inorganic compounds in the water and is no 

longer an effective disinfectant for recreational water. The combined chlorine level is derived by 

subtracting the water’s free chlorine test level from its total chlorine test level. This level is likely 

to include combined compounds in addition to chloramines (see chloramines). 

communitywide outbreak This outbreak typically starts as a focal outbreak associated with one recreational water venue and 

evolves into an outbreak associated with multiple recreational water venues or other settings, such 

as child care centers in a community.

contact time The length of time that recreational water and any pathogens in the water are exposed to a 

disinfectant, usually measured in minutes (e.g., chlorine contact time). 

disinfection A treatment that kills microorganisms (e.g., bacteria, viruses, and parasites); in water treatment, 

a chemical (commonly chlorine, chloramine, or ozone) or physical process (e.g., ultraviolet 

radiation) can be used.

disinfection by-products Chemicals formed in water through reactions between organic or inorganic matter and 

disinfectants. Examples include chloramines, also known as combined chlorines. These chemicals 

might have acute or chronic health effects.

etiology The pathogen, chemical, or toxin causing a waterborne disease outbreak or other health event. 

Infectious etiologies include bacteria, parasites, and viruses.

fill-and-drain pools Small pools or slides that often are constructed of plastic and that might be inflatable. These pools 

and slides are filled with tap water without any ongoing chemical disinfection or filtration. 

filtration In water treatment, the process of passing water through one or more permeable membranes or media 

of small diameter (e.g., sand, anthracite, or diatomaceous earth) to remove suspended particles from 

the water. Filters might be effective in removing pathogens, depending on the type and operation.
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34 MMWR / September 23, 2011 / Vol. 60 / No. 12

free chlorine Chlorine in water (found as an aqueous mixture of hypochlorous acid and hypochlorite anion) 

that has not combined with other constituents; therefore, it is able to serve as an effective 

disinfectant (also referred to as free available chlorine or residual chlorine). Measuring the free 

chlorine level is a common water quality test. 

freshwater Untreated, nonmarine surface water (e.g., water from lakes, rivers, or ponds). 

interactive fountain A fountain or water spray device that is either intended for or accessible for recreational use 

and typically does not have standing water aboveground; recirculated water typically is stored 

in an underground holding tank. These fountains are sometimes called spray pads, splash pads, 

wet decks, or spray grounds. In contrast, a noninteractive (ornamental or decorative) fountain 

intended for public display rather than recreational use is often located in front of buildings and 

monuments, and the water is not easily accessible for public use.

jurisdiction An inclusive term for the U.S. states, District of Columbia, territories, and Freely Associated 

States (FAS) (e.g., the Republic of the Marshall Islands, the Federated States of Micronesia, 

and the Republic of Palau) that report waterborne disease outbreaks to the Waterborne Disease 

and Outbreak Surveillance System (WBDOSS). Waterborne disease outbreaks investigated by 

local public health agencies are reported to the state, territorial, or FAS public health agency. 

Jurisdictions can also report select recreational water–associated health events to WBDOSS.

marine water Untreated recreational water at an ocean or estuarine setting. 

pool chemical–associated 

health events

Injuries or illnesses resulting from exposure to pool chemicals (e.g., halogens or disinfection 

by-products) used to maintain quality of treated recreational water. These events might not 

meet the criteria for a waterborne disease outbreak depending on whether they involve exposure 

to recreational water or two or more persons. These events must be associated with treated 

recreational water venues. 

recreational water venue A body of water used for recreation (e.g., swimming, soaking, or athletics), including any 

structure that encloses this water. It can include a lake, pond, river, spring, ocean, or a man-made 

venue (e.g., swimming pool and spa); some recreational water venues do not include standing 

water (e.g., interactive fountains). 

reservoir, impoundment An artificially maintained lake or other body of water used for the collection and storage of water. 

It can be available as a source of raw water for drinking purposes or recreational use.

setting Location where exposure to contaminated water occurred. 

spa Any structure, basin, chamber, or tank (located either indoors or outdoors) containing a body 

of water that is intended to be used for recreational or therapeutic purposes and that usually 

contains a waterjet or aeration system. It is operated at high temperatures and usually is not 

drained, cleaned, or refilled after each use. It also is referred to as a hot tub or whirlpool.

spray park A recreational water venue consisting of multiple interactive fountains. 

total chlorine A common water quality test that measures the chlorine in water that is free for disinfection (see 

free chlorine) plus that combined with organic or inorganic materials (see combined chlorine 

level). The water’s combined chlorine level is derived by subtracting the free chlorine test level 

from the total chlorine test level. 

treated water Water that has undergone a systematic disinfection process (e.g., chlorination and filtration) 

to maintain good microbiologic quality for recreation. Typically, this refers to any recreational 

water in an enclosed, manufactured structure. This includes water in swimming or wading pools, 

fountains, and spas but might also include water in fill-and-drain pools filled with treated tap 

water or untreated water (e.g., mineral spring water) that receives no further treatment. 

untreated water Water that has not undergone a disinfection or treatment process to maintain good 

microbiological quality for recreation (e.g., lakes, rivers, oceans, and reservoirs). 

user Any person or bather entering recreational water. Might also be referred to as a patron at some 

membership clubs or recreational water venues.
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 35

water quality indicator A microbial, chemical, or physical parameter that indicates the potential risk for infectious 

diseases associated with using the water for drinking, bathing, or recreational purposes. Standards 

might vary based on type and degree of water exposure associated with different water uses. 

Ideally, density or concentration correlates with health effects. Examples include turbidity, 

coliform counts, fecal coliforms, Escherichia coli, enterococci, and free chlorine level.
Surveillance Summaries

36 MMWR / September 23, 2011 / Vol. 60 / No. 12

Appendix B

Descriptions of Select Waterborne Disease Outbreaks 

Associated with Recreational Water Use

Month Year State Etiology 

No. of 

cases Outbreak description

Bacteria

February 2008 Wisconsin Pseudomonas aeruginosa 18 After spending time in a hotel/motel pool and spa, 17 persons reported having 

a rash and one person reported an ear infection. A swab of one person’s skin 

lesion yielded P. aeruginosa. A case-control study found a statistically significant 

association between using the spa on a given Saturday and the development of 

symptoms. An environmental health investigation noted that no water quality 

readings were recorded on the Saturday that case-patients used the spa.

March 2008 Florida Legionella 5 The results of a matched case-control study epidemiologically linked five travelassociated cases of legionellosis to spa exposure at a hotel/motel. The spa had 

two cartridge filters and an automatic chlorine feeder. The environmental health 

investigation noted that the pool and spa had free chlorine levels of <0.5 mg/L 

(or parts per million [ppm]) and the cartridge filters were insufficient for the size 

of the spa. The maintenance logs for the spa were not available. Environmental 

water samples from the spa, hotel boilers, and a room air conditioner all tested 

negative for Legionella.

Viruses

May 2007 Idaho Norovirus genogroup II 50 After exposure to a community pool and wading pool, six persons submitted 

stool specimens that tested positive for norovirus genogroup II. The pools had 

a combined filtration system. An investigation identified multiple contributing 

factors, including high bather load, inaccessible bathroom facilities, and lack of 

oversight by management to handle fecal incidents. 

July 2008 Connecticut Norovirus genogroup I 16 After exposure to a lake that was used by diaper-aged children, six persons 

submitted stool specimens that tested positive for norovirus genogroup I. 

A fecal incident and stagnant water were suspected as contributing factors 

in the outbreak. A case-control study found that the odds of developing 

gastrointestinal symptoms were significantly higher for persons who swam 

in the lake on a particular day, swam in the water for more than an hour, 

or swallowed lake water compared with persons who did not report these 

exposures. Swimming >12 feet from shore also was associated with an increased 

odds of illness (p=0.0004). 

Parasites

June 2007 New Mexico Schistosomes 12 A medical provider’s office reported multiple individuals seen for rashes 

diagnosed as cercarial dermatitis (“swimmer’s itch”) after spending time 

at a freshwater lake. The outbreak investigation included: case interviews, 

environmental assessment, and testing. Cases occurred among individuals 

who entered the water from the shoreline and did not occur in individuals 

who entered the water directly from a boat. Cercariae were isolated from 

snails collected along the shoreline. Low water levels 2 years earlier supported 

increased vegetation and large snail populations along the shoreline. Warning 

signs posted at the lake in English and Spanish listed allergic reaction symptoms 

and prevention measures.

July 2007 Oklahoma

Oklahoma

C. hominis 

C. parvum

93

17

Molecular subtyping of Cryptosporidium isolates led public health officials to 

determine that two distinct outbreaks of cryptosporidiosis had occured in 

neighboring counties during the same month. Persons affected by the first 

outbreak reported swimming in a pool that was the only publicly accessible 

swimming pool in the community, and none of the persons with laboratoryconfirmed cases reported swimming in another pool. Cryptosporidium

oocysts isolated from stool specimens of 11 patients and four liters of pool 

filter backwash were subtyped as C. hominis. Persons affected by the second 

outbreak stayed in state park cabins during a week-long period in mid-July. 

A retrospective cohort study implicated the park’s pool. Molecular typing of 

stool specimens and pool backwash identified C. parvum and provided strong 

supporting evidence that the cases were not part of the first outbreak.
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 37

Month Year State Etiology 

No. of 

cases Outbreak description

August 2007 Iowa Cryptosporidium 34 County public health staff responded quickly to prevent a focal cryptosporidiosis 

outbreak from becoming communitywide following notification of 

two laboratory-confirmed cases in children with recent swimming pool 

exposures. Public health staff immediately initiated case investigations and 

active cryptosporidiosis surveillance. Local health-care providers were sent 

a health alert containing information about how to diagnose and report 

cryptosporidiosis cases. Child care providers were sent guidance about disease 

prevention and control measures. The environmental health staff worked closely 

with local pools, starting on the day that the first two cases were reported. 

Multiple pools were hyperchlorinated immediately. A fact sheet also was shared 

with several neighboring counties for distribution to pool operators. 

July 2008 New Mexico C. hominis 89 Confirmed cases of cryptosporidiosis were identified among competitive 

swimmers who swam at a community aquatic facility. At least one patient 

swam while symptomatic and participated in competitions with hundreds 

of swimmers. Working closely with state and local partners, the New Mexico 

Department of Health coordinated the health communications, epidemiologic 

investigation, and environmental health response to the outbreak. This included 

hyperchlorination of all pools in the aquatic facility as well as any other pools 

that infectious swimmers had used. A total of 34% of ill persons (including 

competitive swimmers, lifeguards, or swim team coaches) reported recreational 

water activity while symptomatic or in the 2 weeks following symptom 

resolution. Stool specimens from multiple persons and recreational water tested 

positive for C. hominis. Clinical isolates were of the same subtype.

July 2008 Arizona C. hominis 9 Four laboratory-confirmed cases of C. hominis infection were linked to the 

same interactive fountain. The interactive fountain was not found to have 

any violations upon initial inspection. Staff complied with health department 

recommendations. Water from the interactive fountain tested positive for 

Cryptosporidium both before and after disinfection guidelines were applied; 

follow-up testing is not recommended by CDC because nonviable oocysts 

present in the treated water might lead to a positive test result via molecular 

testing. Molecular subtyping determined that this outbreak was unique 

from two concurrent recreational water–associated outbreaks in Arizona. 

An ultraviolet disinfection system was added to supplement the bromine 

disinfection system. 

Chemicals/Toxins

January 2008 Illinois Chlorine 20 At least 20 persons were hospitalized and an unknown number of people were 

injured after exposure to chlorine at an indoor waterpark. Case counts and 

symptom data were based on fire department records. The automatic controller 

that managed pool disinfectant and pH levels failed. Pool staff did not recognize 

the problem during routine testing. The free chlorine and pH levels of the 

wading pool measured 18 mg/L and 8.3, respectively, on the day that the event 

occurred.
Surveillance Summaries

38 MMWR / September 23, 2011 / Vol. 60 / No. 12

Surveillance for Waterborne Disease Outbreaks Associated with 

Drinking Water—United States, 2007–2008

Joan M. Brunkard, PhD1

Elizabeth Ailes, PhD2

Virginia A. Roberts, MSPH1

Vincent Hill, PhD1

Elizabeth D. Hilborn, DVM3

Gunther F. Craun, MPH4

Anu Rajasingham, MPH1,5

Amy Kahler, MS1

Laurel Garrison, MPH6

Lauri Hicks, DO6

Joe Carpenter, MS7

Timothy J. Wade, PhD5

Michael J. Beach, PhD1

Jonathan S. Yoder MSW, MPH1

1Division of Foodborne, Waterborne, and Environmental Diseases, National Center for Emerging and Zoonotic Infectious Diseases, CDC

2International Health Resources Consulting (IHRC), Inc., Atlanta, Georgia

3U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 4Gunther F. Craun & Associates, Staunton, Virginia

5Oak Ridge Institute for Science and Education, Oak Ridge, Tennessee 6Division of Bacterial Diseases, National Center for Immunization and Respiratory Diseases, CDC

7Division of Healthcare Quality Promotion, National Center for Emerging and Zoonotic Infectious Diseases, CDC

Abstract

Problem/Condition: Since 1971, CDC, the Environmental Protection Agency (EPA), and the Council of State and Territorial 

Epidemiologists have collaborated on the Waterborne Disease and Outbreak Surveillance System (WBDOSS) for collecting and 

reporting data related to occurrences and causes of waterborne disease outbreaks associated with drinking water. This surveillance 

system is the primary source of data concerning the scope and health effects of waterborne disease outbreaks in the United States.

Reporting Period: Data presented summarize 48 outbreaks that occurred during January 2007–December 2008 and 70 previously unreported outbreaks.

Description of System: WBDOSS includes data on outbreaks associated with drinking water, recreational water, water not intended 

for drinking (WNID) (excluding recreational water), and water use of unknown intent (WUI). Public health agencies in the states, 

U.S. territories, localities, and Freely Associated States are primarily responsible for detecting and investigating outbreaks and 

reporting them voluntarily to CDC by a standard form. Only data on outbreaks associated with drinking water, WNID (excluding 

recreational water), and WUI are summarized in this report. Outbreaks associated with recreational water are reported separately.

Results: A total of 24 states and Puerto Rico reported 48 outbreaks that occurred during 2007–2008. Of these 48 outbreaks, 36 

were associated with drinking water, eight with WNID, and four with WUI. The 36 drinking water–associated outbreaks caused 

illness among at least 4,128 persons and were linked to three deaths. Etiologic agents were identified in 32 (88.9%) of the 36 drinking water–associated outbreaks; 21 (58.3%) outbreaks were associated with bacteria, five (13.9%) with viruses, three (8.3%) with 

parasites, one (2.8%) with a chemical, one (2.8%) with both bacteria and viruses, and one (2.8%) with both bacteria and parasites. 

Four outbreaks (11.1%) had unidentified etiologies. Of the 36 drinking water–associated outbreaks, 22 (61.1%) were outbreaks 

of acute gastrointestinal illness (AGI), 12 (33.3%) were outbreaks of acute respiratory illness (ARI), one (2.8%) was an outbreak 

associated with skin irritation, and one (2.8%) was an outbreak of hepatitis. All outbreaks of ARI were caused by Legionella spp.

A total of 37 deficiencies were identified in the 36 outbreaks associated with drinking water. Of the 37 deficiencies, 22 (59.5%) 

involved contamination at or in the source water, treatment 

facility, or distribution system; 13 (35.1%) occurred at points 

not under the jurisdiction of a water utility; and two (5.4%) 

had unknown/insufficient deficiency information. Among 

the 21 outbreaks associated with source water, treatment, or 

distribution system deficiencies, 13 (61.9%) were associated 

Corresponding author: Joan M. Brunkard, PhD, Division of 

Foodborne, Waterborne, and Environmental Diseases, National Center 

for Emerging and Zoonotic Infectious Diseases, CDC, 1600 Clifton 

Road, N.E., MS C-9, Atlanta, GA 30333. Telephone: 404-639-1700; 

E-mail: healthywater@cdc.gov.
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 39

Introduction

Data on waterborne disease outbreaks in the United States 

have been collected since 1920. Researchers reported these 

statistics during 1920–36 (1), 1938–1945 (2), 1946–1960 (3), 

and 1961–1970 (4). Since 1971, CDC, the U.S. Environmental 

Protection Agency (EPA), and the Council of State and Territorial 

Epidemiologists (CSTE) have collaborated on the Waterborne 

Disease and Outbreak Surveillance System (WBDOSS), which 

tracks the occurrences and causes of outbreaks associated with 

drinking water and other water exposures. The history of 

surveillance for waterborne disease outbreaks in the United States 

has been summarized previously (5). Previously reported data from 

drinking water–associated outbreaks have been reclassified systematically and analyzed for trends (6). This report presents data on 48 

outbreaks that occurred during 2007–2008 and were reported to 

CDC by public health departments in U.S. states, territories, and 

localities as well as a previously unreported outbreak that occurred 

in 2002 and 69 legionellosis outbreaks associated with drinking 

water, water not intended for drinking (WNID), and water of 

unknown intent (WUI) that occurred before 2001. Since 2001, 

with untreated ground water, six (28.6%) with treatment deficiencies, one (4.8%) with a distribution system deficiency, and one 

(4.8%) with both a treatment and a distribution system deficiency. No outbreaks were associated with untreated surface water. Of 

the 21 outbreaks, 16 (76.2%) occurred in public water systems (drinking water systems under the jurisdiction of EPA regulations 

and water utility management), and five (23.8%) outbreaks occurred in individual systems (all of which were associated with 

untreated ground water). Among the 13 outbreaks with deficiencies not under the jurisdiction of a water system, 12 (92.3%) 

were associated with the growth of Legionella spp. in the drinking water system, and one (7.7%) was associated with a plumbing 

deficiency. In the two outbreaks with unknown deficiencies, one was associated with a public water supply, and the other was 

associated with commercially bottled water. The 70 previously unreported outbreaks included 69 Legionella outbreaks during 

1973–2000 that were not reportable previously to WBDOSS and one previously unreported outbreak from 2002.

Interpretation: More than half of the drinking water–associated outbreaks reported during the 2007–2008 surveillance period 

were associated with untreated or inadequately treated ground water, indicating that contamination of ground water remains a 

public health problem. The majority of these outbreaks occurred in public water systems that are subject to EPA’s new Ground 

Water Rule (GWR), which requires the majority of community water systems to complete initial sanitary surveys by 2012. The 

GWR focuses on identification of deficiencies, protection of wells and springs from contamination, and providing disinfection 

when necessary to protect against bacterial and viral agents. In addition, several drinking water–associated outbreaks that were 

related to contaminated ground water appeared to occur in systems that were potentially under the influence of surface water. 

Future efforts to collect data systematically on contributing factors associated with drinking water outbreaks and deficiencies, 

including identification of ground water under the direct influence of surface water and the criteria used for their classification, 

would be useful to better assess risks associated with ground water.

During 2007–2008, Legionella was the most frequently reported etiology among drinking water–associated outbreaks, following 

the pattern observed since it was first included in WBDOSS in 2001. However, six (50%) of the 12 drinking water–associated 

Legionella outbreaks were reported from one state, highlighting the substantial variance in outbreak detection and reporting 

across states and territories. The addition of published and CDC-investigated legionellosis outbreaks to the WBDOSS database 

clarifies that Legionella is not a new public health issue. During 2009, Legionella was added to EPA’s Contaminant Candidate 

List for the first time.

Public Health Actions: CDC and EPA use WBDOSS surveillance data to identify the types of etiologic agents, deficiencies, 

water systems, and sources associated with waterborne disease outbreaks and to evaluate the adequacy of current technologies 

and practices for providing safe drinking water. Surveillance data also are used to establish research priorities, which can lead to 

improved water quality regulation development. Approximately two thirds of the outbreaks associated with untreated ground 

water reported during the 2007–2008 surveillance period occurred in public water systems. When fully implemented, the GWR 

that was promulgated in 2006 is expected to result in decreases in ground water outbreaks, similar to the decreases observed 

in surface water outbreaks after enactment of the Surface Water Treatment Rule in 1974 and its subsequent amendments. One 

third of drinking water–associated outbreaks occurred in building premise plumbing systems outside the jurisdiction of water 

utility management and EPA regulations; Legionella spp. accounted for >90% of these outbreaks, indicating that greater attention is needed to reduce the risk for legionellosis in building plumbing systems. Finally, a large communitywide drinking water 

outbreak occurred in 2008 in a public water system associated with a distribution system deficiency, underscoring the importance 

of maintaining and upgrading drinking water distribution system infrastructure to provide safe water and protect public health.
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40 MMWR / September 23, 2011 / Vol. 60 / No. 12

Drinking water systems

Privately owned home or farm 

wells, springs, or surface water 

sources

Public water systems

Public or private ownership

(subject to EPA regulations)

Noncommunity Community 

Individual water systems

(if regulated, state or local 

regulations)

Bottled water* (regulated by FDA) 

Transient (e.g., gas stations, 

parks, resorts, campgrounds, 

restaurants, and motels with 

their own water systems)

Nontransient (e.g., schools, 

factories, oce buildings, and 

hospitals with their own water 

systems) 

Abbreviations: EPA = Environmental Protection Agency; FDA = Food and Drug Administration.

* In certain instances, bottled water is used in lieu of a community supply or by noncommunity systems.

FIGURE 1. Types of drinking water systems — United States

legionellosis outbreaks associated with drinking water have been 

reported to WBDOSS. Legionellosis outbreaks that occurred 

before 2001 have been added retrospectively to WBDOSS to 

provide a more complete representation of legionellosis outbreaks 

associated with drinking water, WNID, and WUI. The data 

provided in this report represent only a portion of the burden of 

illness associated with exposure to drinking water. They do not 

include either endemic waterborne disease cases (sporadic cases 

not known to be associated with an outbreak) or the estimated 

number of unrecognized and unreported outbreaks.

Background

Environmental Protection Agency Drinking Water 

Regulations

The majority of outbreaks described in this report occurred 

in public drinking water systems (Figure 1). The Safe Drinking 

Water Act (SDWA) of 1974 and its subsequent 1986 and 

1996 amendments authorized EPA to set national standards to 

protect public drinking water and its sources against naturally 

occurring or man-made contaminants (7–9). These standards 

include health-based maximum levels for microbiologic, 

chemical, and other contaminants in drinking water and water 

treatment performance criteria for the removal or inactivation 

of contaminants (Table 1). If needed, EPA can issue guidance 

or a health advisory instead of a regulation. EPA regulations do 

not apply to private, individual water supplies. However, certain 

states and localities might set standards for individual water 

supplies (e.g., driller licensing and registration, well permitting, 

and water testing processes). Standards and requirements for 

private wells vary among states and localities. Commercially 

bottled water is regulated by the Food and Drug Administration 

(FDA) (Figure 1).

Additional rules that protect against exposure to waterborne 

pathogens include the Surface Water Treatment Rule (SWTR) 

and its amendments (10–15), the Total Coliform Rule (TCR) 

(16,17), and the 2006 Ground Water Rule (GWR) (18,19). The 

SWTR and amendments specify water-treatment techniques 

(e.g., filtration and disinfection), monitoring, and performance 

criteria for systems that use surface water sources to protect 

against Giardia and Cryptosporidium contamination. EPA has 

established criteria to assess whether ground water sources are 

under the direct influence of surface water, and, if so, these systems are required to meet provisions of the SWTR and amendments. The TCR requires public water systems to monitor for 

indicators of fecal contamination and take corrective action when 

they are found. In 2007, EPA established an advisory committee 

to provide recommendations on revisions to the TCR and on 
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 41

TABLE 1. Selected U.S. Environmental Protection Agency listings and 

regulations regarding drinking water, by year enacted — United 

States, 1974–2009

Regulation Year

Safe Drinking Water Act (SDWA) 1974

Interim Primary Drinking Water Standards 1975

National Primary Drinking Water Standards 1985

SDWA Amendments 1986

Surface Water Treatment Rule (SWTR) 1989

Total Coliform Rule (TCR) 1989

Chemical Contaminant Rules Phase I 1989

Lead and Copper Regulations 1990

Chemical Contaminant Rules Phase II 1992

Chemical Contaminant Rules Phase IIB 1993

Chemical Contaminant Rules Phase V 1994

SDWA Amendments 1996

Information Collection Rule 1996

Interim Enhanced SWTR 1998

Stage 1 - Disinfectants and Disinfection By-Products (D-DBP) 

Regulation

1998

Drinking Water Contaminant Candidate List 1998

Unregulated Contaminant Monitoring Regulations 1999

Radionuclides Rule 2000

Lead and Copper Rule — action levels 2000

Filter Backwash Recycling Rule 2001

Long Term 1 Enhanced SWTR 2002

Unregulated Contaminant Monitoring Regulations 2002

Drinking Water Contaminant Candidate List 2 2005

Long Term 2 Enhanced SWTR 2006

Stage 2 D-DBP Rule 2006

Ground Water Rule 2006

Drinking Water Contaminant Candidate List 3 2009

information needed to understand better the public health risks 

associated with the degradation of water quality in pipes, storage tanks, and other appurtenances used to distribute drinking 

water to consumers (20). The GWR specifies when corrective 

action, including disinfection, is required for wells and springs to 

protect against bacteria and viruses (Table 1). In addition to the 

rules described above, a program established in the 1986 SDWA 

amendments, the Wellhead Protection Program, requires states 

to develop plans to delineate and manage wellhead protection 

areas and to actively reduce the potential for contamination of 

all public ground water systems (18).

The SDWA amendments of 1996 require EPA to publish 

periodically a list of contaminants that must be evaluated for 

potential regulatory action (21,22) and to establish criteria 

for a program to monitor unregulated contaminants (23–26). 

Contaminant Candidate Lists (CCL1, 2 and 3) were published 

in 1998, 2005, and 2009 (21,22,27). Microbial contaminants are 

selected for inclusion on the CCL on the basis of three criteria: 

1) that the contaminant might have an adverse effect on human 

health, 2) that the contaminant is known to occur or there is a 

substantial likelihood that it will occur in public water systems 

with a frequency and at levels of public health concern, and 3) that 

regulation of the contaminant presents a meaningful opportunity 

to reduce health risk (28). In 2009, Legionella pneumophila was 

added to CCL3 for the first time (22).

Methods

Data Sources

Public health agencies in the states, U.S. territories, localities, 

and Freely Associated States* (FAS) have primary responsibility 

for detecting and investigating outbreaks, which they report 

voluntarily to CDC using a standard form (CDC 52.12, available at http://www.cdc.gov/healthywater/statistics/wbdoss/

nors/forms_archive.html). The form solicits data on characteristics of outbreaks (e.g., number of cases, time, and location), 

results from epidemiologic and environmental investigations, 

and results from clinical-specimen and water-sample testing. 

CDC annually requests reports of outbreaks from persons 

designated as waterborne disease surveillance coordinators 

and obtains additional information regarding epidemiologic 

investigations, water quality, and water treatment to supplement submitted outbreak reports as needed. Numeric and text 

data are abstracted from outbreak report forms and supporting 

documents and entered into a database for analysis. All of the 

outbreaks that occurred during 2007–2008 were reported 

through the paper-based reporting system and analyzed by 

using SAS 9.2 (SAS Institute, Inc. Cary, North Carolina). 

To ensure completeness of legionellosis outbreak data for 

2007–2008, CDC compared data from WBDOSS with data 

from the Travel-Associated Legionellosis in the United States 

System. In addition, data on outbreaks of legionellosis that 

occurred before 2001 were added to the WBDOSS database.

Definitions

Waterborne Disease Outbreak

Two criteria must be met for an event to be defined as an 

outbreak associated with drinking water, WNID (excluding 

recreational water), or WUI: 1) two or more persons must 

be linked epidemiologically by time, location of exposure to 

water, and illness characteristics; and 2) the epidemiologic 

evidence must implicate water as the probable source of illness. Outbreak reports with limited or no environmental data 

might be included in WBDOSS, but outbreak reports that lack 

epidemiologic data linking the outbreak to water are excluded.

Reported outbreaks associated with contaminated drinking 

water, commercially bottled water, ice, beverages made with 

contaminated water, and water contaminated by malfunctions 

in equipment or devices in which water is used or distributed 

(e.g., beverages contaminated by plumbing failures in drink 

mix/soda machines) are classified as drinking water–associated 

*Includes the Republic of the Marshall Islands, the Federated States of 

Micronesia, and the Republic of Palau; formerly part of the U.S.-administered 

Trust Territory of the Pacific Islands.
Surveillance Summaries

42 MMWR / September 23, 2011 / Vol. 60 / No. 12

outbreaks. Outbreaks involving the consumption of beverages 

containing contaminated ingredients (i.e., other than water) 

or ice contaminated through human handling are reported to 

CDC as foodborne disease outbreaks, not waterborne disease 

outbreaks. WBDOSS report data are categorized on the basis 

of the location of the water exposure, not the ill person’s state 

of residence. Outbreaks occurring on cruise ships are not 

reported to WBDOSS; CDC’s Vessel Sanitation Program 

tracks outbreaks of acute gastrointestinal illness (AGI) related 

to cruise ships (29).

Single Cases

Single cases of illness or injury associated with water exposure 

are not classified or analyzed as outbreaks but might be reported 

directly to WBDOSS or through other national surveillance 

systems. Cases that might be reported directly to WBDOSS 

include laboratory-confirmed primary amebic meningoencephalitis (PAM) as a result of Naegleria fowleri infection and 

single cases of chemical or toxin poisoning.

Water Systems

WBDOSS includes data on outbreaks occurring in public 

and individual water systems. EPA defines a public water 

system as a system for the provision of water for human 

consumption through a distribution system that has at least 

15 service connections or that regularly serves at least 25 

persons (30). An individual system is one that does not meet 

EPA’s definition of a public water system. It typically serves 

a single family or farm; individual systems are not subject to 

EPA regulations but might be regulated at the state or local 

level. Public water systems include community water systems 

(serving the same persons year round) and noncommunity 

water systems (which serve the public but generally do not 

serve the same persons year round). Noncommunity water 

systems include transient noncommunity systems (serving 

different persons for >6 months of the year, such as those 

in parks and restaurants) and nontransient, noncommunity 

systems (serving the same persons for >6 months out of 

the year, such as those in schools and factories) (Figure 1). 

Of the approximately 153,530 public water systems in the 

United States, 51,651 (33.6%) are community systems, and 

101,879 (66.4%) are noncommunity systems, including 

83,484 (81.9%) transient systems and 18,395 (18.1%) 

nontransient systems (Figure 1) (31). Community systems 

serve 294.3 million persons; of the 51,651 community water 

systems, 4,156 (8%) are classified by EPA as “large” (serving 

10,001–100,000 persons) or “very large” (serving >100,000 

persons) and provide water to 77% of the U.S. population. 

Nontransient, noncommunity systems provide water to 6.2 

million persons, and transient noncommunity systems provide 

water to 13.3 million persons (by definition, these populations 

also use another type of water system at their residences, except 

for the limited number of permanent residents of nontransient 

systems) (31). Although 78% of community water systems are 

supplied by ground water, more persons (70%) are supplied 

year-round by community water systems that use surface water 

(31). Approximately 15% of the U.S. population (15.8 million 

households) relies on individual water systems that are owned 

privately (32,33). Private wells that serve <25 persons are not 

regulated by EPA under SDWA (34).

Water Sources

Drinking water–associated outbreaks are categorized in 

WBDOSS as having ground water, surface water, or mixed 

water sources. Ground water sources include springs, aquifers, 

and wells. Surface water refers to all water found on the surface 

(e.g., river, lake, or pond) as distinguished from subsurface or 

ground water. A drinking water system that uses both a ground 

water and a surface water source is defined by WBDOSS as 

having a mixed source. Ground water sources that have the 

potential for surface water contamination and are used to supply drinking water systems may be categorized as ground water 

under the direct influence of surface water (GWUDI). EPA 

has specified criteria to identify potential GWUDI, and state 

and local environmental health agencies conduct site-specific 

evaluations of water quality, construction characteristics, and 

geology to determine treatment requirements. 

Deficiencies

To understand the circumstances and water system 

breakdowns that lead to outbreaks, each outbreak is classified 

as having one or more deficiencies (e.g., in water treatment and 

operation, water storage and delivery, or premise plumbing) 

(Table 2). Analyses of outbreak deficiencies provide important 

information about how the water became contaminated, water 

system characteristics, and factors leading to waterborne disease 

outbreaks.

Strength-of-Evidence Classification for 

Waterborne Disease Outbreaks

All outbreaks reported to WBDOSS for 2007–2008 have 

been classified according to the strength of 1) epidemiologic 

and clinical laboratory evidence and 2) environmental evidence 

implicating water as the vehicle of transmission (Table 3). 

The classification (i.e., Classes I–IV), which was first used 

in the 1989–1990 surveillance report (35), is based on the 

epidemiologic and environmental data reported to WBDOSS.

Outbreaks and subsequent investigations occur under 

different circumstances, and not all outbreaks can be investigated 
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 43

TABLE 2. CDC deficiency classification for drinking water, water not intended for drinking (excluding recreational water), and water of unknown 

intent

Contamination of water at/in the water source, treatment facility, or distribution system* 

1: Untreated surface water intended for drinking

2: Untreated ground water intended for drinking

3: Treatment deficiency (e.g., temporary interruption of disinfection, chronically inadequate disinfection, or inadequate or no filtration) 

4: Distribution system deficiency, including storage (e.g., cross-connection, backflow, contamination of water mains during construction or repair)

13: Current treatment processes not expected to remove a chemical contaminant (e.g., pesticide contamination of ground water treated with disinfection only)

A: Surface water

B: Ground water

Contamination of water at points not under the jurisdiction of a water utility or at the point of use†

5: Legionella spp. in water system

A: Water intended for drinking

B: Water not intended for drinking (excluding recreational water)

C: Water of unknown intent

6: Plumbing system deficiency after the water meter or property line (e.g., cross-connection, backflow, or corrosion products)

7: Deficiency in building/home-specific water treatment after the water meter or property line

8: Deficiency or contamination of equipment using or distributing water (e.g., drink-mix machines)

9: Contamination or treatment deficiency during commercial bottling 

10: Contamination during shipping, hauling, or storage

A: Water intended for drinking – tap water

B: Water intended for drinking – commercially bottled water

11: Contamination at point of use

A: Tap

B: Hose

C: Commercially bottled water

D: Container, bottle, or pitcher

E: Unknown

12: Drinking or contact with water not intended for drinking (excluding recreational water) 

Unknown/Insufficient Information

99: Unknown/Insufficient information

A: Water intended for drinking – tap water

B: Water intended for drinking – commercially bottled water

C: Water not intended for drinking (excluding recreational water)

D: Water of unknown intent

* For a community water system, the distribution system refers to the pipes and storage infrastructure under the jurisdiction of the water utility prior to the water 

meter or property line (if the system is not metered). For noncommunity and nonpublic individual water systems, the distribution system refers to the pipes and 

storage infrastructure before entry into a building or house.

† Contamination of drinking water and deficiencies occurring in plumbing and pipes that are not part of the distribution system as defined previously. For community 

systems, this means occurring after the water meter or outside the jurisdiction of a water utility; for noncommunity and nonpublic systems, this means occurring 

within the building or house (e.g., in a service line leading to a house or building, in the plumbing inside a house or building, during shipping or hauling, during 

storage other than in the distribution system, or at point of use).

rigorously. Classifications that do not meet the highest level (I) 

do not necessarily imply that the investigation was inadequate 

or incomplete because multiple factors (e.g., timeliness of 

outbreak detection) contribute to the ability to collect optimal 

epidemiologic, clinical laboratory, and environmental data.

Additional terms used in this report have been defined 

elsewhere (Appendix A).

Changes in the 2007–2008 

Surveillance Summary

Strength-of-Evidence Classification

Molecular epidemiology is used increasingly to understand 

pathogen transmission patterns, detect outbreaks, and identify 

important risk factors and outbreak sources. The criteria used 

to determine the strength-of-evidence classifications have been 

revised to reflect the increasing use of molecular characterization 

of pathogens identified in clinical specimens and environmental 

samples collected during outbreak investigations. Molecular data 

that link multiple persons who had an identical water exposure 

now are considered adequate epidemiologic data to support a 

Class I or Class II assignment; molecular data that link at least 

one person to the implicated water exposure now are considered 

adequate water quality data to support a Class I or Class III assignment. Previously, epidemiologic study data were required to receive 

a strength-of-evidence classification of I or II. The “epidemiologic 

data” and “water quality data” categories have been renamed 

“epidemiologic and clinical laboratory data” and “environmental 

data,” respectively (Table 3).

Number of Cases

Case counts provided in this report were based on the 

estimated number of total cases if sufficient supporting evidence 
Surveillance Summaries

44 MMWR / September 23, 2011 / Vol. 60 / No. 12

TABLE 3. CDC classification of investigations of waterborne disease outbreaks on the basis of strength of evidence implicating water as a vehicle 

of transmission — United States, 2007–2008

Class Epidemiologic and clinical laboratory data Environmental data

I Provided and adequate Provided and adequate

Epidemiologic data provided about exposed and unexposed persons, 

with relative risk or odds ratio ≥2 or p-value ≤0.05; or

Laboratory data or historic information (e.g., reports of chlorinator 

malfunction, a water main break, no detectable free-chlorine residual, or 

the presence of coliforms in the water); or

Molecular characterization of pathogens linked multiple persons who 

had a single identical exposure

Molecular characteristics of pathogens isolated from water and at least 

one clinical specimen were identical

II Provided and adequate Not provided or inadequate

Epidemiologic data provided about exposed and unexposed persons, 

with relative risk or odds ratio ≥2 or p-value <0.05; or

E.g., laboratory testing of water not conducted and no historic information 

available

Molecular characterization of pathogens linked multiple persons who 

had a single identical exposure

III Provided but limited Provided and adequate

Epidemiologic data provided that did not meet the criteria for Class I 

or II or claim made that ill persons had no exposures in common, 

besides water, but no data provided

Laboratory data or historic information (e.g., reports of chlorinator 

malfunction, a water main break, no detectable free-chlorine residual, or 

the presence of coliforms in the water); or

Molecular characteristics of pathogens isolated from water and at least 

one clinical specimen were identical

IV Provided but limited Not provided or inadequate

Epidemiologic data provided that did not meet the criteria for Class I 

or II or claim made that ill persons had no exposures in common, 

besides water, but no data provided

E.g., laboratory testing of water not conducted and no historic information 

available

was provided. For example, this might include applying the 

attack rate found during a cohort study to the entire population 

exposed to contaminated water to estimate the total number 

of ill persons associated with an outbreak. If no “estimated ill” 

number was provided, the actual number of reported cases (e.g., 

laboratory-confirmed and probable cases as reported by the 

state) was used. CDC requests that states report only cases in 

which primary exposure to water occurs, so secondary cases (e.g., 

person-to-person transmission among household members) are 

not included in case counts of waterborne disease in WBDOSS.

Analysis of Deficiencies

Previous WBDOSS surveillance reports limited the descriptive 

analyses to focus on drinking water–associated outbreaks with 

source water, treatment, or distribution system deficiencies (i.e., 

deficiencies 1–4 and 13) (Table 2). However, tables and figures 

in this report present data for all of the drinking water–associated outbreaks and deficiencies, including those associated with 

Legionella spp. and building premise plumbing deficiencies (i.e., 

deficiencies 1–13 and 99).

Legionnaires’ Disease Outbreaks Before 2001

Data concerning previously unreported legionellosis outbreaks 

that occurred before 2001 have been added to the WBDOSS 

database. These data were abstracted from Epidemic Intelligence 

Service outbreak investigation reports and peer-reviewed 

publications. Previously, only Legionnaires’ disease (LD) 

outbreaks that occurred after 2001 were included in WBDOSS.

Results

During 2007–2008, a total of 24 states and Puerto Rico 

reported 48 outbreaks, including 29 for 2007 and 19 for 2008. 

Of these, 36 outbreaks were associated with drinking water, eight 

with WNID, and four with WUI. Outbreaks are tabulated by 

year and state (Tables 4–6). One outbreak that occurred in 2002 

and 69 Legionella outbreaks from 1973–2000 also were included 

as previously unreported outbreaks (Table 7).

Waterborne Disease Outbreaks Associated 

with Drinking Water

Since the surveillance system first started in 1971, drinking 

water–associated outbreaks have been reported every year 

(Figure 2). The 36 outbreaks described in this report (including 

20 in 2007 and 16 in 2008) occurred in 23 states and Puerto 

Rico (Figure 3). Outbreaks occurred predominantly in the 

spring through fall, with only three outbreaks reported during 

November–February. Multiple etiologic agents were implicated 

in the 36 outbreaks (Table 8). Descriptions of selected 

outbreaks have been summarized (Appendix B).

The 36 outbreaks reported during 2007–2008 caused illness 

among at least 4,128 persons and resulted in three deaths. The 

median number of persons affected in an outbreak was 14 

(range: 2–1,663). Four predominant illnesses were reported: 22 

(61.1%) outbreak reports of AGI, 12 (33.3%) outbreak reports 

of acute respiratory illness (ARI), one (2.8%) outbreak report 
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 45

TABLE 4. Waterborne disease outbreaks associated with drinking water (n = 20) — Waterborne Disease and Outbreak Surveillance System, 

United States, 2007

State Month Class* Etiology

Predominant 

illness†

No. of cases§

(n = 2,456)

Type of 

system¶ Deficiency**

Water 

source Setting

California Jul III Giardia intestinalis AGI 46 Ncom 3 Spring Camp/Cabin

Colorado Jun III Norovirus genogroup II AGI 77 Ncom 3 Well Camp/Cabin

Florida Sep I Unidentified AGI 1,663 Com 3 Reservoir Community

Idaho Jun I Campylobacter AGI 15 Ncom 2 Spring†† Camp/Cabin

Massachusetts Apr III Sodium hydroxide§§ Skin 145 Com 3 Well Community

Maryland Jun I Norovirus genogroup II AGI 94 Ncom 3 Well Membership club

Missouri May I Unidentified AGI 51 Com 3 Well Community

New Hampshire Aug I G. intestinalis AGI 35 Com 2 Well Neighborhood/Subdivision

Nevada Oct I Legionella pneumophila serogroup 1 ARI 7 Com 5A Reservoir Condominium

New York Oct III L. pneumophila serogroup 1 ARI 3 Com 5A Reservoir Nursing home

New York Oct IV L. pneumophila serogroups 1, 5 ARI 3 Com 5A Reservoir Hospital/Health-care facility

New York Jun III L. pneumophila serogroup 1 ARI 2 Com 5A Reservoir Nursing home

Ohio Mar IV Unidentified AGI 3 Bottled 99B Reservoir¶¶ Private residence

Ohio Nov III L. pneumophila serogroup 1 ARI 3 Com 5A Lake Hospital/Health-care facility

Tennessee May I Salmonella Newport AGI 2 Ind 2 Well Private Residence

Vermont Oct III L. pneumophila serogroup 1 ARI 3 Ncom 5A Well Hotel/Motel

Washington Jan I Norovirus AGI 32 Ncom 2 Well Restaurant

Wisconsin May I Norovirus genogroup I, 

Campylobacter, Salmonella

AGI 229 Ncom 2 Well Restaurant

West Virginia Oct III Campylobacter AGI 4 Ind 2 Spring Private residence

Wyoming Jan I Unidentified*** AGI 39 Ncom 2 Well Lodge

Abbreviations: AGI = acute gastrointestinal illness; ARI = acute respiratory illness; Skin = illnesses, conditions, or symptoms related to skin; Com = community; Ncom = noncommunity; 

Ind = individual; Bottled = commercially bottled water.

* On the basis of epidemiologic, clinical laboratory, and environmental data (e.g., water quality data) provided to CDC. † The category of illness reported by ≥50% of ill respondents. § No deaths were reported in cases associated with outbreaks reported during 2007. ¶ Community and noncommunity water systems are public water systems that have >15 service connections or serve an average of ≥25 residents for ≥60 days/year. A community water 

system serves year-round residents of a community, subdivision, or mobile home park. A noncommunity water system serves an institution, industry, camp, park, hotel, or business and 

can be nontransient or transient. Nontransient systems serve ≥25 of the same persons for >6 months of the year but not year-round (e.g., factories or schools) whereas transient systems 

provide water to places in which persons do not remain for long periods of time (e.g., restaurants, highway rest stations, and parks). Individual water systems are small systems not owned 

or operated by a water utility that have <15 connections or serve <25 persons.

** Deficiency classification for drinking water, water not intended for drinking (excluding recreational water), and water of unknown intent (see Table 2). †† The spring associated with this outbreak was vulnerable to surface water contamination and is regulated as a surface water source by the state. §§ A chemical overfeed of sodium hydroxide at the water-treatment facility raised the pH of the water in the distribution system. ¶¶ Water from a community water system that used surface water was distilled before distribution.

*** Etiology unidentified; norovirus suspected based upon incubation period, symptoms, and duration of illness.

of hepatitis and one (2.8%) outbreak report of skin irritation 

associated with a chemical exposure. Although the 22 AGI 

outbreaks were caused by a variety of pathogens, all 12 ARI 

outbreaks were caused by Legionella (i.e., LD or Pontiac fever 

[PF]) (Figure 4).

Of the 36 drinking water–associated outbreaks, 17 (47.2%) 

were assigned a strength-of-evidence rank of Class I, two (5.6%) 

were ranked as Class II, 14 (38.9%) were ranked as Class III, and 

three (8.3%) were ranked as Class IV. Drinking water–associated 

outbreaks were tabulated by etiology, type of water system, and 

water source (Table 8); deficiency and type of water system 

(Table 9); and deficiency and water source (Table 10).

Etiology

Of the 36 drinking water–associated outbreaks, 21 (58.3%) 

were caused by bacteria, five (13.9%) were caused by viruses, 

three (8.3%) were caused by parasites, and one (2.8%) was 

caused by a chemical. Two (5.6%) outbreaks had multiple 

etiologies: one (2.8%) was caused by bacteria and viruses, and 

one (2.8%) was caused by bacteria and parasites. Four (11.1%) 

had unidentified etiologies: one was suspected to be caused by 

norovirus (Table 8; Figure 5).

Bacteria. A total of 21 outbreaks were associated with bacterial 

agents and resulted in 1,520 cases of illness: 12 outbreaks, 75 

cases of illness, and two deaths were caused by Legionella; four 

outbreaks and 77 cases of illness were caused by Campylobacter; 

three outbreaks, 1,307 cases of illness, and one death were caused 

by Salmonella; one outbreak and six cases of illness were caused 

by E. coli O157:H7; and one outbreak with 55 cases of illness 

was caused by Providencia spp. (Table 8). An estimated 1,300 

cases of illness were related to Salmonella contamination during 

a single outbreak in Colorado (Table 5; Appendix B).

Viruses. Five outbreaks were associated with viral agents and 

resulted in 274 cases of illness: four outbreaks with 265 cases of 

illness were caused by norovirus, and one outbreak with nine cases 

of illness was caused by hepatitis A. 

Parasites. Three outbreaks were associated with parasitic 

agents and resulted in 163 cases of illness and no reported 

deaths. Two of these outbreaks with 81 cases of illness were 

caused by Giardia intestinalis, and one outbreak with 82 cases 

of illness was caused by Cyclospora cayetanensis. 

Chemicals. One outbreak involved a chemical exposure and 

resulted in an estimated 145 cases of illness following exposure 
Surveillance Summaries

46 MMWR / September 23, 2011 / Vol. 60 / No. 12

TABLE 5. Waterborne disease outbreaks associated with drinking water (n = 16), by state/jurisdiction — Waterborne Disease and Outbreak 

Surveillance System, United States, 2008

State/ 

Jurisdiction Month Class* Etiology

Predominant 

illness†

No. of cases [deaths]

(n = 1,672 [3]§)

Type of 

system¶ Deficiency**

Water 

source Setting

Colorado Mar I Salmonella Typhimurium AGI 1,300 [1] Com 4 Well Community

Connecticut Aug I Providencia†† AGI 55 —§§ Com 2 Well Apartment complex

Georgia Sep III Legionella pneumophila serogroup 1 ARI 6 — Com 5A Reservoir Hospital/Health-care facility

Illinois Jun III L. pneumophila serogroup 1 ARI 4 — Com 5A Well Hospital/Health-care facility

Illinois Oct II Escherichia coli O157:H7 AGI 6 — Ind 2 Well Farm

Illinois Sep I Shigella sonnei, Cryptosporidium, Giardia AGI 41 — Com 6 Lake Boat

New Jersey Aug III L. pneumophila serogroup 1 ARI 9 — Com 5A Reservoir Hospital/Health-care facility

New York Jul III L. pneumophila serogroup 1 ARI 13 [1] Com 5A Well, river Seniors housing complex

New York Aug IV L. pneumophila serogroup 1 ARI 19 — Com 5A Lake Assisted living facility

New York Sep III L. pneumophila serogroup 1 ARI 3 [1] Com 5A Lake Nursing home

Oklahoma Jun I Norovirus genogroup 1.4 AGI 62 — Com 3,4 Well Neighborhood/Subdivision

Puerto Rico Apr II Cyclospora cayetanensis AGI 82 — Com 99A River Community/Municipality

Tennessee Mar I Hepatitis A virus Hep 9 — Ind 2 Well Community/Municipality

Tennessee Aug I Salmonella serotype I 4,5,12:i:- AGI 5 — Ncom 2 Spring Private residence

Utah Jun III Campylobacter AGI 50 — Ncom 2 Spring Camp/Cabin

West Virginia May I C. jejuni AGI 8 — Ind 2 Well Private residence

Abbreviations: AGI = acute gastrointestinal illness, ARI = acute respiratory illness; Hep = hepatitis; Com = community; Ncom = noncommunity; Ind = individual; Bottle = commercially bottled 

water.

* On the basis of epidemiologic, clinical laboratory, and environmental data (e.g., water quality data) provided to CDC.

† The category of illness reported by >50% of ill respondents. § Deaths are included in the overall case count.

¶ Community and noncommunity water systems are public water systems that have ≥15 service connections or serve an average of ≥25 residents for ≥60 days/year. A community water 

system serves year-round residents of a community, subdivision, or mobile home park. A noncommunity water system serves an institution, industry, camp, park, hotel, or business and 

can be nontransient or transient. Nontransient systems serve ≥25 of the same persons for >6 months of the year but not year-round (e.g., factories or schools) whereas transient systems 

provide water to places in which persons do not remain for long periods of time (e.g., restaurants, highway rest stations, and parks). Individual water systems are small systems not owned 

or operated by a water utility that have <15 connections or serve <25 persons.

** Deficiency classification for drinking water, water not intended for drinking (excluding recreational water), and water of unknown intent (see Table 2). †† Six of nine stool specimens tested positive for Providencia. Extensive testing concluded that stool specimens were negative for Salmonella, Shigella, Campylobacter, Escherichia coli, ova and parasites, 

and norovirus. §§ No deaths were reported.

TABLE 6. Waterborne disease outbreaks (n = 12) associated with water not intended for drinking (WNID) (excluding recreational water) and 

water of unknown intent (WUI) , by state — Waterborne Disease and Outbreak Surveillance System, United States, 2007–2008

State

Water 

type Month Year Class* Etiology

Predominant 

illness†

No. of cases 

[deaths]

(n = 79 [4]§) Deficiency¶ Water source Setting

Colorado WUI Aug 2007 III Giardia intestinalis AGI 13 —** 99D Unknown Park

Florida WUI Mar 2007 IV Legionella pneumophila serogroup 1 ARI 2 — 5C Unknown Membership club

Iowa WNID May 2007 IV Unidentified†† AGI 10 — 12 Stream State forest

Maryland WNID Oct 2007 I L. pneumophila serogroup 1§§ ARI 2 — 5B Ornamental fountain Hospital/Health-care facility

Maryland WNID Jul 2008 IV L. pneumophila serogroup 1 ARI 18 — 5B Unknown¶¶ Community/Municipality

New Jersey WUI Jul 2008 IV L. pneumophila serogroup 1 ARI 2 — 5C Unknown Senior housing facility

New York WNID Jun 2007 III L. pneumophila serogroup 1 ARI 2 — 5B Cooling tower Community/Municipality

New York WNID Jun 2007 III L. pneumophila serogroup 1 ARI 2 — 5B Recycled water Vehicle-washing station***

New York WUI Aug 2007 IV L. pneumophila serogroup 1

L. pneumophila serogroup 7

ARI 2 [2] 5C Unknown††† Nursing home

New York WNID Sep 2007 III L. pneumophila serogroup 1 ARI 4 [1] 5B Cooling tower Assisted living facility

New York WNID Jun 2008 I L. pneumophila serogroup 1 ARI 12§§§ [1] 5B Cooling tower Community/Municipality

Wyoming WNID Apr 2007 IV Unidentified AGI 10 — 12 Hydrant, hose Youth facility

Abbreviations: AGI = acute gastrointestinal illness; ARI = acute respiratory illness.

* On the basis of epidemiologic, clinical laboratory, and environmental data (e.g., water quality data) provided to CDC.

† The category of illness reported by >50% of ill respondents. § Deaths are included in the overall case count.

¶ Deficiency classification for drinking water, water not intended for drinking (excluding recreational water), and water of unknown intent (see Table 2).

** No deaths were reported. †† Etiology unidentified; norovirus suspected based upon incubation period, symptoms, and duration of illness. §§ Outbreak occurred at a federal facility and was investigated independently by the facility. ¶¶ Environmental testing found positive results in samples collected at a cooling tower and a decorative fountain. 

*** The outbreak was associated with a mass-transit vehicle washing station. ††† The investigation identified the drinking water system and a cooling tower as possible water exposures in this outbreak. §§§ Eleven persons received a diagnosis of Legionnaires’ disease, and one person received a diagnosis of Pontiac fever.
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 47

TABLE 7. Previously unreported outbreaks (n = 70) associated with drinking water, water not intended for drinking, and water of unknown 

intent, by state — Waterborne Disease and Outbreak Surveillance System, United States, 1973–2002*

State Month/Year Class† Etiology

Predominant 

illness§

No. of cases 

[deaths] 

(n = 1,522 [237¶]) Deficiency** Setting

Virginia Jun 1973 IV Legionella pneumophila ARI 12 —†† 5B Factory/Industrial facility

Virginia Jul 1973 IV L. pneumophila ARI 10 — 5B Factory/Industrial facility

Pennsylvania Sep 1974 II L. pneumophila ARI 11 — 5C Hotel/Motel

Kansas Aug 1975 IV Legionella ARI 3 [3] 5C Hospital/Health-care facility 

Connecticut Nov 1976 I L. pneumophila serogroup 1 ARI 28 [12] 5B Community/Municipality

Pennsylvania Jul 1976 II Legionella ARI 182 [29] 5C Hotel/Motel

California May 1977 II Legionella ARI 49 [15] 5C Hospital/Health-care facility

Indiana May 1977 IV Legionella ARI 39 [4] 5C Hotel/Motel/Lodge

Ohio Jul 1977 II Legionella ARI 14 [1] 5C Community/Municipality

Tennessee Aug 1977 IV Legionella ARI 33 [3] 5C Community/Municipality

Vermont May 1977 IV Legionella ARI 69 [17] 5C Community/Municipality

Virginia Jun 1977 IV L. micdadei ARI 16 [2] 5A Hospital/Health-care facility

Georgia Jul 1978 III L. pneumophila ARI 8 — 5B Membership club

New York Aug 1978 IV L. pneumophila serogroup 1 ARI 57 [3] 5B Factory/Industrial facility

Tennessee Aug 1978 I L. pneumophila serogroup 1 ARI 39 [7] 5B Hospital/Health-care facility

Texas Aug 1978 II L. pneumophila serogroup 1 ARI 18 [2] 5B Hotel/Motel

Illinois Dec 1979 III L. pneumophila serogroup 6 ARI 3 — 5A Hospital/Health-care facility 

New York Jul 1979 III L. pneumophila ARI 6 [1] 5B Factory/Industrial facility

Pennsylvania Feb 1979 IV L. pneumophila ARI 27 [12] 5C Hospital/Health-care facility

Wisconsin Jun 1979 I L. pneumophila serogroup 1 ARI 13 [4] 5B Hotel/Motel

California Mar 1980 III L. pneumophila serogroup 1 ARI 14 — 5B Office

California Dec 1980 III L. pneumophila serogroup 1, L. dumoffi ARI 20 — 5C Hospital/Health-care facility 

Connecticut Dec 1980 IV L. pneumophila ARI 36 [5] 5C Hospital/Health-care facility 

Vermont May 1980 II L. pneumophila serogroup 1 ARI 85 [16] 5B Community/Municipality

Washington Nov 1980 IV L. dumoffi ARI 9 [4] 5C Hospital/Health-care facility 

Iowa Oct 1981 IV L. pneumophila serogroup 1 ARI 12 [1] 5C Community/Municipality

New York Feb 1982 I L. pneumophila serogroup 1 ARI 11 — 5A Hospital/Health-care facility 

New York Jun 1982 IV L. pneumophila ARI 3 — 5A Hospital/Health-care facility

Washington Oct 1982 I L. pneumophila serogroup 3 ARI 15 [2] 5A Hospital/Health-care facility

Rhode Island Jun 1983 I L. pneumophila serogroup 1 ARI 15 [10] 5B Hospital/Health-care facility 

California Jan 1984 IV L. pneumophila, L. dumoffi, L. micdadei ARI 14 [1] 5A Hospital/Health-care facility 

New York Apr 1984 III L. pneumophila serogroup 1 ARI 86 — 5B Office

Ohio Apr 1984 IV L. pneumophila serogroup 1 ARI 7 — 5C Hospital/Health-care facility 

Utah May 1984 I L. pneumophila serogroup 1 ARI 4 [1] 5B Hospital/Health-care facility

Michigan May 1985 IV L. pneumophila serogroup 1 ARI 14 [3] 5C Temporary event

South Dakota Apr 1985 I L. pneumophila serogroup 1 ARI 26 [10] 5A Hospital/Health-care facility 

Wisconsin Aug 1986 I L. pneumophila serogroup 1 ARI 32 [2] 5B Community/Municipality

California Mar 1986 IV L. pneumophila serogroups 1, 4, 6 ARI 7 [5] 5A Hospital/Health-care facility

Maryland May 1986 IV L. pneumophila serogroup 1 ARI 27 [2] 5C Store

Arizona Jan 1987 I L. pneumophila serogroups 1, 5, 6, 10 ARI 25 [12] 5A Hospital/Health-care facility 

Florida Jan 1987 III L. pneumophila serogroup 1 ARI 2 [1] 5A Hospital/Health-care facility 

Washington May 1987 II L. micdadei, L. feelei, L. pneumophila serogroup 3 ARI 7 [5] 5A Hospital/Health-care facility

Michigan Jul 1987 IV L. pneumophila serogroup 1 ARI 7 — 5B Prison/Jail

Pennsylvania Jul 1987 III L. pneumophila serogroup 1 ARI 2 [1] 5B Factory/Industrial facility

Vermont Oct 1987 IV L. pneumophila serogroup 1 ARI 27 [3] 5C Hotel/Motel

California Apr 1988 III L. anisa ARI†† 34 — 5B Hotel/Motel

California Jun 1988 IV L. pneumophila serogroup 1 ARI 3 — 5C Retirement home 

Ohio Jul 1989 III L. pneumophila serogroup 1 ARI 38 [11] 5A Hospital/Health-care facility 

Louisiana Oct 1989 I L. pneumophila serogroup 1 ARI 33 [2] 5A Store

New York Jun 1989 II L. pneumophila serogroups 1, 6 ARI 7 — 5C Hospital/Health-care facility

Texas Apr 1989 II L. pneumophila serogroup 1 ARI 14 [6] 5A Hospital/Health-care facility

California Mar 1991 II L. pneumophila serogroups 4, 5, 6 ARI 5 — 5C Hospital/Health-care facility 

California Aug 1991 IV L. pneumophila serogroup 1 ARI 10 [1] 5C Office

Utah Aug 1991 I L. pneumophila serogroup 1 ARI 11 — 5A Indoor place of work

Florida Jan 1992 I L. pneumophila serogroup 1 ARI 5 — 5B Hotel/Motel

Ohio Jun 1992 II L. pneumophila serogroups 1, 3, 7, L. longbeachae ARI 52 [2] 5C Community/Municipality

Pennsylvania Feb 1992 II L. pneumophila serogroup 1 ARI 2 — 5C Factory/Industrial Facility

Massachusetts Jul 1993 I L. pneumophila serogroup 1 ARI 11 [3] 5B Community/Municipality

Rhode Island Aug 1993 I L. pneumophila serogroup 1 ARI 17 [2] 5B Community/Municipality

Connecticut Apr 1994 I L. pneumophila serogroup 1 ARI 20 — 5A Hospital/Health-care facility 

Delaware Jul 1994 I L. pneumophila serogroup 1 ARI 29 [2] 5B Hospital/Health-care facility 

New York Jan 1995 I L. micdadei ARI 19 [4] 5A Hospital/Health-care facility 

Pennsylvania Jul 1995 I L. pneumophila serogroup 1 ARI 22 [3] 5B Hospital/Health-care facility 

Missouri Sep 1996 III L. pneumophila serogroup 1 ARI 3 — 5C Restaurant/Cafeteria

Rhode Island Aug 1996 III L. pneumophila serogroup 1 ARI 18 [1] 5B Community/Municipality

Texas Apr 1996 III L. pneumophila ARI 4 — 5C Hospital/Health-care facility

See table footnotes on page 48
Surveillance Summaries

48 MMWR / September 23, 2011 / Vol. 60 / No. 12

Multiple

Legionella spp.†

Unidentied

Chemical

Viral

Bacterial§

Parasitic

0

10

20

30

40

50

60

1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007

Year

No. of outbreaks

FIGURE 2. Number of waterborne disease outbreaks associated with drinking water (n = 818),* by year and etiology — Waterborne Disease 

and Outbreak Surveillance System, United States, 1971–2008

* Some outbreaks from prior reporting periods were added, reclassified, or excluded during an extensive review (Craun GF, Brunkard JM, Yoder JS, et al. Causes of 

outbreaks associated with drinking water in the United States from 1971 to 2006. Clin Microbiol Rev 2010;23:507–28); therefore, data are not comparable to figures 

in previous reports. 

† Legionnaires’ disease (LD) was reported to the Waterborne Disease and Outbreak Surveillance System (WBDOSS) beginning in 2001. A review of publications and 

CDC-led investigations during 1971–2000 resulted in the addition of 17 historic LD drinking water outbreaks to WBDOSS.

§ Includes all bacteria except Legionella.

TABLE 7. (Continued) Previously unreported outbreaks (n = 70) associated with drinking water, water not intended for drinking, and water of 

unknown intent, by state — Waterborne Disease and Outbreak Surveillance System, United States, 1973–2002*

State Month Year Class† Etiology

Predominant 

illness§

No. of 

cases[deaths] 

(n = 1,522 [237¶]) Deficiency** Setting

California Nov 1997 II L. pneumophila serogroup 1 ARI 8 [1] 5C Community/Municipality

Missouri Jul 1997 III L. pneumophila serogroup 1, L. bozemanii ARI 4 — 5B Hospital/Health-care facility 

Pennsylvania May 1998 IV L. pneumophila serogroup 1 ARI 7 —  5B Community/Municipality

Georgia Sep 2002 III Mycobacterium mageritense Skin 2 — 12 Nail salon

Abbreviations: ARI = acute respiratory illness; Skin = illnesses, conditions, or symptoms related to skin.

* Previously unreported legionellosis outbreaks included in this table were identified through a review of the published literature on Legionella as well as reports from past CDC-led investigations occurring during 1971–2000. Data are presented on the basis of a review and interpretation of available outbreak investigation reports and published articles.

† On the basis of epidemiologic, clinical laboratory, and environmental data (e.g., water quality data) provided to CDC. § The category of illness reported by ≥50% of ill respondents.

¶ Deaths are included in the overall case count.

** Deficiency classification for drinking water, water not intended for drinking (excluding recreational water), and water of unknown intent (see Table 2). †† No deaths were reported. §§ Some or all cases might have been Pontiac fever.

to water containing high levels of sodium hydroxide. No deaths 

were reported.

Multiple etiologies. Two outbreaks were associated with 

multiple etiologies and resulted in 270 cases of illness; one outbreak with viral and bacterial agents caused by norovirus genogroup I, Campylobacter, and Salmonella resulted in 229 cases of 

illness; one outbreak with parasitic and bacterial agents caused 

by Shigella sonnei, Giardia intestinalis, and Cryptosporidium

resulted in 41 cases of illness. No deaths were reported.

Unidentified etiologies. In four outbreaks an etiologic agent 

was not identified; however, one outbreak had a suspected 
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 49

FIGURE 3. Number of waterborne disease outbreaks associated with 

drinking water (n = 36), by state/jurisdiction — Waterborne Disease 

and Outbreak Surveillance System, United States, 2007–2008*

* These numbers are largely dependent on reporting and surveillance activities 

in individual states/jurisdictions, and do not necessarily indicate the true incidence of waterborne disease outbreaks.

≥4 

3 

2 

1 

0 

etiology of norovirus. These four outbreaks resulted in 1,756 

cases of AGI; no deaths were reported (Table 8).

Deficiencies 

A total of 37 deficiencies were cited in 36 drinking water–

associated outbreaks. Of these, 22 (59.5%) involved the 

source water, treatment facility, or distribution system (eight 

in community, nine in noncommunity, and five in individual 

systems), including 13 associated with untreated ground water 

(deficiency 2), seven associated with treatment deficiencies 

(deficiency 3), and two associated with distribution system 

deficiencies (deficiency 4) (Table 9). No outbreaks associated 

with untreated surface water (deficiency 1) or current treatment 

processes that are not expected to remove a chemical contaminant 

(deficiency 13) were reported during this surveillance period 

(Table 11). Of the 37 deficiencies, 13 (35.1%) occurred in 

public water systems at points not under the jurisdiction of a 

water utility, including 12 deficiencies associated with Legionella

(deficiency 5A) and one associated with a plumbing system 

deficiency (deficiency 6); two outbreaks had an unknown 

deficiency (deficiency 99A, B) (Table 9; Figure 6). No outbreaks 

reported during 2007–2008 were associated with deficiencies 

in treatment after the property line or meter; contamination 

of equipment using or distributing water; contamination 

during commercial bottling, shipping, hauling, or storage; or 

contamination at point of use (deficiencies 7–11) (Table 11).

Contamination of Water at/in the Water 

Source, Treatment Facility, or Distribution 

System

A total of 21 outbreaks were assigned a deficiency classification of 2–4 (untreated ground water, treatment, and distribution system deficiencies, respectively), including one outbreak 

that was assigned two deficiency classifications (treatment 

and distribution system deficiencies). No outbreaks with a 

deficiency classification of 1 (untreated surface water) or 13 

(current treatment processes not expected to remove a chemical 

contaminant) were reported (Table 11).

Etiology

Nine (42.9%) of these 21 outbreaks were associated with 

bacteria, five (23.8%) with viruses, three (14.3%) with unidentified etiologies, two (9.5%) with parasites, one with a chemical 

(4.8%), and one (4.8%) with multiple etiologies (both bacterial 

and viral agents). Cases in one outbreak with an unidentified 

etiology had an incubation period, symptoms, and duration 

of illness that were consistent with norovirus infection.

Water Quality Data

All but one of the 21 outbreaks with a deficiency classification 

of 2–4 (Tables 2–4) had current water quality data (e.g., 

laboratory data regarding the presence of coliform bacteria, 

pathogens, or chemical contaminants) or historic data (e.g., 

levels of disinfectants) available. 

Water Systems

Nine (42.9%) of the 21 outbreaks with deficiencies 2–4 

involved noncommunity water systems, five (23.8%) involved 

individual water systems, and seven (33.3%) involved 

community water systems (Table 9). Among the nine outbreaks 

involving noncommunity water systems, six (66.7%) were 

associated with untreated ground water, and three (33.3%) 

were associated with a treatment deficiency. All five outbreaks 

involving individual water systems were associated with 

untreated ground water. Among the seven outbreaks involving a 

community water system, eight deficiencies were assigned. Two 

(28.6%) outbreaks were associated with untreated ground water, 

three (42.9%) were associated with a treatment deficiency, one 

(14.3%) was associated with a distribution system deficiency, 

and one (14.3%) was associated with both a treatment deficiency 

and a distribution system deficiency (Table 9).

Water Sources

A total of 20 (95.2%) of the 21 outbreaks with deficiencies 2–4 

were associated with ground water sources (i.e., wells or springs); 

one (4.8%) outbreak was associated with surface water derived 

from a reservoir (Table 10). Among the 20 outbreaks and 21 
Surveillance Summaries

50 MMWR / September 23, 2011 / Vol. 60 / No. 12

TABLE 8. Number of waterborne disease outbreaks associated with drinking water (n=36), by etiology, type of water system, and water 

source* — Waterborne Disease and Outbreak Surveillance System, United States, 2007–2008

Type of water system

Total

Etiology

Public water systems† Individual water 

systems

Commercially bottled 

water systems

Ground Surface Other§ Ground Surface

Outbreaks Cases Outbreaks Cases Outbreaks Cases Outbreaks Cases Outbreaks Cases Outbreaks Cases

Bacterium 7 1,432 9 55 1 13 4 20 0 0 21 1,520

Campylobacter spp. 2 65 0 0 0 0 2 12 0 0 4 77

Escherichia coli O157 0 0 0 0 0 0 1 6 0 0 1 6

Legionella pneumophila 2 7 9 55 1 13 0 0 0 0 12 75

Providencia 1 55 0 0 0 0 0 0 0 0 1 55

Salmonella spp. 2 1,305 0 0 0 0 1 2 0 0 3 1,307

Virus 4 265 0 0 0 0 1 9 0 0 5 274

Hepatitis A virus 0 0 0 0 0 0 1 9 0 0 1 9

Norovirus 4 265 0 0 0 0 0 0 0 0 4 265

Parasite 2 81 1 82 0 0 0 0 0 0 3 163

Cyclospora cayetanensis 0 0 1 82 0 0 0 0 0 0 1 82

Giardia intestinalis 2 81 0 0 0 0 0 0 0 0 2 81

Chemical 1 145 0 0 0 0 0 0 0 0 1 145

Sodium hydroxide 1 145 0 0 0 0 0 0 0 0 1 145

Multiple¶ 1 229 1 41 0 0 0 0 0 0 2 270

Unidentified** 2 90 1 1,663 0 0 0 0 1 3 4 1,756

Total 17 2,242 12 1,841 1 13 5 29 1 3 36 4,128

* Only categories of water sources in which outbreaks occurred are presented in this table.

† Includes community and noncommunity water systems that have ≥15 service connections or serve an average of ≥25 residents for ≥60 days/year. A community water system serves 

year-round residents of a community, subdivision, or mobile home park. A noncommunity water system serves an institution, industry, camp, park, hotel, or business and can be nontransient or transient. Nontransient systems serve ≥25 of the same persons for >6 months of the year but not year-round (e.g., factories and schools), whereas transient systems provide 

water to places in which persons do not remain for long periods of time (e.g., restaurants, highway rest stations, and parks). Individual water systems are defined as small systems not 

owned or operated by a water utility that have <15 connections or serve <25 persons. Individual water systems are not under the jurisdiction of the EPA; however, there may be additional 

regulations or operational guidelines that fall under state jurisdiction, depending on the state and size of the water system.

§ Includes outbreaks with mixed or unknown water sources. One outbreak of legionellosis was associated with a mixed public water system.

¶ Defined as outbreaks in which more than one type of etiologic agent is identified in clinical specimens from affected persons, and each etiologic agent is found in ≥5% of positive clinical 

specimens (e.g., an outbreak with Giardia intestinalis [parasites] and Salmonella spp. [bacteria] with each agent identified in ≥5% of stool specimens). Two outbreaks occurred: one with 

229 cases was associated with Norovirus GI, Campylobacter spp., and Salmonella spp, and one with 41 cases was associated with Shigella sonnei, Giardia, and Cryptosporidium.

** For one outbreak with an unidentified etiology, norovirus was suspected on the basis of incubation period, symptoms, and duration of illness.

deficiencies related to ground water sources, 13 (65.0%) outbreaks 

were associated with consumption of untreated ground water. Well 

water sources were used by systems in nine of these outbreaks, 

and springs were water sources in four outbreaks. In one of the 

outbreaks associated with the use of untreated spring water, the 

spring was classified by the State as GWUDI and thus subject 

to EPA regulations for surface water (SWTR and amendments) 

(10–15). In several other outbreaks, evidence of contamination 

suggested that the well or spring was under the direct influence 

of surface water; however, information was not provided on the 

surveillance form about GWUDI testing and classification. 

Six (30.0%) ground water outbreaks were associated with 

treatment deficiencies, including inadequate disinfection or 

filtration, interruption of disinfection, and deficiencies in other 

treatment processes. Two outbreaks in systems using well water 

were associated with distribution system deficiencies. One large 

communitywide outbreak was associated with contamination of a 

distribution system storage tank, and a second outbreak had both 

distribution system and treatment deficiencies, including crossconnections and inadequate disinfection (Table 10; Appendix B).

Contamination of Water at Points Not 

Under the Jurisdiction of a Water Utility or 

at the Point of Use

A total of 13 outbreaks were given a deficiency classification 

of 5A (Legionella spp. in drinking water systems) or 6 

(plumbing system deficiency). Of these 13 outbreaks, 12 

(92.3%) were associated with Legionella spp., and one (7.7%) 

outbreak with multiple etiologies was associated with a 

plumbing system deficiency (Table 11). No outbreaks reported 

for 2007–2008 were associated with a deficiency classification 

of 7–11 (deficiencies in treatment after the property line or 

meter; contamination of equipment using or distributing 

water [e.g., drink-mix machines]; and contamination during 

commercial bottling; shipping, hauling, or storage; and at 

point of use) (Table 11).

Water Quality Data

Water quality data were available for all 13 outbreaks with 

a deficiency of 5A or 6. Legionella spp. were isolated from the 

implicated water source in all of the 12 legionellosis outbreaks.
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 51

FIGURE 4. Percentage of waterborne disease outbreaks asscociated with drinking water, by predominant illness and etiology — Waterborne 

Disease and Outbreak Surveillance System, United States, 2007–2008

ARI¶ 33.3%

Multiple§

 9.1%

All illness (n = 36)

AGI only (n = 22)

Bacteria 40.9%

Parasites 13.6%

Viruses 18.2%

Unidentied** 18.2%

Skin†

 2.8% Hep* 2.8%

AGI 61.1%

Abbreviations: AGI = acute gastrointestinal illness; ARI = acute respiratory illness; Hep = viral hepatitis; Skin = illnesses, conditions, or symptoms related to skin. 

* All hepatitis infections were attributed to hepatitis A virus.

† One outbreak was associated with exposure to water that contained elevated levels of sodium hydroxide.

§ One outbreak involved bacterial and viral agents. A second outbreak involved bacterial and parasitic agents.

¶ All acute respiratory illness was attributed to Legionella spp.

** Three outbreaks. Norovirus was suspected in one outbreak on the basis of incubation period, symptoms, and duration of illness.

Legionella in Drinking Water

A total of 12 outbreaks were related to multiplication of 

Legionella spp. in building plumbing systems. Five (41.7%) 

of the 12 drinking water–associated legionellosis outbreaks 

occurred in hospitals, three in nursing homes (25.0%), two in 

residential buildings (16.7%), one in a hotel (8.3%), and one 

in an assisted living facility (8.3%). The majority of cases of 

legionellosis were diagnosed by urinary antigen testing, which 

is specific for L. pneumophila serogroup 1 (36).

Deficiencies 6–11. One outbreak that had multiple etiologies (both bacterial and parasitic agents were identified) was 

associated with a plumbing system deficiency (deficiency 6).

Deficiency 99A–B. The deficiency involved in two (5.6%) 

of the 36 outbreaks could not be identified because the cause 

of water contamination was unknown. One of the outbreaks 

occurred in a rural community in Puerto Rico in which 

persons became ill with Cyclospora cayetanensis. Although an 

epidemiologic investigation implicated drinking water as the 

source of exposure in the outbreak, the point at which water 

was contaminated was not determined, and a deficiency could 

not be assigned. The second outbreak was associated with 

commercially bottled water and was assigned a deficiency of 

99B. A private residence used a five-gallon water dispenser 

to supply drinking water. Three persons who replaced an 

empty container and consumed water from the new container 

developed gastrointestinal symptoms. Water from the container 

was not tested, and no etiologic agent was identified. An FDA 

inspection of the water facility did not find any problems with 

the bottling process.

Waterborne Disease Outbreaks Associated 

with Water Not Intended for Drinking 

(WNID) and Water of Unknown 

Intent (WUI)

A total of 12 outbreaks were associated with either WNID 

(n = eight) or WUI (n = four) (Table 6). The 12 WNID/WUI 

outbreaks caused illness among ≥79 persons, resulting in four 

deaths, all of which were associated with legionellosis. Of the 

12 outbreaks, nine (75.0%) were categorized as ARI and three 

(25.0%) as AGI. Two (16.7%) of the 12 outbreaks were assigned 

a strength-of-evidence Class I ranking. No outbreaks were ranked 

as Class II, four (33.3%) were ranked as Class III, and six (50.0%) 

were ranked as Class IV.

Etiology

Nine (75.0%) of the 12 WNID/WUI outbreaks were 

attributed to L. pneumophila serogroup 1, with one of these 
Surveillance Summaries

52 MMWR / September 23, 2011 / Vol. 60 / No. 12

TABLE 10. Number and percentage of waterborne disease outbreaks associated with drinking water (n = 36), by type of deficiency (n = 37) and 

water source — Waterborne Disease and Outbreak Surveillance System, United States, 2007–2008*

Type of deficiency

Water source†

Ground water Surface water Mixed source Total

No. (%) No. (%) No. (%) No. (%)

2: Untreated ground water intended for drinking 13 (56.5) 0 (0) 0 (0) 13 (35.1)

3: Treatment deficiency 6 (26.1) 1 (7.7) 0 (0) 7 (18.9)

4: Distribution system deficiency, includes storage 2 (8.7) 0 (0) 0 (0) 2 (5.4)

5A: Legionella in drinking water system 2 (8.7) 9 (69.2) 1 (100.0) 12 (32.4)

6: Plumbing system deficiency 0 (0) 1 (7.7) 0 (0) 1 (2.7)

99A: Unknown/insufficient information 0 (0) 1 (7.7) 0 (0) 1 (2.7)

99B: Unknown/insufficient information (bottled water) 0 (0) 1 (7.7) 0 (0) 1 (2.7)

Total (%) 23 (100.0) 13 (100.0) 1 (100.0) 37 (100.0)

* Includes all deficiencies for drinking water–associated outbreaks reported for 2007–2008. Some outbreaks have multiple deficiencies that are tabulated separately. 

This table reports 37 deficiencies from 36 outbreaks.

† No water outbreaks associated with unknown or unidentified drinking water sources were reported for 2007–2008.

TABLE 9. Number and percentage of waterborne disease outbreaks associated with drinking water (n = 36), by type of deficiency (n = 37)* and 

type of water system — Waterborne Disease and Outbreak Surveillance System, United States, 2007–2008

Type of deficiency

Type of water system†§

Community Noncommunity Individual Bottled Total

No. (%) No. (%) No. (%) No. (%) No. (%)

2: Untreated ground water intended for drinking 2 (9.5) 6 (60) 5 (100.0) 0 (0) 13 (35.1)

3: Treatment deficiency 4 (19.0) 3 (30) 0 (0) 0 (0) 7 (18.9)

4: Distribution system deficiency, includes storage 2 (9.5) 0 (0) 0 (0) 0 (0) 2 (5.4)

5A: Legionella in drinking water system 11 (52.4) 1 (10) 0 (0) 0 (0) 12 (32.4)

6: Plumbing system deficiency 1 (4.8) 0 (0) 0 (0) 0 (0) 1 (2.7)

99A: Unknown/insufficient information 1 (4.8) 0 (0) 0 (0) 0 (0) 1 (2.7)

99B: Unknown/insufficient information (bottled water) 0 (0) 0 (0) 0 (0) 1 (100.0) 1 (2.7)

Total (%) 21 (100.0) 10 (100.0) 5 (100.0) 1 (100.0) 37 (100.0)

* Includes all deficiencies for drinking water–associated outbreaks reported for 2007–2008. Some outbreaks have multiple deficiencies that are tabulated separately. 

This table reports 37 deficiencies from 36 outbreaks.

† Community and noncommunity water systems are public water systems that have ≥15 service connections or serve an average of ≥25 residents for ≥60 days/year. A 

community water system serves year-round residents of a community, subdivision, or mobile home park. A noncommunity water system serves an institution, industry, 

camp, park, hotel, or business and can be nontransient or transient. Nontransient systems serve ≥25 of the same persons for >6 months of the year but not year-round 

(e.g., factories and schools) whereas transient systems provide water to places in which persons do not remain for long periods of time (e.g., restaurants, highway rest 

stations, and parks). Individual water systems are small systems not owned or operated by a water utility that have <15 connections or serve <25 persons. 

§ No outbreaks associated with mixed drinking water systems were reported for 2007–2008.

outbreaks also being attributed to L. pneumophila serogroup 7; 

the nine legionellosis outbreaks affected 46 persons and resulted 

in four deaths. One (8.3%) of the WNID/WUI outbreaks was 

attributed to G. intestinalis. Of the two outbreaks that did not 

have an identified etiology, one outbreak was suspected to be 

caused by norovirus (Table 6).

Deficiencies 

Each of the 12 WNID/WUI outbreaks was assigned one 

deficiency: six (50.0%) WNID outbreaks involved Legionella

spp. in the water system (deficiency 5B), three (25.0%) 

WUI outbreaks involved Legionella spp. (deficiency 5C), 

two (16.7%) WNID outbreaks of AGI involved unidentified 

etiologies (deficiency 12), and one (8.3%) WUI outbreak 

involved contamination of an unknown water source 

(deficiency 99D) (Tables 2 and 6).

Previously Unreported Outbreaks

A total of 70 previously unreported outbreaks that occurred 

from 1973–2002 were added to WBDOSS during 2010. One 

previously unreported WNID (deficiency 12) outbreak of 

Mycobacterium mageritense occurred in 2002 (Table 7). Two 

patients were diagnosed with M. mageritense infections on their 

feet following separate visits to a nail salon. Environmental 

samples from the nail salon isolated M. mageritense in foot 

bath, drain, and hand sink samples. M. goodii and M. smegma

also were isolated in foot bath samples (37).

A total of 69 legionellosis outbreaks that occurred during 

1973–2000 were added to the surveillance system in 2010 

(Table 7). These data were abstracted from Epidemic 

Intelligence Service outbreak investigation reports and peerreviewed publications (38–75). Seventeen outbreaks were 

associated with drinking water, 26 with WNID, and 26 with 
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 53

FIGURE 5. Percentage of waterborne disease outbreaks associated with drinking water, by etiology, water system, and water source — Waterborne 

Disease and Outbreak Surveillance System, United States, 2007–2008*

* Not limited to deficiencies 1–4, and therefore figure is not comparable to those in previous summaries.

† Percentages do not add up to 100% due to rounding. 

§ One outbreak involved bacterial and viral agents. A second involved bacterial and parasitic agents.

¶ Other than Legionella spp.

Community 55.6% 

Etiology (n = 36)

Chemical 2.8% Bottled 2.8%

Individual†

 13.9% 

Water system (n = 36)†

Water source (n = 36)†

Parasites 8.3%

Multiple§

 5.6%

Viruses 13.9%

Unidentied 11.1%

Bacteria¶

25.0%

Legionella spp. 33.3%

Mixed water 2.8%

Surface water 36.1% Ground water 61.1% 

Noncommunity 27.8% 

WUI (Figure 7). Among these 69 outbreaks, 1,520 cases and 

237 deaths were reported from 27 states.

Surveillance Reports Not Classified as 

Waterborne Disease Outbreaks

Two outbreak reports about occupational exposures did not 

meet the criteria for inclusion in the surveillance system because 

either water did not appear to be the source of exposure or evidence 

was insufficient. In one event, five persons were injured at a water 

treatment plant; chlorine gas was released following an accidental 

mixing of sodium hypochlorite and hydrofluorosilicic acid. The 

second event involved two housecleaners who poured ammonia 

into a toilet suspected to have contained water with high concentrations of chlorine. One person later called a local poison control 

center (PCC) to report eye pain and respiratory irritation; however, 

insufficient epidemiologic and water quality data were available to 

warrant inclusion of this report as a waterborne disease outbreak. 

Six additional single case reports that were associated with known 

or suspected chemical exposures occurring primarily at the point of 

water use also were excluded because single cases do not meet the case 

definition for an outbreak. These single cases were identified by one 

state that conducted an independent review of supplemental data 

sources for occupational diseases and chemical poisonings, including 

PCC inquiries and reports submitted to the Hazardous Substances 

Emergency Events Surveillance (HSEES) system. Although single 

cases are not classified as outbreaks, the case reports highlight the 

potential to detect waterborne illness and injury at a state level 

through collaboration with other existing surveillance systems.
Surveillance Summaries

54 MMWR / September 23, 2011 / Vol. 60 / No. 12

TABLE 11. Waterborne disease outbreaks associated with drinking water (n = 36), by type of deficiency (n = 37)* — Waterborne Disease and 

Outbreak Surveillance System, United States, 2007–2008

Contamination of water at/in the water source, treatment facility, or distribution system† 22

1: Untreated surface water intended for drinking 0

2: Untreated ground water intended for drinking 13

3: Treatment deficiency (e.g., temporary interruption of disinfection, chronically inadequate disinfection, or inadequate or no filtration) 7

4: Distribution system deficiency, including storage (e.g., cross-connection, backflow, contamination of water mains during construction or 

repair)

2

13: Current treatment processes not expected to remove a chemical contaminant (e.g., pesticide contamination of ground water treated with 

disinfection only)

A: Surface water 0

B: Ground water 0

Contamination of water at points not under the jurisdiction of a water utility or at the point of use§ 13

5: Legionella spp. in water system

A: Water intended for drinking 12

6: Plumbing system deficiency after the water meter or property line (e.g., cross-connection, backflow, or corrosion products) 1

7: Deficiency in building/home-specific water treatment after the water meter or property line 0

8: Deficiency or contamination of equipment using or distributing water (e.g., drink-mix machines) 0

9: Contamination or treatment deficiency during commercial bottling 0

10: Contamination during shipping, hauling, or storage

A: Water intended for drinking – tap water 0

B: Water intended for drinking – commercially bottled water 0

11: Contamination at point of use

A: Tap 0

B: Hose 0

C: Commercially bottled water 0

D: Container, bottle, or pitcher 0

E: Unknown 0

Unknown/Insufficient Information 2

99: Unknown/Insufficient information

A: Water intended for drinking – tap water 1

B: Water intended for drinking – commercially bottled water 1

Total 37

* More than one deficiency might have been identified during the investigation of a single waterborne disease outbreak.

† For a community water system, the distribution system refers to the pipes and storage infrastructure under the jurisdiction of the water utility prior to the water 

meter or property line (if the system is not metered). For noncommunity and nonpublic water systems, the distribution system refers to the pipes and storage infrastructure prior to entry into a building or house.

§ Contamination of drinking water and deficiencies occurring in plumbing and pipes that are not part of the distribution system as previously defined. For community 

systems, this means after the water meter or outside the jurisdiction of a water utility, and for noncommunity and nonpublic systems, this means within the building 

or house (e.g., in a service line leading to a house or building, in the plumbing inside a house or building, during shipping or hauling, during storage other than in 

the distribution system, or at point of use).

Discussion

The outbreaks reported during this surveillance period 

highlight several important public health challenges associated 

with drinking water in the United States. The large proportion 

of outbreaks associated with untreated or inadequately treated 

ground water in particular indicate that additional efforts are 

needed to monitor and protect ground water sources from 

contamination and to ensure that adequate, continuous 

treatment is provided when it is needed. Legionella continues 

to be the most frequently reported etiology among drinking 

water–associated outbreaks and was also the predominant 

etiology among WNID and WUI outbreaks. However, 

more than half of the legionellosis outbreaks reported during 

2007–2008 were from one state, demonstrating the substantial 

variance in outbreak detection, investigation and reporting 

across states. All deaths except one reported during this surveillance 

period were associated with Legionella, underscoring the need 

for improved methods of elimination and control of Legionella, 

particularly in settings with vulnerable populations. In addition, 

a large communitywide outbreak associated with contamination 

of a storage tank underscores the importance of protecting and 

maintaining drinking water distribution system infrastructure. This 

report also includes the addition of data concerning 69 legionellosis 

outbreaks, providing a more complete representation of legionellosis 

outbreaks in the United States before 2001.
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 55

FIGURE 6. Percentage of waterborne disease outbreaks associated with drinking water, by deficiency* and water system† — Waterborne 

Disease and Outbreak Surveillance System, United States, 2007–2008

* There were 36 waterborne disease outbreaks but 37 deficiencies. See Table 2 for a list of all deficiencies. 

† For deficiencies 2, 3, and 5A only. See Table 9 for a summary of all 37 outbreak deficiencies by water system.

§ Percentages do not add up to 100.0% due to rounding.

¶ Deficiencies 99A and 99B.

All deciencies (n = 37)§

Untreated ground water 35.1% 

Plumbing system 2.7% 

Treatment deciency 18.9%

Distribution system 5.4% 

 

Unknown 5.4%¶

Noncommunity

46.2% 

Individual 38.5%

Community 15.4% 

Untreated

ground water

(n = 13)

Noncommunity 

42.9% 

Community 57.1% 

Treatment

deciency

(n = 7)

Community 91.7% 

Noncommunity 8.3%

Legionella spp. in water 

system 32.4% 

Legionella spp.

(n = 12)

Outbreaks Associated with Drinking Water

Illness and Etiology 

During 2007–2008, AGI was the dominant illness type 

(associated with 61% of drinking water–associated outbreaks) 

in contrast with the prior surveillance period (2005–2006) 

in which half of drinking water–associated outbreaks were 

associated with ARI (78). Of the 36 outbreaks, 24 (67%) 

were caused by bacteria, five (14%) by viruses, three (8%) by 

parasites, one (3%) by a chemical, and two (6%) by multiple 

etiologies. Four (11%) outbreaks had unidentified etiologies.

Bacteria

Since its addition to WBDOSS in 2001, Legionella has 

been the single most commonly reported pathogen identified 

in drinking water–associated outbreaks. During 2007–2008, 
Surveillance Summaries

56 MMWR / September 23, 2011 / Vol. 60 / No. 12

FIGURE 7. Number of waterborne disease outbreaks associated with drinking water, water not intended for drinking (WNID) (excluding recreational water), and water use of unknown intent (WUI), by year and etiology — Waterborne Disease and Outbreak Surveillance System, 

United States, 1971–2008 

* Since 2001, legionellosis outbreaks associated with drinking water, WNID, and WUI have been reported to WBDOSS by public health jurisdictions, and Legionella is 

presented as its own bacterial etiologic category. Outbreaks that occurred before 2001 were identified through a literature review of journal publications and CDCled investigation reports.

† Includes all bacteria except Legionella. No. of outbreaks

60

50

40

30

20

10

0

60

50

40

30

20

10

0

No. of outbreaks

1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007

1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007

Multiple

Legionella spp.

Unidentied

Chemical

Viral

Bacterial†

Parasitic

Multiple

Legionella spp.

Unidentied

Chemical

Viral

Bacterial†

Parasitic

Before the addition of pre-2001 legionellosis outbreaks (n = 867)*

After the addition of pre-2001 legionellosis outbreaks (n = 936)*

Year

Year

a total of 12 (33.3%) of the 36 reported drinking water–

associated outbreaks involved Legionella spp.; half of these 

outbreaks were reported by one state (New York). Unlike other 

waterborne bacterial outbreaks that also might be foodborne, 

zoonotic, or spread person-to-person, legionellosis outbreaks 

are almost exclusively associated with exposure to colonized 

water; therefore, Legionella spp. might be more likely than 

other waterborne pathogens to be associated successfully with 

a water source during outbreak investigations. All legionellosis 

outbreaks reported herein occurred as a result of colonization 

of premise plumbing and pipes (i.e., infrastructure that neither 

is under the jurisdiction of a water utility nor is regulated by 

EPA). EPA has no contaminant level specific to Legionella

spp. but believes that if parasites and viruses are removed 

or inactivated according to the treatment techniques in the 

SWTR and amendments, Legionella will be controlled at 
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 57

the water source (10). However, this does not ensure control 

within the water distribution system. Because Legionella is the 

predominant drinking water–related pathogen in WBDOSS, 

increased attention is needed to understand its ecology, the 

characteristics contributing to its pathogenicity, the need 

for improved diagnostics (e.g., molecular tests) and national 

laboratory testing capacity, and the interventions that are most 

effective to prevent ongoing disease transmission (6,76–78). 

EPA’s recent inclusion of Legionella in CCL3 might lead to 

additional research that improves risk-reduction interventions.

During the 2007–2008 surveillance period, nine drinking 

water–associated outbreaks involved only bacteria (excluding 

Legionella spp.) compared with two during 2005–2006, five 

during 2003–2004, and three during 2001–2002. The nine 

bacterial outbreaks involved chlorine-sensitive pathogens (e.g., 

Campylobacter, Salmonella, Providencia, and E. coli), including 

five outbreaks associated with ground water sources in public 

water systems, highlighting the importance of protecting 

ground water sources and providing adequate treatment and 

filtration where needed. The ongoing occurrence of bacterial 

outbreaks, despite available and efficacious treatment practices, 

underscores the need for source water protection and adequate 

disinfection when needed for ground water systems (79) in 

community, noncommunity, and individual water systems. 

A large outbreak of Salmonella Typhimurium occurred in a 

community water system using untreated ground water. The 

outbreak was associated with contamination in a storage reservoir 

and emphasizes the importance of protecting water quality in the 

distribution system, particularly in systems that use untreated 

ground water sources or have other system vulnerabilities. 

The outbreak of Providencia is the first documented drinking 

water outbreak of this bacterium known to be reported in the 

United States. An epidemiologic investigation indicated that 55 

persons developed gastroenteritis associated with drinking water 

from a community water system served by a well. Six of nine 

stool samples from ill persons tested positive for Providencia and 

negative for other bacteria (Campylobacter, Shigella, and E. coli), 

norovirus, and parasites. Previously thought to be nonpathogenic 

(80), Providencia was identified as the cause of a large foodborne 

outbreak in Japan in 1996 (81) and is thought to be a potential 

source of travelers’ diarrhea (82). Whether Providencia is a 

true human pathogen or an indicator of exposure to fecally 

contaminated water or food remains unclear (83).

Viruses

Five outbreaks involving only viruses were reported during 

2007–2008, four involving norovirus and one involving 

hepatitis A virus. All five outbreaks involved contaminated 

ground water that was either untreated or treated improperly 

(inadequate or interrupted chlorination as the only treatment 

provided). These types of events are anticipated to decrease as 

the GWR is fully implemented. However, one of these viral 

outbreaks occurred in an individual water system, and the 

GWR does not apply to private systems.

Parasites

Parasites were identified in three outbreaks reported during 

2007–2008. An outbreak linked epidemiologically to a drinking 

water system in Puerto Rico was caused by C. cayetanensis

(84), a rarely reported source of drinking water outbreaks in 

developed countries. This is the first known reported outbreak 

of cyclosporiasis associated with drinking water in the United 

States or associated territories since 1990. A previous outbreak 

of cyclosporiasis that occurred in Chicago in 1990 was suspected 

to be associated with water exposure (85). In the 2008 outbreak, 

residents reported recent interruptions in the community water 

supply and changes in water quality. A pumping station was 

determined to be damaged, and a water tanker had been used to 

haul treated water from a neighboring system to fill a water tank 

during the same time period. In addition to the C. cayetanensis

outbreak, two outbreaks of giardiasis occurred in public water 

systems using ground water; one outbreak was associated with 

the improper installation of a filter (86) and the other with a 

well that was possibly GWUDI (87).

Chemicals

Only one outbreak during 2007–2008 involved a chemical 

exposure. The outbreak occurred within a public water system 

after a sodium hydroxide overfeed occurred at the watertreatment facility, raising the pH level of the drinking water 

supply and injuring an estimated 145 persons. Although a 

mechanism was in place to monitor pH levels at the treatment 

plant, the system was not designed to provide automatic 

notification of offsite staff regarding the problem. The response 

effort required collaboration among local and state agencies 

to flush the contaminated water from the distribution system, 

provide guidance to the community, and make safe drinking 

water available to local residents. This event highlights the need 

for appropriate remote monitoring systems in water treatment 

facilities when staff members are not onsite and the value for 

public water systems of developing comprehensive incident 

response plans with involvement from state and local agencies.

Multiple Etiologies and Unidentified Etiologies

Two outbreaks involving multiple etiologies occurred during 

2007–2008; one outbreak was caused by norovirus genogroup 

I, Campylobacter spp. and Salmonella spp., and the other was 

caused by S. sonnei, G. intestinalis, and Cryptosporidium. The 

occurrence of multiple etiologic agent outbreaks emphasizes 

the importance of considering more than one etiology 
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58 MMWR / September 23, 2011 / Vol. 60 / No. 12

in outbreak investigations, collecting appropriate clinical 

and environmental specimens, and requesting appropriate 

diagnostic testing for each agent type. One outbreak was 

associated with sewage contamination of a well, underscoring 

the importance of proper waste management and water system 

protection, and the other outbreak was associated with a 

plumbing system and cross-connection deficiency. A dinner 

cruise boat connected to a municipal water supply from a dock 

had a cross-connection that might have allowed backsiphonage 

of lake water contaminated with sewage into the boat’s potable 

water supply (88). The outbreak occurred after a period of 

heavy rainfall and flooding that resulted in the release of a large 

volume of storm water containing highly diluted sewage into 

the lake in which the boats were located.

The etiologies of four outbreaks could not be identified, 

although norovirus was suspected in one outbreak on the basis of 

symptoms, incubation period, and duration of illness. These four 

outbreaks represent 11.1% of the 36 drinking water–associated 

outbreaks reported during 2007–2008, which is among the lowest 

proportion of outbreaks caused by an unidentified etiology since 

the beginning of the surveillance system in 1971. This continues 

a decreasing trend in the proportion of outbreaks caused by an 

unidentified etiology (6).

The identification of etiologic agents depends on the ability of 

investigators to recognize the outbreak in a timely manner and for 

appropriate clinical and environmental samples to be collected and 

analyzed for the organism, chemical, or toxin of interest. WBDOSS 

data suggest that these capabilities are improving. During 1971–

1996, the etiologic agent was unidentified in 48% (312/645) of 

drinking water–associated outbreaks whereas during 1997–2008, 

the etiologic agent was unidentified in 23% (40/174) of outbreaks. 

This decrease likely reflects improved diagnostic capabilities of 

laboratories and improvements in outbreak investigations, resulting 

in more rapid and appropriate specimen collection. Reasons for 

improved etiologic attribution might also include increased testing 

and testing capabilities for viral agents in clinical specimens and 

water samples and improved water sampling and testing methods.

Deficiencies

Outbreaks associated with untreated or inadequately treated 

source water and distribution system contamination are 

assigned deficiencies 1–4 and 13. EPA regulations associated 

with the Safe Drinking Water Act are applicable to public 

water systems from the water source up to the water meter, the 

property line, or entry into the building or house, and these 

deficiencies are important in assessing regulatory strategies. 

Although individual water systems are not regulated by EPA, 

the same problems might affect these systems including 

contamination of pipes or storage infrastructure.

During 2007–2008, 58.3% of all drinking water–related 

outbreaks (n = 21) and 59.5% of deficiencies (n = 22) involved 

deficiencies 2–4. One outbreak was associated with more than 

one deficiency (deficiencies 3 and 4). Deficiencies 1 and 13 were 

not implicated in any outbreak during the 2007–2008 surveillance period. No outbreaks in public water systems have been 

associated with the use of untreated surface water since 1990.

Untreated Ground Water

Of the 36 drinking water–associated outbreaks reported 

during 2007–2008, a total of 13 (36.1%) were associated with 

contaminated ground water, indicating that contamination of 

ground water remains a public health problem. Eight (61.5%) of 

these 13 outbreaks occurred in public water systems, including six 

in noncommunity water systems and two in community water 

systems. Factors potentially contributing to these outbreaks included 

improperly constructed or sited wells, improperly maintained or 

placed septic systems, contamination by wild or domestic animals, 

periods of heavy rainfall, and contamination of wells through 

limestone or fissured rock. Included in the outbreaks associated 

with improper well construction was a large outbreak of giardiasis 

in which 35 persons in New Hampshire reported becoming ill after 

their community drinking water system’s well became contaminated 

with G. intestinalis. The well was approximately 40 feet from a 

brook that had evidence of beaver habitation and was likely under 

the influence of surface water (87). New Hampshire Department 

of Environmental Services regulations require that wells be placed 

≥50 feet from sources of surface water (89), but the well owner had 

not sought a permit before construction. Following an investigation, 

the well was disconnected from the water system.

In another outbreak, five persons who drank from a 

noncommunity water system using untreated, contaminated 

spring water became ill with salmonellosis (Salmonella serotype 

I 4,5,12:i:-) (90). Although this serotype has been identified in 

foodborne disease outbreaks reported to CDC, this is the first 

report to WBDOSS of an outbreak with this serotype. Several 

factors potentially contributed to the outbreak, including the 

poor protection of the spring from contamination (including 

by runoff and wildlife) and lack of disinfection. Pulsed-field 

gel electrophoresis patterns for a patient’s stool and tap water 

collected from the patient’s residence and another distribution 

system sample were identical. In addition, investigators noted 

that the geology of the area (porous limestone [karst]) might 

have increased the likelihood that the spring was under the 

influence of surface water (90). The spring water tested positive 

for E. coli (it was not tested for Salmonella), which suggested 

that the spring was the source of the contamination.

Outbreaks can occur even when wells and septic systems are 

built according to existing codes. An extensive investigation 

of a multiple pathogen (norovirus, Campylobacter, and 
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MMWR / September 23, 2011 / Vol. 60 / No. 12 59

Salmonella) outbreak that sickened 229 persons in Wisconsin 

revealed that these pathogens were likely introduced into a 

noncommunity water system through fissures in the underlying 

dolomite rock that made the system particularly vulnerable to 

interconnections between the septic system and the well. The 

investigators noted that the septic system and well were built in 

accordance with state codes, so it is unlikely that the source of 

the outbreak would have been identified had the investigators 

not conducted a rigorous multifaceted investigation (including 

epidemiologic, laboratory, and hydrologic examinations). As a 

result of the outbreak, the well water now is being treated with 

a combination of UV and chlorination (91). 

Eight of thirteen outbreaks associated with contaminated 

ground water during 2007–2008 occurred in noncommunity 

or community water systems that are subject to EPA’s GWR. 

In 2006, the GWR was promulgated for all public systems 

that use ground water as a source of drinking water. The 

GWR establishes a risk-based approach to target ground 

water systems that are vulnerable to fecal contamination and 

comprises four major components: 1) sanitary surveys, 2) 

source water monitoring to test for the presence of indicators 

of fecal contamination in the ground water source, 3) 

corrective action, and 4) compliance monitoring to ensure 

that the treatment technology installed to treat drinking water 

reliably achieves ≥99.99% (4 log) inactivation and/or removal 

of viruses (92). States are required to perform initial sanitary 

surveys on the majority of community water systems by 2012; 

the remainder of the community water systems (the best 

performing systems) and all noncommunity drinking water 

systems must be surveyed by the end of 2014 (92). Operators 

of ground water systems that are identified as being at risk for 

fecal contamination must take corrective action to reduce the 

potential for illness from exposure to microbial pathogens.

The remaining five outbreaks associated with untreated 

ground water deficiencies occurred in individual water systems. 

Potential contributing factors in these outbreaks included septic 

system problems, contamination of wells by domestic or wild 

animals, flooding and heavy rain, and cracks in the well casing. 

These systems are not subject to the GWR. Approximately 15 

million households in the United States have private wells (33). 

To safeguard the quality of well water, homeowners should 

seek information on protective measures and implement 

recommended operation and maintenance guidelines for 

private well usage. Recommendations for protecting private 

wells have been published previously (93–95).

Ground Water Under the Direct Influence of Surface 

Water

In several outbreaks associated with the use of untreated 

ground water or treatment deficiencies, the water source 

appeared to be GWUDI (e.g., microscopic particulate analysis 

testing identified algae and diatoms, the well was located <50 

feet from a surface water source, or the spring was not protected 

from surface runoff). If a drinking water source is determined 

to be GWUDI, it is subject to SWTR requirements and 

amendments and may be classified as a surface water source by 

the state. Systems using GWUDI must disinfect their water and 

either provide filtration or meet criteria for avoiding filtration. 

EPA has developed a consensus method for assessing whether 

ground water is under the direct influence of surface water (98). 

However, WBDOSS currently does not collect information 

on whether ground water systems are under the influence of 

surface water, nor do states have standard case definitions and 

criteria for categorizing a system as GWUDI. Establishing a 

more standardized system for capturing data on GWUDI will 

be particularly important in better understanding the potential 

contributing factors and risks between ground water systems 

and those that have unique challenges associated with surface 

water influence.

Treatment Deficiencies

Seven outbreaks (19.4%) were associated with treatment 

deficiencies (deficiency 3) during 2007–2008. Six (85.7%) 

of these treatment deficiencies were associated with failures 

to treat contaminated ground water adequately. Multiple 

treatment-related factors potentially contributed to these 

outbreaks, including temporary interruption of disinfection, 

chronically inadequate disinfection, lack of disinfection, and 

inadequate filtration. When these deficiencies are considered 

with deficiency 2, contaminated ground water was determined 

to be the single largest contributing factor in outbreaks 

associated with drinking water in 2007–2008, underscoring 

the need for source water protection and adequate disinfection 

of ground water sources.

Two outbreaks were reported in public systems that provided filtration and disinfection, but both systems experienced 

treatment failures preceding the outbreak. One outbreak was 

reported in a noncommunity system using contaminated 

spring water and was responsible for 46 cases of giardiasis (86). 

This spring was classified as GWUDI and therefore subject 

to EPA’s SWTR and amendments. Although the system had 

installed a slow sand filter and chlorinator, inadequate time was 

allowed for formation of a schmutzdecke† biologic layer on the 

surface of the filter, which is important for effective treatment 

performance of slow sand filters. When a slow sand filter is first 

constructed, the bed of clean sand cannot, strictly speaking, 

be called a filter because the vital living organisms on which 

†Gelatinous biologic layer on the surface of a slow sand filter, consisting of a 

complex microbial community (including bacteria, fungi, protozoa, and rotifera 

and other aquatic organisms) and organic particulate matter.
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60 MMWR / September 23, 2011 / Vol. 60 / No. 12

effective filtration depends are not yet present. Building up the 

biologic content of a new filter is a slow but necessary process. 

Until the filter is sufficiently “ripened,” the filter should not be 

put into service. Water samples collected 1 week after the filter 

was put into operation indicated considerably higher levels of 

total coliforms and turbidity in filtered water samples than in 

the spring water samples. On the basis of these samples, this 

system did not meet the treatment requirements of the TCR 

and SWTR and amendments (97). However, 6 weeks after 

operation of the filter, a schmutzdecke layer formed, and water 

samples met requirements.

The second outbreak was reported in a community water 

system that used surface water from a reservoir. The water was 

treated conventionally with coagulation, settling, filtration, and 

disinfection; however, operator error and the interruption of 

chlorination for 1–2 hours allowed partially disinfected water 

to enter the distribution system directly through a bypass 

pipe. During this time, the system did not meet the treatment 

requirements for disinfection as defined by the SWTR and 

amendments. The system was recycling filter backwash water 

as required by EPA (98); however, before the outbreak, the 

backwash water erroneously bypassed the recovery basin and 

ozonation before filtration, thereby adversely affecting filter 

performance. This investigation combined monitoring data 

with modeling of the distribution system to identify potential 

system vulnerabilities and sources of contamination. In conjunction with a boil water order notice, the health department 

asked that residents in the affected area report symptoms of 

gastrointestinal illness. A cross-sectional, randomly selected 

household survey was conducted in the affected area to assess 

potential associations between tap water consumption and 

gastrointestinal illness. The study identified increased risk for 

gastrointestinal illness among households that consumed tap 

water during the period before the boil water notice and also 

identified substantial dose-response effects and statistically 

significant trends, with higher rates of illness associated with 

increased tap water consumption. An extensive list of shortand long-term recommendations was provided through an 

independent investigation to address system deficiencies and 

operational issues at the treatment plant.

Distribution System Deficiencies

During 2007–2008, two drinking water–related outbreaks 

involving distribution system deficiencies occurred, including a large communitywide outbreak of salmonellosis that 

required local, regional, state, and federal emergency response. 

An estimated 1,300 persons became ill; 122 infections were 

laboratory-confirmed, and one person died. The likely source 

of the outbreak was animal contamination of a storage tank that 

had numerous cracks and entry points (99). The post outbreak 

inspection of the water system also identified >100 possible 

cross-connections in the distribution system, although these 

were not thought to contribute directly to the outbreak (99). 

This was one of the largest outbreaks in a community water 

system in recent years and highlights the critical importance of 

robust inspection of storage facilities, identification of potential 

cross-connections during required sanitary surveys, and the 

staffing and resources for adequate follow-up to ensure that 

deficiencies have been addressed. The other outbreak associated 

with a distribution system deficiency was related to a crossconnection between a well serving a residential community 

and a noncommunity water system. Additional deficiencies 

and contributing factors included inadequate disinfection and 

filtration along with a sewage lift that was out of service; this 

was the only outbreak reported during this surveillance period 

with two deficiencies noted.

Cross-connections and backsiphonage represented the largest underlying contributing factors among distribution system 

deficiencies identified in drinking water outbreaks reported 

during 1971–2006 (6), indicating that greater attention should 

be focused on cross-connection and backflow prevention 

and on maintaining the integrity of the distribution system. 

Drinking water quality within the distribution systems of 

public water supplies is assessed by monitoring requirements 

under EPA’s Total Coliform Rule. The 2010 revisions to the 

TCR include a focus on research and information collection 

needs in the distribution system to protect public health 

(100). Seven high-priority areas needing additional focus 

(cross-connections, storage, water main breaks and pressure 

transients, intrusion, nitrification, contaminant accumulation, and biofilms) were identified by a steering committee 

selected by EPA and the Water Research Foundation as part 

of the Research and Information Collection Partnership (101).

Legionella

Legionellosis includes two clinically distinct syndromes: 

LD, characterized by severe pneumonia, and PF, a febrile illness that includes cough and does not progress to pneumonia. 

Legionellosis outbreaks accounted for 33.3% of all drinking 

water–associated outbreaks reported during 2007–2008 and 

91.7% of all deficiencies occurring outside the jurisdiction 

of regulations or water utility management, indicating that 

legionellosis is a serious public health issue. Approximately 

8,000–18,000 cases of LD occur each year in the United 

States (102), and incidence appears to be increasing (103). 

Cases typically manifest as LD rather than PF, but regardless 

of the syndrome, the source of legionellosis outbreaks typically 

share common features (e.g., warm stagnant water, inadequate 

biocide concentrations, and aerosolization, which provides the 

mechanism for inhalation).
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The outbreaks of legionellosis reported during this 

surveillance period highlight the challenges related to its 

detection and prevention. Surveillance for legionellosis is 

passive, and the approach to surveillance, outbreak response, 

and reporting varies substantively by state. For example, six 

of 12 LD outbreaks associated with drinking water during 

this surveillance period were reported from one state, 

emphasizing the variance in outbreak detection, investigation, 

and reporting across states. However, there appear to be true 

differences in disease incidence across the United States, with 

the highest rates of disease reported in the northeast (103). 

LD is underdiagnosed because the majority of patients with 

community-acquired pneumonia are treated empirically with 

broad-spectrum antibiotics (104). However, because Legionella

spp. are not transmitted from person-to-person and are always 

acquired from an environmental source, even a single case 

of LD implies the presence of a contaminated water source 

to which others can be exposed. Certain host factors (e.g., 

underlying lung disease and immunodeficiencies) influence 

the development and severity of legionellosis. As a result of 

underdiagnosis and underreporting, identification of two or 

more cases of LD in association with a potential source is 

adequate justification for conducting an investigation.

During 2007–2008, nine (75.0%) of 12 legionellosis 

outbreaks associated with drinking water occurred in healthcare or long-term–care settings, demonstrating the propensity 

for Legionella spp. to colonize potable water systems and 

underscoring the importance of maintaining a high index of 

suspicion for legionellosis in health-care settings and in other 

settings with vulnerable populations. Although Legionella

spp. can live in a free state, they often colonize biofilms and 

free-living protozoa frequently found inside large, complex 

hospital plumbing systems (79,105). This protects Legionella

from biocides and temperature extremes and allows the bacteria 

to amplify to levels sufficient to be transmitted and cause 

disease. Persons in hospitals or elderly living facilities are at 

increased risk as a result of advanced age and a high prevalence 

of underlying chronic medical conditions.

An outbreak of legionellosis in a health-care setting should 

prompt both epidemiologic and environmental investigations. 

Additional cases might point to water exposures that contributed 

to the outbreak. Environmental sampling of the potable water 

system and devices that aerosolize water (e.g., cooling towers) 

can confirm the source of the outbreak and lead to targeted 

interventions that prevent additional cases. Each health-care 

facility should develop a plan for legionellosis prevention to address 

conditions that support Legionella growth in the potable water 

supply. Guidelines for reducing the risk for legionellosis associated 

with building water systems are available (106).

Waterborne Disease Outbreaks Associated 

with Water Not Intended for Drinking and 

Water of Unknown Intent

During 2007–2008, a total of 12 outbreaks occurred that 

were associated with WNID or WUI. Nine (75.0%) of these 

outbreaks were associated with Legionella spp., three (33.3%) 

with cooling towers, one (11.1%) with an ornamental 

fountain, and one (11.1%) with recycled water from a vehicle 

washing station. Four (44.4%) legionellosis outbreaks had 

an unidentified source of exposure. Of the nine legionellosis 

outbreaks associated with WNID/WUI, two occurred in longterm–care or assisted living facilities, two occurred at hospitals, 

and one occurred at an apartment complex for seniors. As noted 

previously, it is important to address Legionella colonization 

in and near buildings that house high-risk populations. 

Although potable water systems within buildings are implicated 

frequently in health-care–associated outbreaks, other sources of 

aerosolized water on site and in the surrounding community 

(e.g., cooling towers and decorative fountains) also should 

be considered. Aerosols containing Legionella can travel great 

distances; an investigation of an outbreak among residents of 

a long-term–care facility implicated a cooling tower that was 

0.4 km from the facility (107).

Previously Unreported Outbreaks

This report incorporates 69 outbreaks of legionellosis from 

27 states during 1973–2000 that were added to the surveillance 

system in 2010 and one previously unreported outbreak of 

M. mageritense from 2002. Legionellosis outbreaks associated 

with drinking water, WNID, and WUI were not included in 

WBDOSS before 2001. During 2009–2010, CDC conducted 

a comprehensive search of the peer-reviewed scientific literature 

and CDC outbreak investigation reports to capture the historic 

record of legionellosis outbreaks since the disease was first 

recognized in 1976; additional legionellosis outbreaks occurring 

before 1976 were identified later. Legionellosis outbreaks that 

occurred after 1971 were included in the analysis to correspond 

with the period when the surveillance system was started 

(Table 7). The legionellosis outbreaks identified (covering 1971–

2000) are likely an underestimate of legionellosis outbreaks that 

occurred during this period, and caution should be used when 

interpreting the reported data (Figures 2 and 7). For example, 

because reporting mechanisms have changed over time (e.g., 

only published outbreaks and Epi-Aids are included before 2001, 

whereas all outbreaks reported by state health departments are 

included as of 2001), comparisons over time cannot be made, 

and these data might not be representative of the number of 

outbreaks over time. However, the addition of the pre-2001 
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62 MMWR / September 23, 2011 / Vol. 60 / No. 12

outbreaks to WBDOSS underscores the point that legionellosis 

outbreaks have occurred consistently over the past 3 decades.

Limitations

WBDOSS and waterborne disease outbreak reporting is subject 

to at least four main limitations. First, the level of surveillance 

and reporting activity varies across states and localities. Therefore, 

determining whether an increase or decrease in reporting reflects 

an actual change in the incidence of outbreaks or reflects a 

change in the sensitivity of surveillance practices is unknown. 

Outbreak reporting might increase as waterborne disease becomes 

better recognized, water system deficiencies are identified, and 

state surveillance activities and laboratory capabilities increase 

(108–110). Environmental testing and laboratory capacity also 

vary substantially across states and localities. 

Second, detection, investigation, and reporting of outbreaks 

are incomplete. Multiple factors contribute to the ability of state 

and local public health agencies to recognize, investigate, and 

report outbreaks. Public health agencies must have the financial 

and personnel resources to investigate outbreaks; they must 

recognize and link cases of illness to a common water source, 

which requires appropriate epidemiologic, environmental, and 

laboratory capacity to conduct investigations. For example, 

analyses for specific pathogens and indicators of water 

contamination depend upon the availability of certified or 

approved laboratories. Although many laboratories are certified 

to conduct standard analyses for fecal indicators and chemicals, 

few laboratories have capabilities for identifying waterborne 

pathogens, and these tests can be expensive. Collecting water 

samples for pathogen identification often requires sampling large 

quantities of water or filtering large volumes of water through 

special membranes. Methods for concentrating large volumes 

of water for testing are being developed and implemented for 

use in outbreak investigations (89,111–113).

Third, outbreaks associated with drinking water are inherently 

difficult to detect because most persons have daily exposure to 

tap water. For this reason, case-control and other epidemiologic 

studies are less likely to find statistically significant associations 

unless the outbreaks are communitywide or environmental 

investigations provide supporting data to identify deficiencies 

in water treatment or distribution systems that can help identify 

drinking water as the potential exposure. 

Finally, gaps exist in the types of data that are systematically 

collected and reported during outbreak investigations. 

In particular, data from environmental investigations 

on contributing factors associated with outbreaks (e.g., 

distribution system vulnerabilities, classification of GWUDI 

systems including testing criteria, or details on septic system 

placement and maintenance) are often not collected or 

reported to CDC. Water samples also often are not collected 

or are collected late in an investigation, limiting the ability 

to link clinical and environmental data to establish a waterrelated exposure through molecular epidemiologic or other 

laboratory testing. Success in the detection of a pathogen in 

water samples is highly dependent on a timely investigation 

and sample collection before contamination is flushed from 

the water system or inactivated by a disinfectant.

Conclusion

Data collected as part of the national WBDOSS are used to 

describe the epidemiology of waterborne disease outbreaks in 

the United States. Trends regarding water systems and deficiencies implicated in these outbreaks are used to assess whether 

regulations for water treatment and water quality monitoring 

are adequate to protect public health. Trends regarding the 

etiologic agents responsible for these outbreaks are used to 

assess the need for different interventions and changes in policies and resource allocations.

The data provided in this report highlight two primary 

findings. The first is the high proportion of outbreaks associated 

with contaminated ground water, whether consumed untreated 

or with inadequate treatment. This is consistent with data from 

1971–2006 indicating that ground water outbreaks comprised 

the majority of drinking water outbreaks and showed no 

decrease over time (6). The second finding is that Legionella

was again the most frequently reported etiology among 

drinking water–associated outbreaks, following the pattern 

observed since it was first included in WBDOSS in 2001 (6). 

The addition of published and CDC-investigated legionellosis 

outbreaks to the database demonstrates that legionellosis is not 

a new public health issue.

Federal drinking water regulations have focused on 

protecting consumers from contaminated surface water, in 

part because of previous outbreaks and analyses that suggested 

needed improvements for surface water systems and the large 

percentage of the population served by surface water (70% of 

those using community water systems) (31). These regulations 

have likely contributed to the decrease in the number and 

proportion of reported outbreaks associated with contaminated 

surface water during the previous twenty years. Similar 

protections for ground water were absent until promulgation 

of the GWR in 2006. The GWR focuses on identification 

of deficiencies, protection of wells and springs from 

contamination, and providing disinfection where necessary 

to protect against bacterial and viral agents. Outbreaks in 

disinfected ground water systems emphasize the importance 

of maintaining adequate, continuous disinfection. Outbreaks 
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MMWR / September 23, 2011 / Vol. 60 / No. 12 63

in untreated ground water systems underscore the importance 

of assessing contamination sources to determine if the ground 

water source is at risk of contamination from fecal sources or 

is under the direct influence of surface water where additional 

treatment is needed to protect against parasites. When fully 

implemented, the GWR is expected to reduce ground waterassociated outbreaks, as seen in surface water outbreaks after 

promulgation of the SWTR and its amendments. 

Surveillance, prevention, and control activities for outbreaks 

occur primarily at the local and state levels (including 

territories and FAS). CDC and other federal agencies provide 

technical assistance with laboratory testing and epidemiologic 

and environmental investigations when requested by states 

and territories (Box 1). Efforts to improve the detection, 

investigation, and reporting of outbreaks at the local, state, 

and national levels include the 2006 CSTE position statement 

that made waterborne disease outbreaks, as a unit of reporting, 

nationally notifiable and reportable to CDC starting in 

2007. To improve timeliness and completeness of outbreak 

reporting, in 2009, CDC transitioned to electronic outbreak 

reporting through the National Outbreak Reporting System 

(NORS) for outbreaks occurring on or after January 1, 2009 

(114,115). NORS is a collaborative project that is currently 

used for national surveillance of waterborne disease outbreaks, 

foodborne disease outbreaks, and enteric disease outbreaks 

associated with zoonotic, person-to-person, environmental, 

and undetermined exposures. Efforts to develop and enhance 

NORS functionality focus on the need to support public 

health agencies and researchers working to identify the causes 

of outbreaks and to understand the environmental factors 

contributing to these outbreaks.

Adequate programmatic funding and human resources to 

investigate outbreaks are essential for an effective surveillance 

system. Improved communication among local and state public 

health departments, regulatory agencies, and water utilities, 

along with routine reporting or sharing of water quality data 

within the health and environmental departments will aid 

in the detection and control of outbreaks. Additional data 

are needed to better understand some of the contributing 

factors associated with many drinking water outbreaks (e.g., 

a GWUDI classification with details on criteria used to 

determine the influence of surface water). Further research and 

exploration of other contributing factors and antecedent events 

associated with untreated ground water outbreaks are needed, 

including systematic documentation and reporting of the role 

of septic system contamination, extreme precipitation events, 

improper well construction, main breaks, cross-connections, 

and treatment failures. 

Measures to reduce the occurence of drinking water 

outbreaks in the United States have been outlined (Box 2).

Additional efforts are needed to improve outbreak detection 

and investigations. Such efforts include enhancing surveillance 

activities, increasing laboratory support for clinical specimen and 

water sample testing, conducting environmental investigations 

to assess outbreak deficiencies and contributing factors, and 

providing adequate resources and staff to state and local health 

departments to monitor, detect, and prevent outbreaks.
Surveillance Summaries

64 MMWR / September 23, 2011 / Vol. 60 / No. 12

BOX 1. Federal organizations that provide assistance with investigations of waterborne disease outbreaks

State and territorial health departments can request epidemiologic assistance, water quality assessment, and laboratory 

testing from CDC during waterborne disease outbreaks. 

Collection of large-volume water samples might be required 

to identify pathogens that require special protocols for 

their recovery. The EPA Safe Drinking Water Hotline can 

be consulted for information about drinking water rules, 

guidance, and regulations or to identify state and local 

laboratories certified for drinking water quality testing. The 

U.S. Geological Survey can be consulted for assistance with 

hydrogeologic investigations of outbreaks when untreated 

ground water is suspected.

Requests for assistance with outbreak investigations 

(e.g., epidemiologic assistance, water testing, diagnosis 

of free-living amebas, or molecular characterization of

Cryptosporidium and Giardia)

Waterborne Disease Prevention Branch

Division of Foodborne, Waterborne, and Environmental 

Diseases

National Center for Emerging and Zoonotic Infectious 

Diseases, CDC

Telephone: 404-639-1700

Email: healthywater@cdc.gov

Internet: http://www.cdc.gov/healthywater

Requests for diagnostic testing for viral organisms

Division of Viral Diseases

National Center for Immunization and Respiratory 

Diseases, CDC

Telephone: 800-232-4636

Requests for diagnostic testing for enteric bacterial 

organisms

Enteric Diseases Laboratory Branch

Division of Foodborne, Waterborne, and Environmental 

Diseases

National Center for Emerging and Zoonotic Infectious 

Diseases, CDC

Telephone: 404-639-3334

Requests for information for diagnostic testing for parasites (except for Cryptosporidium, Giardia, or free-living 

amebas)

Division of Parasitic Diseases and Malaria

Center for Global Health, CDC

Telephone: 404-718-4745

Internet: http://www.cdc.gov/parasites

Requests for information or testing for Legionella

Division of Bacterial Diseases

National Center for Immunization and Respiratory 

Diseases, CDC

Telephone: 404-639-2215

Internet: http://www.cdc.gov/legionella

Information regarding drinking water and public health

CDC

Internet: http://www.cdc.gov/healthywater/drinking

—Drinking water health communication and education resources for the general public

—Information on maintaining individual wells and 

effectively disinfecting water when camping, hiking, or traveling

—Outbreak investigation toolkit and technical information concerning laboratory diagnostics

Safe Drinking Water Hotline

Environmental Protection Agency

Telephone: 800-426-4791

E-mail: hotline-sdwa@epa.gov

Internet: http://www.epa.gov/safewater

Microbiological and chemical exposure assessment

Environmental Protection Agency 

Microbiological and Chemical Exposure Assessment 

Research Division

National Exposure Research Laboratory

Telephone: 513-569-7303

Information about groundwater resources

U.S. Geological Survey

Internet: http://water.usgs.gov/ogw
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 65

Acknowledgments

This report is based in part by contributions from state, local, and 

territorial waterborne disease surveillance coordinators; state, local 

and territorial epidemiologists; and environmental health personnel.

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•	 Increase monitoring and prevention efforts to reduce

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particularly unchlorinated ground water systems.

•	 Conduct research to improve understanding of the

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•	 Systematically collect and analyze data on contributing

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•	 Provide guidance and technical assistance to help

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•	 Maintain efforts focused on water systems operations

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•	 Provide additional resources and capacity building for

outbreak detection, investigation and reporting of 

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MMWR / September 23, 2011 / Vol. 60 / No. 12 69

Appendix A

Glossary of Definitions

action level A specified concentration of a contaminant in water. If this concentration is reached or 

exceeded, certain actions (e.g., further treatment and monitoring) must be taken to comply 

with a drinking water regulation.

aquifer A geologic formation or part of a formation (e.g., gravel, sand, or porous stone) that yields 

water to wells or springs.

backflow A hydraulic condition caused by a difference in water pressure that causes nonpotable water 

or other liquid to enter the potable water system by either backpressure or backsiphonage. See 

cross-connection.

backsiphonage A hydraulic condition caused by negative or subatmospheric pressure within a water system, 

resulting in backflow.

biofilm Microbial cells that adhere to a moist or water-covered surface through a matrix of primarily 

polysaccharide materials in which they are encapsulated. Biofilms can grow on piping and 

surfaces of water systems and can be very difficult to remove. They protect microbes from 

disinfectants (e.g., chlorine) in the water.

boil water advisory A statement to the public advising that tap water must be boiled before drinking.

bottled water Commercially produced bottled water.

class Waterborne disease outbreaks are classified according to the strength of the epidemiologic 

and clinical laboratory data, and environmental data implicating water as the source of the 

outbreak (see Table 3).

coliforms All aerobic and facultative anaerobic, gram-negative, nonspore-forming, rod-shaped bacteria 

that ferment lactose with gas formation within 48 hours at 95°

F (35°C). Coliforms are mostly 

harmless bacteria that live in soil and water as well as the gut of humans and animals.

community water system A public water system that has at least 15 service connections used by year-round residents or 

that regularly serves at least 25 year-round residents. The system might be owned by a private 

or public entity providing water to a community, subdivision, or mobile home park.

cross-connection Any actual or potential connection between a drinking water supply and a possible source 

of contamination or pollution (i.e., nonpotable water). Under this condition, contaminated 

water might flow back into the drinking water system. See backflow and backsiphonage.

deficiency An antecedent event or situation contributing to the occurrence of a waterborne disease or 

outbreak. Outbreaks associated with water intended for drinking, water not intended for 

drinking and water of unknown intent are assigned deficiency codes, as categorized in Table 2.

dermatitis Inflammation of the skin. In this report, dermatitis denotes a broad category of skin-related 

symptoms (e.g., folliculitis, cellulitis, chemical burns, or rash).

disinfection A treatment that kills microorganisms (e.g., bacteria, viruses, and protozoa); in water 

treatment, a chemical (commonly chlorine, chloramine, or ozone) or physical process (e.g., 

ultraviolet light) can be used.

disinfection by-products Chemicals formed in water through reactions between organic or inorganic matter and 

disinfectants. Examples include chloramines, also known as combined chlorines. These 

chemicals might have acute or chronic health effects.
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70 MMWR / September 23, 2011 / Vol. 60 / No. 12

distribution system Water pipes, storage reservoirs, tanks, and other means used to deliver drinking water to 

consumers or to store finished water before delivery to a customer. In community water 

systems, the distribution system is under the jurisdiction of a water utility and ends 

at the water meter or at the customer’s property line (if the system is not metered). In 

noncommunity and nonpublic individual water systems, the distribution system ends at the 

point where water enters the building or house. See plumbing.

etiology The pathogen, chemical, or toxin causing a waterborne disease or outbreak or other health 

event. Infectious etiologic agents include bacteria, parasites, and viruses.

fecal coliforms Coliform bacteria that grow and ferment lactose to produce gas at 112.1°

F (44.5°C) within 

24 hours. These bacteria are associated with human and animal wastes, and their presence in 

water might be an indication of recent sewage or animal waste contamination.

filtration In water treatment, the process of passing water through one or more permeable membranes 

or media of small diameter (e.g., sand, anthracite, and diatomaceous earth) to remove 

suspended particles from the water. Filters might be effective in removing pathogens, 

depending on the type and operation.

finished water The water (e.g., drinking water) delivered to the distribution system after treatment (if any 

treatment occurred).

free chlorine Chlorine in water (found as an aqueous mixture of hypochlorous acid and hypochlorite anion) 

that has not combined with other constituents; therefore, it is able to serve as an effective 

disinfectant (also referred to as free available chlorine or residual chlorine). Measuring the free 

chlorine level is a common water quality test.

ground water Water that is contained in interconnected pores in an aquifer. 

ground water system A drinking water system that uses water extracted from an aquifer (i.e., a well or spring) as its 

source.

ground water under the direct 

influence of surface water 

(GWUDI)

As defined by the U.S. Environmental Protection Agency (EPA): Any water beneath the 

surface of the ground with: 1) significant occurrence of insects or other macroorganisms such 

as algae or large-diameter pathogens such as Giardia lamblia or, 2) significant and relatively 

rapid shifts in water characteristics such as turbidity, temperature, conductivity, or pH 

which closely correlate to climatologic or surface water conditions. Direct influence must be 

determined for individual sources in accordance with criteria established by the state. The state 

determination of direct influence may be based on site-specific measurements of water quality 

and/or documentation of well construction characteristics and geology with field evaluation. 

Source: http://water.epa.gov/aboutow/ogwdw/glossary_technical.cfm#G.

individual water system A water system that does not meet the EPA definition for a public water system. The system 

might regularly serve as many as 24 persons or 14 connections or as few as a single person or 

connection (e.g., a single family or farm not having access to a public water system). States are 

responsible for regulating these water systems.

karst aquifer An aquifer characterized by water-soluble limestone and similar rocks in which fractures or 

cracks have been widened by the dissolution of the carbonate rocks by ground water; the 

aquifer might contain sinkholes, tunnels, or even caves.

mixed water source outbreak More than one type of source water is implicated in the outbreak (e.g., a combination of 

ground water and surface water).

noncommunity water system A public water system that is not a community system; it does not serve year-round residents. 

There are two types: transient and nontransient noncommunity systems.
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MMWR / September 23, 2011 / Vol. 60 / No. 12 71

nontransient noncommunity 

water system 

A public water system that regularly supplies water to ≥25 of the same persons for ≥6 months 

per year but not year-round (e.g., schools, factories, office buildings, or hospitals with their 

own water systems).

plumbing Water pipes, storage reservoirs, tanks, and other means used to deliver drinking water to 

consumers inside buildings or houses or to store drinking water inside buildings or houses 

before consumption. In community water systems, the plumbing begins after the water 

utility’s water meter or at the property line (if the distribution system is not metered). In 

noncommunity and nonpublic (i.e., individual) water systems, the plumbing begins at the 

point where water enters the building or house. See distribution system.

predominant illness The category of illness reported by >50% of ill respondents (e.g., acute gastroenteritis, 

dermatitis, or acute respiratory illness). When more than one illness category is reported 

for a single outbreak, they are listed together as predominant illnesses. These mixed illness 

outbreaks are analyzed separately from outbreaks with single illnesses.

primary water exposure For use in this report, a classification used for the source of contaminated water in outbreaks 

involving water not intended for drinking or water of unknown intent.

public water system A system that provides piped water to the public for human consumption and is regulated 

under the Safe Drinking Water Act. Such a system must have at least 15 service connections or 

regularly serve at least 25 persons daily for at least 60 days per year. Each public water system 

is further classified as either a community water system or a noncommunity water system.

raw water Surface water or ground water that has not undergone a disinfection or treatment process for 

the purpose of making it safer for consumption in any way. See untreated water.

reservoir, impoundment An artificially maintained lake or other body of water used for the collection and storage of 

water. This body of water can be available as a source of raw water for drinking purposes or 

recreational use. In certain instances, a finished water storage facility in the distribution system 

might also be called a reservoir.

schmutzdecke Gelatinous biologic layer on the surface of a slow sand filter, consisting of a complex microbial 

community (including bacteria, fungi, protozoa, and rotifera and other aquatic organisms) and 

organic particulate matter.

setting Location in which exposure to contaminated water occurred (e.g., restaurant, hospital, or 

hotel).

source water Untreated ground or surface water (i.e., raw water) used to produce drinking water. Source 

water may or may not be treated prior to human consumption.

surface water All water on the surface of the earth (e.g., lakes, rivers, reservoirs, ponds, and oceans) as 

distinguished from subsurface or ground water.

total coliforms The combined count of fecal and nonfecal coliforms that are detected in water using a 

standard test. The extent to which total coliforms are present in water can indicate the general 

quality of that water and the likelihood that the water is contaminated fecally by animal and/

or human sources.

transient noncommunity 

water system

A public water system that provides water in a place such as a gas station or campground 

where persons do not remain for long periods.

untreated water Water that has not undergone a disinfection or treatment process for the purpose of making it 

safer for consumption (i.e., raw water).
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72 MMWR / September 23, 2011 / Vol. 60 / No. 12

water not intended for 

drinking (WNID)

Water that has not been treated for human consumption in conformance with EPA drinking 

water standards and that is provided for uses other than for drinking. This category includes 

water used in industrial settings; untreated water from lakes, springs, and creeks used as drinking 

water by campers and boaters; irrigation water; and other nonpotable water sources with or 

without taps. This category does not include exposure to recreational water or flood water.

water of unknown intent 

(WUI)

Water for which there is insufficient information to determine for what purpose it is being 

provided or used and whether it has been treated for human consumption in conformance 

with EPA drinking water standards.

water system A system for the provision of water for human consumption through pipes or other 

constructed conduits. This includes any collection, treatment, storage, and distribution 

facilities used primarily in connection with such a system.
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 73

Appendix B

Descriptions of Selected Waterborne Disease Outbreaks Associated with Drinking Water, 

Water Not Intended for Drinking, and Water of Unknown Intent

Month Year

State/Jurisdiction 

in which outbreak 

occurred Etiology

No. of cases 

(deaths) Description of outbreak

Bacteria

October 2007 Nevada Legionella pneumophila

serogroup 1

7 During November and December of 2007, two cases of Legionnaire’s 

disease (LD) occurring at the same timeshare condominium were identified 

through CDC’s surveillance system for travel-associated LD; two additional 

cases were identified in 2008. A subsequent review identified a single 

case in 2006, and a previous outbreak of legionellosis in 2001 at the same 

condominium. Seven laboratory-confirmed cases were reported during 

2007–2008. The condominium has three towers with individual low- and 

high-rise potable water systems. Water samples were collected from guest 

rooms, water heaters, cooling towers, decorative features and recreational 

facilities, as well as the municipal water source at the water meter, prior to 

the implementation of extensive remediation efforts. Environmental samples 

from showerheads and sinks in one of the towers were found to have L. 

pneumophila that was an identical molecular match to the single clinical 

isolate available in 2001. Laboratory findings, combined with evidence of 

sporadic transmission between the first and second outbreak, indicated that 

long-term colonization of the drinking water system with L. pneumophila 

had likely occurred at the complex and highlights the importance of 

travel-associated surveillance systems in linking disease occurrence among 

geographically-dispersed travelers. No cases were reported at the time-share 

condominium in 2009 following completion of the second remediation 

attempt.

July 2008 New York L. pneumophila serogroup 1 13 (1) Thirteen residents of an apartment complex for seniors received a diagnosis 

of Legionnaires’ disease. Several water samples from apartment bathroom 

taps and a hot water cartridge were culture positive for L. pneumophila 

serogroup 1. The environmental isolates matched patient clinical isolates via 

pulse-field gel electrophoresis (PFGE). No single event was determined to 

have led to the outbreak. Potential contributing factors included a previous 

water main break and subsequent loss in water pressure, disruption and 

mobilization of an existing biofilm colonized with Legionella, and a hot water 

heater temperature setting that may have been supportive of Legionella 

growth.

August 2008 Connecticut Providencia 55 Fifty-five persons in an apartment complex supplied by water from a 

community water system became ill with gastrointestinal symptoms. Six 

of nine stool specimens tested positive for Providencia. None of the stool 

specimens tested positive for Salmonella, Shigella, Campylobacter, E. coli, 

parasites, or norovirus. The investigation found a statistically significant 

association between tap water consumption and illness. Seven well water 

samples tested negative for Providencia and six of the samples tested positive 

for E. coli. Raw sewage was visible on the ground after two septic pumps 

located uphill of the water system wells failed and the septic tank overflowed. 

Contributing factors in the outbreak included the downhill movement of 

sewage towards the wells by rainfall and a cracked well casing. 

March 2008 Colorado Salmonella Typhimurium 1,300 (1) An estimated 1,300 persons became ill when a community drinking water 

system became contaminated with Salmonella Typhimurium. At the time of 

the outbreak, a state waiver allowed the distribution of untreated ground 

water. Twenty persons were hospitalized and one person died. Epidemiologic 

and environmental findings implicated the community water system. 

Clinical specimens and tap water tested positive for identical subtypes of S. 

Typhimurium using pulse-field gel electrophoresis. The likely source of the 

outbreak was animal contamination of a storage tank that had numerous cracks 

and entry points. The outbreak response required local, regional, state, and 

federal emergency assistance over a period of three weeks during which bulk 

water was distributed by the National Guard while the distribution system was 

hyperchlorinated. At the time of the outbreak, a new water-treatment plant was 

under construction, primarily for the purpose of abating naturally high levels of 

arsenic. Since the outbreak, water is now continuously disinfected at this new 

plant.
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74 MMWR / September 23, 2011 / Vol. 60 / No. 12

Month Year

State/Jurisdiction 

in which outbreak 

occurred Etiology

No. of cases 

(deaths) Description of outbreak

August 2008 Tennessee Salmonella serotype I 

4,5,12:i:-

5 Five persons were infected with Salmonella serotype I 4,5,12:i:- following 

consumption of untreated water from a local spring that supplied five homes 

and a church. The initial case was an infant who was seen by a physician for 

bloody diarrhea. The regional health department investigated the illness and 

determined that tap water used to mix powdered formula was the primary 

risk factor. Tap water collected from the infant’s home tested positive for 

total coliforms and E. coli. The water also tested positive for Salmonella

and was a pulse-field gel electrophoresis (PFGE) match to isolates from the 

infant’s stool. A subsequent water sample from the church matched the 

clinical and tap water samples by PFGE. As a result of the investigation, the 

Tennessee Department of Environment and Conservation (TDEC) and the 

regional health department then worked with the affected homes, church 

and spring owner to provide education and bring the drinking water system 

into compliance.

Viruses

March 2008 Tennessee Hepatitis A virus 9 Nine persons tested positive and four persons were hospitalized for infection 

with hepatitis A virus after spending time at a lakeside residential and 

vacation community without municipal water or sewer services. Cases were 

identified among relatives and friends of two resident families. The index 

case occurred in early March, and the cluster of secondary cases occurred 

>6 weeks later, in April. The epidemiologic study indicated that person-toperson transmission did not occur. Water samples collected in May from 

two wells, one supplying the residence of three persons with secondary 

cases, detected hepatitis A virus by PCR that was the same strain as that in 

the persons from whom virus specimens were collected. The untreated well 

water was likely contaminated by a faulty septic system used by the index 

case-patient, who lived in a mobile home nearby. The public health response 

included provision of vaccine at three vaccination clinics and provision of 

postexposure immunoglobulin to >1,500 residents and visitors. Education 

regarding karst geology and the risks of drinking water from shallow private 

wells without disinfection was provided to homeowners in the area, and 

local and state government offices were encouraged to consider extending 

water utility services to the affected area.

Parasites

April 2008 Puerto Rico Cyclospora cayetanensis 82 The Puerto Rico Department of Health investigated an outbreak of recurrent 

diarrhea in a rural community. Of 82 case-patients that were identified, 

seven were hospitalized. Clinical testing identified Cyclospora cayetanensis 

in 20 of 26 case-patient stool specimens. An epidemiologic study showed 

that interruptions in the water service during the past six months, changes 

in drinking water quality and appearance, and food purchases at a local 

supermarket were associated with illness; no increased risk was associated 

with consumption of fruits or vegetables. Interruptions in water service 

occurred prior to the outbreak, including one interruption during the 

estimated exposure period when a pumping station was found to be 

damaged and water was delivered in trucks from another water system and 

stored in a water tank. Breaks in water supply and changes in water quality 

were reported by the community during the same time period. Although 

the source of contamination was not identified, this is the first outbreak of 

C. cayetanensis in Puerto Rico in which water consumption is considered the 

probable source of transmission.

July 2007 California Giardia intestinalis 46 A giardiasis outbreak at a camp that used a spring as its water source 

resulted in 46 cases of illness. The spring was classified as ground water 

under the direct influence of surface water and therefore subject to the 

Environmental Protection Agency’s Surface Water Treatment Rule and 

amendments. An epidemiologic study conducted implicated eating a 

garden salad and showering as risk factors for illness. Although a slow sand 

filter and chlorinator had been installed, inadequate time was allowed for 

formation of a schmutzdecke biological layer on the surface of the filter, 

which is important for effective treatment performance of slow sand filters. 

Water samples collected one week after the filter was put into operation 

showed considerably higher levels of total coliforms and turbidity in filtered 

water samples than in the spring water samples. Six weeks after operation of 

the filter a schmutzdecke layer formed and water samples met requirements.
Surveillance Summaries

MMWR / September 23, 2011 / Vol. 60 / No. 12 75

Month Year

State/Jurisdiction 

in which outbreak 

occurred Etiology

No. of cases 

(deaths) Description of outbreak

Chemicals

April 2007 Massachusetts Sodium hydroxide 145 An estimated 145 persons experienced chemical burns after a sodium 

hydroxide overfeed altered the pH balance of water passing through 

a water treatment facility. The pH imbalance occurred overnight when 

treatment lines were left on a manual setting after routine maintenance. 

The event activated an onsite alarm system but the facility did not have an 

automatic notification system for off-site staff. Detection and remediation 

of the problem at the water treatment facility occurred early the following 

morning. The emergency response was a coordinated effort that included 

the local health department and multiple emergency response agencies. 

Response measures included a Do Not Use order, community education and 

outreach, consultation with local healthcare facilities and inspection of food 

and retail establishments prior to re-opening.

Multiple 

etiologies

May 2007 Wisconsin Norovirus genogroup I, 

Campylobacter, Salmonella

229 Gastrointestinal illness in 229 persons was associated with drinking water 

exposure at a local restaurant; three stool specimens were positive for enteric 

pathogens: one for norovirus genogroup 1, one for Campylobacter, and one 

for Salmonella. Well water tested positive for E. coli and a boiled/bottled water 

advisory was issued. Subsequent tests found norovirus genogroup I in the 

water that was identical to the strain in clinical specimens. The restaurant and 

surrounding residences were located in an area with karst geological features. 

Tracer dye testing implicated a septic tank as a source of contamination. 

Underground seepage of sewage and contamination through limestone or 

fissured rock were thought to contribute to the outbreak.

September 2008 Illinois Shigella sonnei, 

Cryptosporidium, Giardia

41 Of 72 persons who gathered for weekend activities that included a dinner 

cruise on a lake, 41 developed illness attributed to infections with S. sonnei, 

Giardia, and Cryptosporidium beginning the following day. Environmental 

inspection revealed conditions and equipment that could have contributed 

to lake water contaminating the hose used to load potable water onto the 

boat. Heavy rainfall and flooding the same weekend resulted in the release 

of a large volume of storm water containing rainwater and highly diluted 

sewage into the lake. Ice consumption was epidemiologically linked with 

illness. S. sonnei was isolated from a surface swab of an ice container.

Unidentified 

etiology

September 2007 Florida Unidentified 1,663 An estimated 1,663 cases of gastrointestinal illness occurred in a community 

water system supplied by surface water and conventionally treated with 

coagulation, settling, filtration, and disinfection. A boil water advisory was 

issued two days after fecal coliforms and E. coli were initially found in the 

water during routine testing. A cross-sectional, random telephone survey of 

households affected by the boil water advisory found statistically significant 

associations between water consumption and illness. An independent 

assessment identified numerous operation and maintenance deficiencies 

in water disinfection and filtration processes, as well as a segment of 

outdated pipe that bypassed disinfection steps because it was not known 

to still be connected to the distribution system. The system was recycling 

filter backwash water as required by EPA; however, before the outbreak, 

the backwash water bypassed the recovery basin and ozonation before 

filtration, thereby adversely affecting filter performance. 


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