CDC

Surveillance

Summaries

April 12, 1996 / Vol. 45 / No. SS-1

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Public Health Service

Centers for Disease Control

and Prevention (CDC)

Atlanta, Georgia 30333

Surveillance for

Waterborne-Disease Outbreaks —

United States, 1993–1994
The MMWR series of publications is published by the Epidemiology Program Office,

Centers for Disease Control and Prevention (CDC), Public Health Service, U.S. Department of Health and Human Services, Atlanta, GA 30333.

Centers for Disease Control and Prevention.......................... David Satcher, M.D., Ph.D.

Director 

The production of this report as an MMWR serial publication was coordinated in:

 Epidemiology Program Office.................................... Stephen B. Thacker, M.D., M.Sc.

Director

Richard A. Goodman, M.D., M.P.H.

Editor, MMWR Series

Scott F. Wetterhall, M.D., M.P.H.

Associate Editor, CDC Surveillance Summaries

 Scientific Information and Communications Program

 CDC Surveillance Summaries ...................................... Suzanne M. Hewitt, M.P.A.

Managing Editor

M. William Park, Ph.D., M.P.H.

Project Editor 

Sandra L. Ford

Morie M. Higgins

Beverly J. Holland

Peter M. Jenkins

Visual Information Specialists

SUGGESTED CITATION

General: Centers for Disease Control and Prevention. CDC Surveillance Summaries, April 12, 1996. MMWR 1996;45(No. SS-1).

Specific: [Author(s)]. [title of particular article]. In: CDC Surveillance Summaries, April 12, 1996. MMWR 1996;45(No. SS-1):[inclusive page

numbers].

Copies can be purchased from Superintendent of Documents, U.S. Government

Printing Office, Washington, DC 20402-9325. Telephone: (202) 783-3238.

Use of trade names and commercial sources is for identification only and does not

imply endorsement by the Public Health Service or the U.S. Department of Health

and Human Services.
Contents

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

EPA Regulations for Drinking Water ...................................................................3

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

Sources of Data ...............................................................................................3

Definitions of Terms........................................................................................4

Classification of Outbreaks ............................................................................5

Results ...................................................................................................................6

1993–1994 Outbreaks Associated With Drinking Water.............................6

Etiologic Agents ...........................................................................................6

Cryptosporidium......................................................................................6

Giardia ....................................................................................................11

Bacteria...................................................................................................12

Chemicals...............................................................................................12

Unidentified Etiologic Agent.................................................................13

Water-Quality Data.....................................................................................13

Water System and Water Source..............................................................14

1993–1994 Outbreaks Associated With Recreational Water....................15

Previously Unreported Outbreaks ..............................................................19

Outbreaks Not Classified as Waterborne-Disease Outbreaks .................20

Discussion ...........................................................................................................20

General Considerations About Surveillance Data for WaterborneDisease Outbreaks.........................................................................................20

1993–1994 Outbreaks Associated With Drinking Water...........................21

1993–1994 Outbreaks Associated With Recreational Water....................26

Conclusions.........................................................................................................27

References...........................................................................................................28

Glossary...............................................................................................................31

Vol. 45 / No. SS-1 MMWR i
Abortion NCCDPHP 1995; Vol. 44, No. SS-2

AIDS/HIV

 Distribution by Racial/Ethnic Group NCID 1988; Vol. 37, No. SS-3

 Among Black & Hispanic Children & 

 Women of Childbearing Age NCEHIC 1990; Vol. 39, No. SS-3

Behavioral Risk Factors NCCDPHP 1991; Vol. 40, No. SS-4

Birth Defects

 B.D. Monitoring Program (see also Malformations) NCEH 1993; Vol. 42, No. SS-1

 Contribution of B.D. to Infant Mortality 

 Among Minority Groups NCEHIC 1990; Vol. 39, No. SS-3

Breast & Cervical Cancer NCCDPHP 1992; Vol. 41, No. SS-2

Campylobacter NCID 1988; Vol. 37, No. SS-2

Chancroid NCPS 1992; Vol. 41, No. SS-3

Chlamydia NCPS 1993; Vol. 42, No. SS-3

Cholera NCID 1992; Vol. 41, No. SS-1

Congenital Malformations, Minority Groups NCEHIC 1988; Vol. 37, No. SS-3

Contraception Practices NCCDPHP 1992; Vol. 41, No. SS-4

Cytomegalovirus Disease, Congenital NCID 1992; Vol. 41, No. SS-2

Dengue NCID 1994; Vol. 43, No. SS-2

Dental Caries & Periodontal Disease Among 

 Mexican-American Children NCPS 1988; Vol. 37, No. SS-3

Diabetes Mellitus NCCDPHP 1993; Vol. 42, No. SS-2

Dracunculiasis NCID 1992; Vol. 41, No. SS-1

Ectopic Pregnancy NCCDPHP 1993; Vol. 42, No. SS-6

Elderly, Hospitalizations Among NCCDPHP 1991; Vol. 40, No. SS-1

Endometrial & Ovarian Cancers EPO, NCCDPHP 1986; Vol. 35, No. 2SS

Escherichia coli O157 NCID 1991; Vol. 40, No. SS-1

Evacuation Camps EPO 1992; Vol. 41, No. SS-4

Family Planning Services at Title X Clinics NCCDPHP 1995; Vol. 44, No. SS-2

Foodborne Disease NCID 1990; Vol. 39, No. SS-1

Gonorrhea & Syphilis, Teenagers NCPS 1993; Vol. 42, No. SS-3

Hazardous Substances Emergency Events ATSDR 1994; Vol. 43, No. SS-2

Health Surveillance Systems IHPO 1992; Vol. 41, No. SS-4

Hepatitis NCID 1985; Vol. 34, No. 1SS

Homicide NCEHIC 1992; Vol. 41, No. SS-3

Homicides, Black Males NCEHIC 1988; Vol. 37, No. SS-1

Hysterectomy NCCDPHP 1986; Vol. 35, No. 1SS

Infant Mortality (see also National Infant Mortality;

 Birth Defects; Postneonatal Mortality) NCEHIC 1990; Vol. 39, No. SS-3

Influenza NCID 1993; Vol. 42, No. SS-1

Injury

 Death Rates, Blacks & Whites NCEHIC 1988; Vol. 37, No. SS-3

 Drownings NCEHIC 1988; Vol. 37, No. SS-1

 Falls, Deaths NCEHIC 1988; Vol. 37, No. SS-1

 Firearm-Related Deaths, Unintentional NCEHIC 1988; Vol. 37, No. SS-1

 Head & Neck NCIPC 1993; Vol. 42, No. SS-5

*Abbreviations

ATSDR Agency for Toxic Substances and Disease Registry

CIO Centers/Institute/Offices

EPO Epidemiology Program Office

IHPO International Health Program Office

NCCDPHP National Center for Chronic Disease Prevention and Health Promotion

NCEH National Center for Environmental Health

NCEHIC National Center for Environmental Health and Injury Control

NCID National Center for Infectious Diseases

NCIPC National Center for Injury Prevention and Control

NCPS National Center for Prevention Services

NIOSH National Institute for Occupational Safety and Health

NIP National Immunization Program

Reports Published in CDC Surveillance Summaries Since January 1, 1985

Responsible

Subject CIO/Agency* Most Recent Report 

ii MMWR April 12, 1996
 In Developing Countries NCEHIC 1992; Vol. 41, No. SS-1

 In the Home, Persons <15 Years of Age NCEHIC 1988; Vol. 37, No. SS-1

 Motor Vehicle-Related Deaths NCEHIC 1988; Vol. 37, No. SS-1

 Objectives of Injury Control, State & Local NCEHIC 1988; Vol. 37, No. SS-1

 Objectives of Injury Control, National NCEHIC 1988; Vol. 37, No. SS-1

 Residential Fires, Deaths NCEHIC 1988; Vol. 37, No. SS-1

 Tap Water Scalds NCEHIC 1988; Vol. 37, No. SS-1

Lead Poisoning, Childhood NCEHIC 1990; Vol. 39, No. SS-4

Low Birth Weight NCCDPHP 1990; Vol. 39, No. SS-3

Malaria NCID 1995; Vol. 44, No. SS-5

Maternal Mortality NCCDPHP 1991; Vol. 40, No. SS-2

Measles NCPS 1992; Vol. 41, No. SS-6

Meningococcal Disease NCID 1993; Vol. 42, No. SS-2

Mining NIOSH 1986; Vol. 35, No. 2SS

Mumps NIP 1995; Vol. 44, No. SS-3

National Infant Mortality (see also Infant Mortality;

 Birth Defects) NCCDPHP 1989; Vol. 38, No. SS-3

Neisseria gonorrhoeae, Antimicrobial Resistance in NCPS 1993; Vol. 42, No. SS-3

Neural Tube Defects NCEH 1995; Vol. 44, No. SS-4

Nosocomial Infection NCID 1986; Vol. 35, No. 1SS

Occupational Injuries/Disease

 Asthma NIOSH 1994; Vol. 43, No. SS-1

 Hazards, Occupational NIOSH 1985; Vol. 34, No. 2SS

 In Meatpacking Industry NIOSH 1985; Vol. 34, No. 1SS

 Silicosis NIOSH 1993; Vol. 42, No. SS-5

 State Activities NIOSH 1987; Vol. 36, No. SS-2

Parasites, Intestinal NCID 1991; Vol. 40, No. SS-4

Pediatric Nutrition NCCDPHP 1992; Vol. 41, No. SS-7

Pertussis NCPS 1992; Vol. 41, No. SS-8

Plague NCID 1985; Vol. 34, No. 2SS

Plague, American Indians NCID 1988; Vol. 37, No. SS-3

Poliomyelitis NCPS 1992; Vol. 41, No. SS-1

Postneonatal Mortality NCCDPHP 1991; Vol. 40, No. SS-2

Pregnancy Nutrition NCCDPHP 1992; Vol. 41, No. SS-7

Pregnancy, Teenage NCCDPHP 1993; Vol. 42, No. SS-6

Rabies NCID 1989; Vol. 38, No. SS-1

Racial/Ethnic Minority Groups Various 1990; Vol. 39, No. SS-3

Respiratory Disease NCEHIC 1992; Vol. 41, No. SS-4

Rotavirus NCID 1992; Vol. 41, No. SS-3

Salmonella NCID 1988; Vol. 37, No. SS-2

Sexually Transmitted Diseases in Italy NCPS 1992; Vol. 41, No. SS-1

Smoking NCCDPHP 1990; Vol. 39, No. SS-3

 Smoking-Attributable Mortality NCCDPHP 1994; Vol. 43, No. SS-1

 Tobacco Control Laws, State NCCDPHP 1995; Vol. 44, No. SS-6

 Tobacco-Use Behaviors NCCDPHP 1994; Vol. 43, No. SS-3

Streptococcal Disease (Group B) NCID 1992; Vol. 41, No. SS-6

Sudden Unexplained Death Syndrome Among

 Southeast Asian Refugees NCEHIC, NCPS 1987; Vol. 36, No. 1SS

Suicides, Persons 15–24 Years of Age NCEHIC 1988; Vol. 37, No. SS-1

Syphilis, Congenital NCPS 1993; Vol. 42, No. SS-6

Syphilis, Primary & Secondary NCPS 1993; Vol. 42, No. SS-3

Tetanus NCPS 1992; Vol. 41, No. SS-8

Trichinosis NCID 1991; Vol. 40, No. SS-3

Tuberculosis NCPS 1991; Vol. 40, No. SS-3

Waterborne-Disease Outbreaks NCID 1996; Vol. 45, No. SS-1

Years of Potential Life Lost EPO 1992; Vol. 41, No. SS-6

Youth Risk Behaviors NCCDPHP 1995; Vol. 44, No. SS-1

Reports Published in CDC Surveillance Summaries Since January 1, 1985 — Continued

Responsible

Subject CIO/Agency* Most Recent Report 

Vol. 45 / No. SS-1 MMWR iii
Surveillance for Waterborne-Disease Outbreaks —

United States, 1993–1994

Michael H. Kramer, M.D., M.P.H.1, 2

Barbara L. Herwaldt, M.D., M.P.H.2

Gunther F. Craun, P.E., D.E.E., M.P.H.3

Rebecca L. Calderon, Ph.D., M.P.H.4

Dennis D. Juranek, D.V.M., M.Sc.2

1

 Epidemic Intelligence Service, Epidemiology Program Office, CDC 2

 Division of Parasitic Diseases, National Center for Infectious Diseases, CDC

3

 Global Consulting for Environmental Health, Radford, VA 4

 Human Studies Division, National Health and Environmental Effects

Laboratory, U.S. Environmental Protection Agency

Summary

Problem/Condition: Since 1971, CDC and the U.S. Environmental Protection Agency

have maintained a collaborative surveillance system for collecting and periodically

reporting data that relate to occurrences and causes of waterborne-disease outbreaks

(WBDOs).

Reporting Period Covered: This summary includes data for January 1993 through

December 1994 and for previously unreported outbreaks in 1992.

Description of the System: The surveillance system includes data about outbreaks

associated with water intended for drinking (i.e., drinking water) and those associated

with recreational water. State, territorial, and local public health departments are primarily responsible for detecting and investigating WBDOs and voluntarily reporting

them to CDC on a standard form. 

Results: For the 2-year period 1993–1994, 17 states and one territory reported a total

of 30 outbreaks associated with drinking water. These outbreaks caused an estimated

405,366 persons to become ill, including 403,000 from an outbreak of cryptosporidiosis in Milwaukee, the largest WBDO ever documented in the United States,

and 2,366 from the other 29 outbreaks. No etiologic agent was identified for five

(16.7%) of the 30 outbreaks. The protozoan parasites Giardia lamblia and Cryptosporidium parvum caused 10 (40.0%) of the 25 outbreaks for which the etiologic agent

was identified. Two outbreaks of cryptosporidiosis occurred in large metropolitan areas (i.e., Milwaukee and Las Vegas/Clark County) and were associated with deaths

among immunocompromised persons. The waterborne nature of these two outbreaks

was not recognized until at least 2 weeks after the onset of the Milwaukee outbreak

and until after the end of the Las Vegas outbreak. Campylobacter jejuni was implicated

for three outbreaks and the following pathogens for one outbreak each: Shigella sonnei, Shigella flexneri, non-O1 Vibrio cholerae (in a U.S. territory; the vehicle was

commercially bottled water), and Salmonella serotype Typhimurium (the outbreak

was associated with seven deaths). Eight outbreaks of chemical poisoning were

reported: three were caused by lead (one case each), two by fluoride, two by nitrate,

Vol. 45 / No. SS-1 MMWR 1
and one by copper. Twenty (66.7%) of the 30 outbreaks were associated with a wellwater source.

Fourteen states reported a total of 26 outbreaks associated with recreational water,

in which an estimated 1,714 persons became ill. Fourteen (53.8%) of these 26 were

outbreaks of gastroenteritis. The etiologic agent in each of these 14 outbreaks was

identified; 10 (71.4%) were caused by G. lamblia or C. parvum. Six of these 10 were

associated with chlorinated, filtered pool water, and three with lake water. One of

the latter was the first reported outbreak of cryptosporidiosis associated with the

recreational use of lake water. Four outbreaks of lake water-associated bacterial gastroenteritis were reported, two caused by S. sonnei, one by S. flexneri, and one by

Escherichia coli O157:H7. Nine outbreaks of hot tub-, whirlpool-, or swimming poolassociated pseudomonas dermatitis were reported. Two outbreaks of swimming

pool-associated dermatitis had a suspected chemical etiology. The child who had the

one reported case of primary amebic meningoencephalitis, caused by infection with

Naegleria fowleri, died. 

Interpretation: The number of WBDOs reported annually has been similar for each

year during 1987–1994, except for an increase in 1992. Protozoan parasites, especially

C. parvum and G. lamblia, remain important etiologic agents of WBDOs. The outbreaks of cryptosporidiosis in Milwaukee and Las Vegas demonstrate that WBDOs can

occur in large metropolitan areas. Surveillance methods are needed that expedite the

detection of WBDOs and the institution of preventive measures (e.g., boil-water advisories).

Actions Taken: Surveillance data that identify the types of water systems, their deficiencies, and the etiologic agents associated with outbreaks are used to evaluate the

adequacy of current technologies for providing safe drinking and recreational water.

In addition, they are used to establish research priorities and assist in improving

water-quality regulations.

INTRODUCTION

National statistics on outbreaks associated with water intended for drinking (i.e.,

drinking water) have been available since 1920 (1 ). Since 1971, CDC, the U.S.

Environmental Protection Agency (EPA), and the Council of State and Territorial Epidemiologists have maintained a collaborative surveillance system with the collection

and periodic reporting of data on the occurrences and causes of waterborne-disease

outbreaks (WBDOs) (2,3 ). The surveillance system includes data about outbreaks associated with drinking water and recreational water. This summary includes data for

1993 and 1994 and for previously unreported outbreaks in 1992.

CDC’s and EPA’s efforts related to waterborne-disease surveillance have the following goals: a) to characterize the epidemiology of WBDOs; b) to identify the etiologic

agents of WBDOs and to determine why the outbreaks occurred; c) to train public

health personnel how to detect and investigate WBDOs; and d) to collaborate with

local, state, federal, and international agencies on initiatives to prevent waterborne

diseases. The data gathered through this surveillance system are useful for evaluating

the adequacy of current technologies for providing safe drinking and recreational

water. Surveillance information also influences research priorities and may lead to

improved water-quality regulations.

2 MMWR April 12, 1996
EPA REGULATIONS FOR DRINKING WATER

Public water systems are regulated under the Safe Drinking Water Act of 1974 (4 ),

as amended in 1986 (5 ). In 1989, to minimize microbial transmission, EPA promulgated the Surface Water Treatment Rule (SWTR; in effect since 1991 [6 ]) and in 1990

made the Total Coliform Rule more stringent (7,8 ). To limit the public’s exposure to

pathogens, the SWTR requires water utilities to disinfect surface water and groundwater that is at risk for contamination by surface water; human enteric viruses and

Giardia lamblia cysts are target organisms for assessing the adequacy of treatment. In

addition, utilities that use such source waters must provide filtration, unless specific

criteria are met (e.g., with respect to turbidity, total or fecal coliforms in source water,

and the operation of a watershed-control program to minimize contamination). The

SWTR requires the monitoring of turbidity and various other parameters to demonstrate the adequacy of filtration and disinfection, respectively.

The Total Coliform Rule currently requires, under certain conditions, periodic field

inspections of water systems, testing water for Escherichia coli, and if water samples

are positive for total coliforms, additional sampling to evaluate the water quality. No

more than 5% of the samples collected in a given month can be positive for total coliforms. For systems in which <40 samples per month are required, no more than one

sample can be positive. In addition, a maximum-contaminant-level goal for fecal coliforms or E. coli is specified. Repeated sampling and testing within 24 hours is required

if these limits are exceeded.

The Enhanced Surface Water Treatment Rule, proposed July 29, 1994 (9 ), and the

Ground Water Disinfection Rule (to be proposed) also will address the prevention of

waterborne diseases. The former may require additional treatment beyond that mandated by the SWTR for systems with source waters of poor quality. It also may include

specific treatment requirements to ensure the removal of Cryptosporidium parvum

oocysts. The latter rule will be designed to protect groundwater systems from microbial contamination, particularly by viruses.

To emphasize the importance of having treatment plants operate optimally, EPA,

in conjunction with operators of water-treatment plants, has initiated a voluntary program. Under this program, water utilities can have peer review, in addition to their

self-assessment, of the performance of their treatment processes. Additionally, on

February 10, 1994, EPA proposed the Information Collection Rule (10 ) for the collection of data about the presence of several pathogens and disinfection by-products

(and selected precursors thereof) in water, as well as about the ability of treatment

plants to remove these substances. This rule would require utilities that obtain water

from surface-water sources and that provide service to ≥100,000 persons to test, for

18 months, source water (in some instances, also finished water) for C. parvum and

G. lamblia, total culturable viruses, total and fecal coliforms or E. coli, coliphages, and

Clostridium perfringens.

METHODS

Sources of Data

State, territorial, and local public health agencies have the primary responsibility

for detecting and investigating WBDOs and voluntarily reporting them to CDC on a

Vol. 45 / No. SS-1 MMWR 3
standard form (CDC form 52.12 [3 ]). CDC periodically requests reports from the state

and territorial epidemiologists or from persons designated as coordinators of WBDO

surveillance. When needed, additional information about water quality and treatment

is obtained from the state’s drinking-water agencies.

Definitions of Terms*

The surveillance system for WBDOs resembles that used for foodborne-disease

outbreaks and differs from other systems in that the unit of analysis is an outbreak

rather than an individual case of a particular disease. Two criteria must be met for an

event to be defined as a WBDO. First, at least two persons must have experienced a

similar illness after the ingestion of drinking water or after exposure to water used for

recreational purposes. Second, epidemiologic evidence must implicate water as the

probable source of the illness. The stipulation that at least two persons be ill is waived

for single cases of laboratory-confirmed, primary amebic meningoencephalitis and for

single cases of chemical poisoning if water-quality data indicate contamination by the

chemical. If primary and secondary cases are distinguished on the outbreak-report

form, only primary cases are included in the total number of cases. Outbreaks caused

by contamination of water or ice at the point of use (e.g., a contaminated serving container) are not classified as WBDOs and are, instead, reported to the surveillance

system for foodborne-disease outbreaks.

Public water systems, classified as either community or noncommunity water systems, provide piped water to the public for general consumption and are regulated

under the Safe Drinking Water Act. A community water system serves year-round residents of a community, subdivision, or mobile-home park that has ≥15 service

connections or an average of ≥25 residents. A noncommunity water system serves an

institution, industry, camp, park, hotel, or business that is used by the general public

for ≥60 days per year and has ≥15 service connections or serves an average of ≥25 persons. Of the approximately 215,000 water systems in the United States classified as

public systems, 158,000 (73.5%) are noncommunity systems, serving transients

(133,000 systems) and nontransients (25,000), and 57,000 (26.5%) are community systems (EPA, unpublished data, 1995). Individual water systems are defined as small

water systems, not owned or operated by a water utility, that serve <15 residences or

farms that do not have access to a public water system. Community and individual

water systems serve approximately 243 million persons in the United States (91% of

the U.S. population) and 24 million (9%), respectively. In addition, millions of persons

use noncommunity systems while traveling or working.

Each drinking-water system associated with a WBDO is classified as having one of

the following deficiencies:

1 = untreated surface water; 

2 = untreated groundwater; 

3 = treatment deficiency (e.g., temporary interruption of disinfection, chronically

inadequate disinfection, and inadequate or no filtration); 

*Additional terms are defined in the glossary.

4 MMWR April 12, 1996
4 = distribution system deficiency (e.g., cross-connection, contamination of

water mains during construction or repair, and contamination of a storage

facility); and

5 = unknown or miscellaneous deficiency (e.g., contaminated bottled water).

If more than one deficiency is noted on the report form for an outbreak, only the one

most likely to have caused the outbreak is noted.

Recreational waters encompass swimming pools, whirlpools, hot tubs, spas, water

parks, and naturally occurring fresh and marine surface waters. Although the surveillance system includes whirlpool- and hot tub-associated outbreaks of dermatitis

caused by Pseudomonas aeruginosa, it does not include wound infections resulting

from waterborne organisms (e.g., Aeromonas species).

Classification of Outbreaks

The surveillance system classifies WBDOs according to the strength of the evidence implicating water (Table 1). The classification numbers (i.e., I–IV) are based on

the epidemiologic data provided and the presence or absence of water-quality data on

the report form. Epidemiologic data are weighted more heavily than water-quality

data. Thus, although some outbreaks without water-quality data were included in this

summary, reports of the use of contaminated water were omitted if they did not provide supporting epidemiologic data. The classification numbers are included in the

line listings to indicate what data were available.

A classification of I means that adequate epidemiologic and water-quality data

were reported but does not necessarily imply that the investigation was optimal. Classification numbers of II–IV do not necessarily imply that the investigations were

flawed; the circumstances of each outbreak differ, and not all outbreaks can or should

be rigorously investigated. Outbreaks of pseudomonas dermatitis and single cases of

TABLE 1. Classification of investigations of waterborne-disease outbreaks — United

States

Class* Epidemiologic data Water-quality data 

I Adequate†

a) Data were provided about exposed

and unexposed persons; and 

b) the relative risk or odds ratio was ≥2,

or the p-value was ≤0.05.

Provided and adequate

Could be historical information or

laboratory data (e.g., the history that a

chlorinator malfunctioned or a water

main broke, no detectable free-chlorine

residual, or the presence of coliforms in

the water).

II Adequate Not provided or inadequate

(e.g., stating that a lake was crowded). 

III Provided, but limited

a) Epidemiologic data were provided

that did not meet the criteria for Class I; or

b) the claim was made that ill persons had

no exposures in common besides water,

but no data were provided.

Provided and adequate

IV Provided, but limited Not provided or inadequate

*The classification is based on the epidemiologic and water-quality data that were provided on

the outbreak-report form.

†Adequate data were provided to implicate water as the source of the outbreak.

Vol. 45 / No. SS-1 MMWR 5
primary amebic meningoencephalitis or of illness resulting from chemical poisoning

are not classified according to this scheme.

RESULTS

1993–1994 Outbreaks Associated With Drinking Water

For the period 1993–1994, 17 states and one territory reported a total of 30 outbreaks associated with drinking water. Eighteen outbreaks were reported for 1993 and

12 for 1994.

Thirteen (43.3%) of the 30 outbreak reports were classified as Class I (i.e., adequate

epidemiologic and water-quality data were provided), three (10.0%) as Class II, and

10 (33.3%) as Class III; three single cases of lead poisoning and one single case of

nitrate poisoning were not classified (13.3%). Outbreaks are listed individually by state

(Tables 2 and 3) and are tabulated by the etiologic agent and type of water system

(Table 4) and by the type of deficiency and type of water system (Table 5).

The months with the greatest number of outbreaks (five per month each) were

March, June, and August (Figure 1). The months of onset are not known for two

outbreaks. Seven (23.3%) of the 30 outbreaks were reported by two states, Pennsylvania (four outbreaks) and Minnesota (three). The outbreaks caused an estimated

405,366 persons to become ill: 403,000 persons during one outbreak and 2,366 during

the other 29. The median outbreak size was 33.5 persons (range: 1–403,000). An estimated 4,442 persons were hospitalized; 4,419 of these persons had cryptosporidiosis,

22 had bacterial gastroenteritis, and one had acute gastrointestinal illness of unknown

etiology (AGI). Seven persons infected with Salmonella serotype Typhimurium died,

as did an unknown number of persons who had cryptosporidiosis.

Etiologic Agents

Twenty-two (73.3%) of the 30 outbreaks were known or suspected to be associated

with infectious agents and eight (26.7%) with chemical contaminants. A protozoan

parasite (G. lamblia or C. parvum) was identified as the causative agent for 10 outbreaks, representing 33.3% of the 30 outbreaks and 40.0% of the 25 WBDOs for which

the etiologic agent was determined.

Cryptosporidium. Five outbreaks of cryptosporidiosis, which resulted in illness in

an estimated 403,271 persons, occurred in Minnesota (one outbreak), Nevada (one),

Washington (two), and Wisconsin (one). The outbreaks occurred in March (one), April

(one), August (two), and December (one). Three were associated with community

water systems and one each with either a noncommunity or an individual system;

three systems used lake water that was filtered and chlorinated, and two used untreated well water.

The outbreak of cryptosporidiosis in Milwaukee was the largest documented

WBDO in the United States since record keeping began in 1920. An estimated

403,000 persons became ill, of whom 4,400 were hospitalized (11,12 ). Accurate estimates of the number of deaths are not currently available. The outbreak was

associated with water that had been filtered and chlorinated after it was obtained

from Lake Michigan. Deterioration in raw-water quality and decreased effectiveness

of the coagulation-filtration process led to an increase in the turbidity of treated

6 MMWR April 12, 1996
TABLE 2. Waterborne-disease outbreaks associated with drinking water — United States, 1993 (N=18)* State Month Class† Etiologic agent No. of cases Type of system§ Deficiency¶ Source 

Arizona Jul —** Lead 1 Ind 4 Water-storage tank Arizona Aug — Lead 1 Ind 4 Water-storage tank California Nov — Lead 1 Ind 4 Water-storage tank Hawaii Oct I Fluoride 9 Com 4 Well Indiana ?†† I Nitrate 3§§ Ind 2 Well Minnesota Aug II Cryptosporidium parvum 27 NCom 5¶¶ Lake Minnesota Nov I Campylobacter jejuni 32 NCom 4*** Well 

Missouri Nov I Salmonella serotype Typhimurium 625 Com 4*** Well 

Mississippi Aug I Fluoride 34 Com 3 Well Nevada††† Dec§§§ II C. parvum 103¶¶¶ Com 5¶¶ Lake 

New York Jun III Campylobacter jejuni 172 Com 5 Well 

Pennsylvania Jan III Giardia lamblia 20 Com 3¶¶ Well 

Pennsylvania Mar I AGI**** 65 NCom 3 Well Pennsylvania Mar I Copper 43 Com 4¶¶ Reservoir South Dakota Sep III G. lamblia 7 Com 2 Well 

South Dakota Sep I AGI 40 NCom 2 Well Washington Apr III C. parvum 7 Ind 2 Well Wisconsin†††† Mar I C. parvum 403,000 Com 3§§§§ Lake 

 * Refer to the Methods section for a description of the reporting variables. † Refer to Table 1 for information concerning the classification of outbreaks.

§ NCom=noncommunity; Com=community; Ind=individual; refer to the Methods section for definitions of the types o¶ Refer to the Methods section for the classification of water-system deficiencies.

 ** According to the classification scheme, single cases of illness resulting from chemical poisonings are not classif

†† The month of onset is not known for this outbreak.

§§ Three pregnant women had a total of six miscarriages. ¶¶ The water treatment included filtration.

 *** The outbreak was associated with a contaminated water-storage tower. ††† The outbreak occurred in Clark County, which includes Las Vegas. §§§ The outbreak period was estimated to extend from December 1993 through June 1994. ¶¶¶ The outbreak involved mainly HIV-infected persons; estimates of the total number of infected persons in Clark 

available (refer to the text for details).

**** AGI=acute gastrointestinal illness of unknown etiology. †††† The outbreak occurred in Milwaukee.

§§§§ The water was inadequately filtered, but the treated water met all existing state and federal water-quality stand

that time.
TABLE 3. Waterborne-disease outbreaks associated with drinking water — United States, 1994 (N=12)* State† Month Class§ Etiologic agent No. of cases Type of system¶ Deficiency** Source 

Idaho Jun III Shigella flexneri 33 Ind 2 Well Indiana ?†† —§§ Nitrate 1¶¶ Ind 2 Well Indiana Mar II AGI*** 118 NCom 2 Well Maine Aug III AGI 72 NCom 2 Well Minnesota Jun I Campylobacter jejuni 19 NCom 2 Well 

New Hampshire May III Giardia lamblia 18 Com 3††† Reservoir 

New Hampshire May III G. lamblia 36 Com 3††† Lake New York Jun III Shigella sonnei 230 NCom 2 Well 

Northern Mariana Islands (Saipan) Jun III Non-O1 Vibrio cholerae 11 Com 5 Wells§§§ Pennsylvania Sep I AGI 200 NCom 3 Well Tennessee Mar I G. lamblia 304 Com 4 Reservoir¶¶¶ Washington Aug I Cryptosporidium parvum 134 Com 2 Well 

 *Refer to the Methods section for a description of the reporting variables. † This also includes one territory.

§ Refer to Table 1 for information concerning the classification of outbreaks.

¶ NCom=noncommunity; Com=community; Ind=individual; refer to the Methods section for definitions of the types o

 **Refer to the Methods section for the classification of water-system deficiencies. †† The month of onset is not known for this outbreak.

§§ According to the classification scheme, single cases of illness resulting from chemical poisonings are not classifie

¶¶ One pregnant woman had two miscarriages.

***AGI=acute gastrointestinal illness of unknown etiology. ††† Unfiltered surface water was used.

§§§ Water was treated with reverse osmosis; refer to the text for details of the outbreak.

¶¶¶ In addition, spring water, which was chlorinated but not filtered, was used for source water.
TABLE 4. Waterborne-disease outbreaks associated with drinking water, by etiologic

agent and type of water system — United States, 1993–1994 (N=30)*

Etiologic agent

Type of water system†

Community Noncommunity Individual Total

Outbreaks Cases Outbreaks Cases Outbreaks Cases Outbreaks Cases

Cryptosporidium 

parvum 3 403,237 1 27 1 7 5 403,271

AGI§ 0 0 5 495 0 0 5 495

Giardia lamblia 5 385 0 0 0 0 5 385

Campylobacter

jejuni 1 172 2 51 0 0 3 223

Lead 0 0 0 0 3 3 3 3

Fluoride 2 43 0 0 0 0 2 43

Nitrate 0 0 0 0 2 4 2 4

Salmonella

serotype

Typhimurium 1 625 0 0 0 0 1 625

Shigella sonnei 0 0 1 230 0 0 1 230

Copper 1 43 0 0 0 0 1 43

Shigella flexneri 0 0 0 0 1 33 1 33

Non-O1 Vibrio

cholerae 1 11 0 0 0 0 1 11

Total 14 404,516 9 803 7 47 30 405,366

(Percentage¶) (46.7) (99.8) (30.0) (<1.0) (23.3) (<1.0) (100.0) (100.0)

* Ordered by total number of outbreaks and secondarily by total number of cases.

† Refer to the Methods section for definitions of the types of water systems.

§ AGI=acute gastrointestinal illness of unknown etiology.

¶ The percentage is based on 30 outbreaks or 405,366 cases.

TABLE 5. Waterborne-disease outbreaks associated with drinking water, by type of

deficiency and type of water system — United States, 1993–1994 (N=30)

Type of deficiency†

Type of water system*

 Community Noncommunity Individual Total

No. (%) No. (%) No. (%) No. (%)

Untreated surface water 0 ( 0.0) 0 ( 0.0) 0 ( 0.0) 0 ( 0.0)

Untreated groundwater 2 ( 14.3) 5 ( 55.6) 4 ( 57.1) 11 ( 36.7)

Treatment 5 ( 35.7) 2 ( 22.2) 0 ( 0.0) 7 ( 23.3)

Distribution system 4 ( 28.6) 1 ( 11.1) 3 ( 42.9) 8 ( 26.7)

Unknown 3 ( 21.4) 1 ( 11.1) 0 ( 0.0) 4 ( 13.3)

Total 14 (100.0) 9 (100.0) 7 (100.0) 30 (100.0)

*Refer to the Methods section for definitions of the types of water systems.

†Refer to the Methods section for the classification of water-system deficiencies.

Vol. 45 / No. SS-1 MMWR 9
water* and to inadequate removal of C. parvum oocysts (11 ). Although the treated

water met all state and federal quality standards that were then in effect, C. parvum

oocysts were found in ice blocks that were made during the outbreak period. The

original environmental source of the oocysts was not definitively determined.

Factors that contributed to the recognition of the Milwaukee outbreak included

widespread absenteeism among hospital employees, students, and school teachers;

increased numbers of emergency room visits for diarrheal illness; and a citywide

shortage of antidiarrheal drugs. The etiologic agent and the waterborne nature of the

outbreak were not identified until at least 2 weeks after the onset of the outbreak.

Thereafter, a boil-water advisory was issued.

The outbreak of cryptosporidiosis in Clark County, Nevada, which includes

Las Vegas, lasted approximately 7 months and was first recognized among persons

infected with the human immunodeficiency virus (HIV) (13 ). Two major factors contributing to the detection of the outbreak were that cryptosporidiosis had been a

reportable disease in Nevada for several years, and that many persons who have acquired immunodeficiency syndrome live in Clark County. Cryptosporidiosis is more

likely to be diagnosed in such persons because they usually have severe symptoms

and often have physicians who are knowledgeable about this disease and know how

to diagnose it. Testing for C. parvum is not done routinely in most microbiology laboratories.

J FMAMJ J ASOND

0

1

2

3

4

5

6

7

Month

Number of outbreaks

Infectious or suspected

infectious etiology

Chemical poisoning

Drinking water (N=28 ) Recreational water (N=26)

Encephalitis

Dermatitis

Gastrointestinal illness

J F M A M J J A S O N D

0

1

2

3

4

5

6

7

Number of outbreaks

Month

†

*The number of outbreaks for each month is the total for that month during the 2-year period.

†Two outbreaks of nitrate poisoning are not included.

FIGURE 1. Number of waterborne-disease outbreaks, by type of water, type of

etiologic agent or illness, and month — United States, 1993–1994*

*Reported values were as high as 1.7 nephelometric turbidity units; for the previous 10 years,

the recorded values had never exceeded 0.4 (11 ). At the time of the outbreak, national

standards for treated water required that 95% of all daily turbidity measurements in a month

not exceed 1.0 nephelometric turbidity unit (6 ).

10 MMWR April 12, 1996
HIV-infected persons who were diagnosed with cryptosporidiosis during the outbreak period had an increased likelihood of dying by the end of the outbreak period

but did not have an increased 1-year mortality rate. Data obtained during the investigation of the outbreak indicated that cryptosporidiosis also affected non-HIV–infected

persons. However, no estimates of the total number of persons in the general population who had outbreak-related illness are available. Drinking water was not implicated

until after the end of the outbreak; therefore, a boil-water advisory was not issued.

The Nevada outbreak of cryptosporidiosis was associated with water obtained

from Lake Mead that was filtered and chlorinated during treatment. Before and during

the outbreak, the water quality was much better than that required by current national

standards. For example, in the 3 years before the outbreak, the mean daily turbidity

levels for source and treated water had been <0.2 and <0.1 nephelometric turbidity

units, respectively, with no marked increases before or during the outbreak.* Although no C. parvum oocysts were detected in the source or finished water during the

outbreak period, presumptive oocysts were found in both types of water and in filter

backwash that was sampled thereafter. As noted previously, the outbreak of cryptosporidiosis in Minnesota also was associated with filtered and chlorinated lake water;

C. parvum oocysts and G. lamblia cysts were found in lake water but not in finished

water.

The two outbreaks of cryptosporidiosis in Washington were associated with well

water. The first outbreak, which occurred in 1993, was associated with untreated water

from a shallow well that was contaminated by surface water. The second outbreak, in

1994, was associated with a damaged irrigation system that allowed treated waste

water to flow into a well. For both outbreaks, presumptive C. parvum oocysts were

found in the well water. In addition, for one of these outbreaks, fecal coliforms were

found; for the other, total coliforms were found.

Giardia. The five outbreaks of giardiasis associated with drinking water affected an

estimated 385 persons and were reported from New Hampshire (two outbreaks),

Pennsylvania (one), South Dakota (one), and Tennessee (one). The outbreaks occurred

in January (one), March (one), May (two), and September (one). All were associated

with community water systems; three were associated with surface water and two

with well water.

The two outbreaks of giardiasis in New Hampshire were associated with unfiltered,

chlorinated surface water. For one outbreak, water from a reservoir tested positive for

G. lamblia. For the other outbreak, finished water was positive for total coliforms. For

both outbreaks, some of the cases were geographically clustered close to the sites

where the water was chlorinated (i.e., 27.8% of the former and 50.0% of the latter

cases were clustered along proximal portions of the main lines of the respective distribution systems). This suggests that, for these cases, the chlorine contact time was

inadequate.

In Tennessee, a cross-connection between potable and waste-water lines was associated with an outbreak of giardiasis in a correctional facility (14 ). Potable water was

used to cool the seals of a waste-water pump; a fall in pressure in the potable-water

system probably caused waste water to flow back into the potable-water line. Subsequently, high levels of G. lamblia cysts were found in tap water (581 cysts/L). Of

*Current national standards for treated water require that 95% of all daily turbidity measurements in a month not exceed 0.5 nephelometric turbidity units (6 ).

Vol. 45 / No. SS-1 MMWR 11
424 stool specimens collected from persons in the correctional facility, 110 (25.9%)

were positive for G. lamblia and 42 others (9.9%) for Entamoeba histolytica. 

The outbreak of giardiasis in Pennsylvania was associated with filtered and chlorinated well water that was contaminated with sewage. Tests indicated that G. lamblia

and E. coli were present in tap water. The outbreak in South Dakota was attributed to

untreated well water contaminated by water from a nearby creek; G. lamblia was

found in the well water, and fecal coliforms were found in both well and tap water.

Bacteria. Seven outbreaks were caused by bacteria; five of these outbreaks were

associated with untreated well water, one with chlorinated well water, and one with

bottled water. The outbreak in Missouri that was caused by Salmonella serotype

Typhimurium resulted in illness in an estimated total of 625 persons, including 15 persons who were hospitalized and seven who died. The most likely source for the

outbreak was the larger of two storage towers, which was inadequately protected

from wild-bird droppings. S. Typhimurium was isolated from the sediment of one of

the towers, and tap water was positive for fecal coliforms. The 1993 outbreak in Minnesota that was caused by Campylobacter jejuni was associated with untreated well

water, which probably was contaminated in a storage tower that had been cleaned the

previous month. During the investigation of this outbreak, fecal coliforms were found

in water from the storage tower.

In Saipan, Northern Mariana Islands, an outbreak caused by non-O1 Vibrio cholerae

and associated with contaminated bottled water caused illness in 11 persons, four of

whom were hospitalized. Because public tap water in most locales in Saipan is too

salty to be drinkable, two commercial bottled-water plants treat municipal water* with

reverse osmosis to supply drinking water to the public. Although the bottles were to

have been cleaned by machine or manually with hot water and a chlorine solution, the

bottling plants had occasionally been cited for the cursory handling of returned bottles

(e.g., for only rinsing them with treated water). During the outbreak period, bottled

water tested positive for fecal coliforms. The source of the contamination was not

determined.

Chemicals. Eight outbreaks of chemical poisoning were reported, three of which

were individual cases of lead poisoning. Two of these three cases were detected by a

lead-screening program. The three cases were in infants who had blood-lead levels of

15 μg/dL, 37 μg/dL, and 42 μg/dL. CDC’s level of concern is 10 μg/dL (15 ). For all three

cases, lead had leached from brass fittings and lead-soldered seams in drinking-water

storage tanks (16 ). The three water systems used highly demineralized drinking water

that had intensified the leaching process. The cases are counted separately (Table 2)

because it is not known whether lead had leached from the storage tanks because of

the way they were manufactured (at least two of the storage tanks were made by the

same company) or because of repairs or other changes made to them after they left

the manufacturer(s).

In the two outbreaks of fluoride poisoning (i.e., in Mississippi and Hawaii), more

than half the exposed persons became ill (34 of 62 and 9 of 16, respectively). Symptoms included nausea, diarrhea, abdominal pain, headache, and dizziness. For both

outbreaks, the water was accidentally overdosed with fluoride because the fluoridation chemical was siphoned through the pump into the drinking water. This resulted in

*The municipal water supply uses several dozen wells, some of which tested positive for fecal

coliforms.

12 MMWR April 12, 1996
tap-water levels of fluoride in the Mississippi outbreak of 48–200 mg/L and in Hawaii

of 220 mg/L. When levels exceed 10 mg/L, the fluoridation system should be turned off

immediately (17 ).

Elevated copper levels in tap water were associated with gastrointestinal illness in

at least 43 persons at a hotel. With the exception of one water sample that had a

copper level of 156 mg/L, the highest copper level documented for the many other

tested samples was 4.7 mg/L (EPA’s action level is 1.3 mg/L [18 ]). The source of copper

was the plumbing system within the building. Prolonged stagnation of the water in the

pipes contributed to the outbreak.

In Indiana, two episodes of nitrate poisoning were associated with eight miscarriages. In the first episode, three women living within 1 mile (1.6 km) of each other

reported a total of six miscarriages during 1993 and 1994 (W.F. Grant, personal communication, 1996). Five of the miscarriages occurred in the eighth gestational week,

and one occurred in the eleventh. All three women drank water from private wells on

their premises; all three wells tapped the same aquifer, which probably was contaminated by a leaking waste pit on a nearby hog farm. Water from the three wells had

high nitrate (nitrate-nitrogen or NO3-N) levels: 19.0 mg/L, 19.2 mg/L, and 26.0 mg/L.

EPA’s maximum-contaminant level is 10 mg/L (4 ). Additional testing of well water did

not demonstrate elevated levels of volatile or semi-volatile organics, pesticides, heavy

metals, or coliform bacteria. All three women delivered full-term babies after switching to nitrate-free sources of drinking water. During the same period, five women who

lived in the same geographic area and who had wells with nitrate levels ranging from

1.6 mg/L to 8.4 mg/L (median: 1.7 mg/L) had five normal pregnancies.

In the second episode (1994), a woman with five children had two miscarriages

within a 6-month period after moving to a new residence in Indiana, which was located about 12 miles (19 km) from the residences of the three women described in the

first episode. Both of these pregnancies terminated in their eighth week. The private

well at her residence had a nitrate level of 28.7 mg/L and was thought to have been

contaminated by the family’s septic system.

Unidentified Etiologic Agent. No etiologic agent was identified for five (16.7%) of

the 30 WBDOs associated with drinking water. The illnesses associated with two of

these outbreaks had incubation periods, durations, and symptom complexes that

were consistent with viral syndromes. For the four outbreaks for which testing was

done, stool specimens were negative for bacterial pathogens. Three outbreaks were

associated with untreated well water and the other two with inadequate chlorination

of unfiltered well water. For four outbreaks, fecal coliforms were found in water

samples.

Water-Quality Data

Water-quality data (e.g., information as to the presence of coliform bacteria, pathogens, or chemical contaminants) were obtained ≤1 month after the onset of the

outbreak for 28 outbreaks and later for the two episodes of nitrate poisoning. Water

samples were tested for coliform bacteria for 21 of the 22 WBDOs that had a known or

suspected infectious etiology; the samples were positive for total or fecal coliforms for

16 (76.2%) of the 21 outbreaks. Coliforms were detected for 11 (91.7%) of the 12 outbreaks of bacterial or unknown etiology but for only five (55.6%) of the nine protozoan

outbreaks for which testing was done. For these nine outbreaks, coliforms were

Vol. 45 / No. SS-1 MMWR 13
detected for two of the six systems with chlorinated water but for all three with untreated water. For three of the nine outbreaks, water was positive for C. parvum; for

another three, it was positive for G. lamblia. For one (14.3%) of the seven outbreaks

with a bacterial etiology, the pathogen (Salmonella) was isolated from the water.

Water System and Water Source

Fourteen (46.7%) of the 30 WBDOs were associated with community systems, nine

(30.0%) with noncommunity systems, and seven (23.3%) with individual systems

(Tables 4, 5, Figure 2). In contrast, for 1991–1992, more of the reported WBDOs were

associated with noncommunity (64.3%) than with community systems (26.2%) (2 , updated with data from previously unreported outbreaks). During 1993–1994, outbreaks

in noncommunity systems were more likely than those in community systems to be

associated with untreated water (55.6% versus 14.3%). Eight (88.9%) of the nine outbreaks in the noncommunity systems were associated with well-water sources, as

were eight (57.1%) of the 14 community outbreaks.

Of the 22 outbreaks with a known or suspected infectious etiology, 16 (72.7%)

occurred in systems using well water, and six (27.3%) were associated with a surfacewater source. For 11 (68.8%) of the 16 well-water systems associated with the

outbreaks, the water was untreated. For four (25.0%) of these 16 systems, all of which

used chlorine for disinfection, inadequate or interrupted disinfection was the deficiency identified or suspected (e.g., coliforms, which are chlorine sensitive, were

present in tap water). For one of these four systems, deficiencies in filtration also were

suspected. For one (6.3%) of the 16 systems (i.e., in Saipan), no treatment deficiency

was noted.

Water Source

Well

66.7%

Surface water

23.3%

Other

10.0%

Water System

Noncommunity

30.0%

Individual

23.3%

Community

46.7%

Deficiency

Untreated

groundwater

36.7%

Treatment

deficiency

23.3%

Unknown

13.3%

Etiologic Agent

Parasitic

33.3% Chemical

26.7%

Bacterial

23.3%

Unidentified

16.7%

Distribution

system

26.7%

N=30

FIGURE 2. Waterborne-disease outbreaks associated with drinking water, by etiologic

agent, water system, water source, and deficiency — United States, 1993–1994 

14 MMWR April 12, 1996
Six (85.7%) of the seven outbreaks associated with a lake or reservoir (i.e., surfacewater sources) were caused by C. parvum (three outbreaks) or G. lamblia (three). Each

of the six systems associated with these six outbreaks provided chlorination, and four

also provided filtration. In the filtered systems, deficiencies in the distribution system

were identified for one outbreak, inadequate filtration was identified for one, and no

deficiencies were identified for two. In the two unfiltered systems associated with outbreaks of giardiasis, an inadequate contact time with the disinfectant was suspected.

1993–1994 Outbreaks Associated With Recreational Water

For the period 1993–1994, 14 states reported a total of 26 outbreaks associated with

recreational water (Tables 6,7). Fourteen outbreaks were reported for 1993 and 12 for

1994. Three of the 14 states submitted 14 (53.8%) of the 26 reports: Minnesota

(seven outbreaks), Wisconsin (four), and New Jersey (three). For the 2-year reporting

period, the months with the greatest number of outbreaks were July (six) and August

(five) (Figure 1). All 14 outbreaks of gastrointestinal illness occurred during the

months of April through September. Eight of the 11 outbreaks of dermatitis (i.e., rash

or folliculitis), which were associated with hot tubs, whirlpools, and swimming pools,

occurred during the relatively colder months of November through March.

The 26 outbreaks caused illness in an estimated 1,714 persons. The median

outbreak size was 32.5 persons (range: 1–418). The types of reported illnesses included gastroenteritis (14 outbreaks), dermatitis (11), and meningoencephalitis (one).

Fifteen persons reportedly were hospitalized. The one child who had amebic meningoencephalitis died.

The etiologic agent was identified for all 14 outbreaks of swimming-associated gastroenteritis (Table 6, Figure 3). Ten (71.4%) of the 14 outbreaks were caused by the

protozoan parasites C. parvum (six outbreaks) and G. lamblia (four). Five of the

six C. parvum outbreaks were associated with motel (three) or community (two)

swimming pools that were filtered and chlorinated (19,20 ). For four of these five

outbreaks, no treatment deficiencies were identified in the pools; for the other, a malfunctioning filter was found. For two of these five, water was tested and was negative

for C. parvum. The tested water, however, had been obtained approximately 2 weeks

after the onset of the outbreaks; for one of the outbreaks, water samples were not

collected until after the pool had been hyperchlorinated.

The first reported U.S. outbreak of cryptosporidiosis linked to the recreational use

of lake water occurred in New Jersey during the summer (1994) and lasted >4 weeks.

In this outbreak, C. parvum was detected in lake-water samples obtained 5 weeks after

the end of the outbreak period.

The four G. lamblia outbreaks were related to unintentional ingestion of water from

two lakes (two outbreaks), a river (one), and a community swimming pool and wading

pool (one). In the latter outbreak, which lasted >3 months, intermittent breakdowns of

the swimming pool’s filter and lack of water filtration in the wading pool were noted.

Water samples from the two lakes that were associated with outbreaks were negative

for G. lamblia. However, samples from one lake were obtained 2 weeks after the onset

of the outbreak, and samples from the other lake were obtained 4 weeks after the

outbreak began.

Four outbreaks of gastroenteritis were attributed to bacterial pathogens and

were associated with swimming in lakes. S. sonnei was implicated for two of these

Vol. 45 / No. SS-1 MMWR 15
TABLE 6. Waterborne-disease outbreaks of gastroenteritis and meningoencephalitis associated with recreational States, 1993–1994 (N=15) State Year Month Class* Illness Etiologic agent No. of cases Source Indiana 1994 Jun I Gastroenteritis Giardia lamblia 80 Pool 

Maryland 1993 Jul II Gastroenteritis G. lamblia 12 Lake Minnesota 1994 May II Gastroenteritis Shigella flexneri 35 Lake Missouri 1994 Jul I Gastroenteritis Cryptosporidium parvum 101 Pool New Jersey 1993 Sep IV Gastroenteritis G. lamblia 43 Lake New Jersey 1994 Jun II Gastroenteritis Shigella sonnei 242 Lake New Jersey 1994 Jul I Gastroenteritis C. parvum 418 Lake New York 1994 Jul II Gastroenteritis Escherichia coli O157:H7 166 Lake 

Ohio 1993 Jul III Gastroenteritis S. sonnei 160 Lake 

Texas 1994 Jul —† Meningoencephalitis Naegleria fowleri 1 Pond/river Washington 1993 Aug III Gastroenteritis G. lamblia 6 River Wisconsin 1993 Apr II Gastroenteritis C. parvum 51 Pool Wisconsin 1993 Aug II Gastroenteritis C. parvum 5 Pool Wisconsin 1993 Aug II Gastroenteritis C. parvum 54 Pool Wisconsin 1993 Aug II Gastroenteritis C. parvum 64 Pool 

*Refer to Table 1 for information concerning the classification of outbreaks.

†According to the classification scheme, single cases of primary amebic meningoencephalitis are not classified.
TABLE 7. Waterborne-disease outbreaks of dermatitis associated with recreational water — United States, 19State Year Month Class* Etiologic agent No. of cases Source SettAlaska 1993 Aug

—† Pseudomonas

aeruginosa§

 4¶ Hot tub Bed 

Alaska 1993 Dec — P. aeruginosa 2 Pool/whirlpool ResoColorado 1994 Feb — P. aeruginosa 10 Hot tub Ski rMinnesota 1993 Jan — P. aeruginosa§ 14 Hot tub HoteMinnesota 1993 Mar — P. aeruginosa§ 6 Hot tub HoteMinnesota 1993 Apr — P. aeruginosa§ 15 Pool SchoMinnesota 1993 May — P. aeruginosa 53¶ Pool SchoMinnesota 1994 Jan II Chemical** 9 Pool HoteMinnesota 1994 Feb — P. aeruginosa 30 Pool HoteSouth Dakota 1994 Mar — P. aeruginosa 113†† Pool/hot tub LodgUtah 1994 Nov IV Chemical** 20 Pool Hosp

 *Refer to Table 1 for information concerning the classification of outbreaks. †According to the classification scheme, outbreaks of pseudomonas dermatitis are not classified.

§The clinical syndrome was consistent with that caused by P. aeruginosa. ¶This includes patients with otitis externa.

**The clinical syndrome was suggestive of a chemical etiology. †† This includes patients with otitis externa and pharyngitis.
outbreaks, S. flexneri for one, and E. coli O157:H7 for one. The latter was the second

reported outbreak in the United States of E. coli O157:H7 associated with recreational

water (2 ). 

For the outbreak in New Jersey that was caused by S. sonnei, water samples obtained 4 weeks after the onset of the outbreak were negative for the etiologic agent.

For the outbreak in Ohio that was caused by S. sonnei, water samples demonstrated

high fecal-coliform counts. The presence of many bathers and poor water exchange

were contributing factors for at least one of the four outbreaks. For another, lake water

used in a shower frequented by toddlers was circulated back into the lake; high fecalcoliform counts were found in water from the shower’s drain basin.

An estimated 247 persons were affected in nine outbreaks of dermatitis that were

associated with hot tubs, whirlpools, or swimming pools. These nine outbreaks had a

known or suspected infectious etiology (Table 7). P. aeruginosa was confirmed as the

etiologic agent in five of the outbreaks and was suspected (based on the clinical

syndrome) in the other four. In six of these nine investigations, water samples demonstrated low chlorine levels, the presence of Pseudomonas, or both. 

The two outbreaks of probable chemical dermatitis affected an estimated 29 persons. These outbreaks were associated with incorrect dosing of acidifying chemicals

used for correcting the pH of the swimming-pool water or with addition of unspecified

chemicals for removing excess chloramines.

One case of fatal primary amebic meningoencephalitis was associated with swimming in both a waste-water holding pond and in the Rio Grande River. The etiologic

 spp.

21.4%

Lake

50%

River

7.1%

Cryptosporidium

parvum

Giardia

lamblia

Etiologic Agent

Escherichia

coli 

Type of Exposure

Swimming pool

42.9%

O157:H7

28.6%

42.9%

7.1%

N=14

Shigella

FIGURE 3. Waterborne-disease outbreaks of gastroenteritis associated with

recreational water, by etiologic agent and type of exposure — United States,

1993–1994

18 MMWR April 12, 1996
agent, Naegleria fowleri, was found in the child’s cerebrospinal fluid and in both the

pond and the river.

Previously Unreported Outbreaks

Reports of 12 previously unpublished WBDOs from 1992 also were received

(Tables 8, 9). For the eight outbreaks associated with drinking water, an estimated

220 persons were reported ill, six of whom were hospitalized. One outbreak caused by

S. sonnei was associated with untreated water from a noncommunity well and

another with an individual cistern. An outbreak caused by hepatitis A virus was associated with an untreated supply of well water (noncommunity) and is the only

outbreak in this surveillance summary in which a viral pathogen was identified (21 ).

Two outbreaks of AGI were associated with noncommunity well-water systems; for

both, coliforms were found in tap water. For one of these outbreaks, the clinical features were consistent with a viral syndrome. Three outbreaks of gastrointestinal

illness were associated with newly installed, copper plumbing systems. For these

three outbreaks, tap water had elevated levels of copper (22 ).

The four WBDOs associated with recreational water caused an estimated total of

203 persons to become ill, one of whom was hospitalized. Two outbreaks of gastroenteritis caused by S. sonnei were associated with swimming in lakes; high total

coliform counts were found in the swimming areas of both lakes. In the New Jersey

outbreak caused by S. sonnei, a bulkhead had been placed in the lake, which essentially created a swimming pool that had insufficient exchange of water. In a third

report, no etiologic agent was identified for an outbreak of AGI associated with swimming in a pool at a water park; the clinical features were consistent with a viral

syndrome. Before the outbreak, muddy run-off from a landscaping project had caused

the pool’s filter to break down. In a fourth report, a clinical syndrome consistent with

schistosomal dermatitis (i.e., swimmer’s itch) was noted for a New Jersey outbreak

associated with swimming in a lake. 

TABLE 8. Waterborne-disease outbreaks associated with drinking water that were not

included in the previous surveillance summary — United States, 1992 (N=8)*

State Month Class†

Etiologic

agent

No. of

cases

Type of

system§ Deficiency¶ Source Setting

Minnesota May I AGI** 70 NCom 5 Well Hotel

Missouri Jan III Shigella

sonnei

 5 Ind 4 Cistern Private

home

Missouri Apr III Hepatitis

A virus

46 NCom 2 Well School/

church

Missouri Jul I S. sonnei 11 NCom 4 Well Campground

Pennsylvania May I AGI 50 NCom 3 Well Camp

Wisconsin Jan I Copper 24 Com 4 Well Subdivision

Wisconsin Sep I Copper 8 Com 4 Lake Building

Wisconsin Dec III Copper 6 Com 5 Well House

 *Refer to the Methods section for a description of the reporting variables.

†Refer to Table 1 for information concerning the classification of outbreaks.

§NCom=noncommunity; Com=community; Ind=individual; refer to the Methods section for

definitions of the types of water systems.

¶Refer to the Methods section for the classification of water-system deficiencies.

** AGI=acute gastrointestinal illness of unknown etiology.

Vol. 45 / No. SS-1 MMWR 19
Outbreaks Not Classified as Waterborne-Disease Outbreaks

Outbreaks attributed to drinking water that was contaminated at its point of use

rather than at its source or in its distribution system traditionally are not classified as

WBDOs. Three such outbreaks of AGI were reported; they caused illness in a total of

376 persons. The outbreaks were associated with the contamination of two water

jugs, a water hose, or ice from an ice chest.

Data relating to 10 other possible WBDOs were not included. One of these outbreaks was more likely to have been foodborne than waterborne, and one was a

report of two cases of leptospirosis among persons exposed to flood waters during

clean-up operations (23 ). For eight of these possible WBDOs, inadequate data were

provided (i.e., the outbreaks did not meet the criteria for Classes I–IV).

DISCUSSION

General Considerations About Surveillance Data for

Waterborne-Disease Outbreaks

The surveillance data, which identify the types of water systems, their deficiencies,

and the respective etiologic agents associated with the outbreaks, are useful for

evaluating the adequacy of current technologies for providing safe drinking and recreational water. However, the data in this surveillance summary are subject to at least

one important limitation: they probably do not reflect the true incidence of WBDOs or

the relative incidence of outbreaks caused by various etiologic agents. Not all WBDOs

may be recognized, investigated, and/or reported to CDC or EPA, and the extent to

which WBDOs are unrecognized and underreported is unknown.

The likelihood that individual cases of illness will be epidemiologically linked and

associated with water varies considerably among locales and is dependent on factors

such as public awareness, physician interest, availability of laboratory-testing facilities, and surveillance activities of state and local health and environmental agencies.

Therefore, the states that report the most outbreaks may not be those in which the

most outbreaks occur. Recognition of WBDOs also is dependent on certain outbreak

characteristics; outbreaks involving serious illness are most likely to receive the attention of health authorities. In cities, large outbreaks are more likely to be recognized

TABLE 9. Waterborne-disease outbreaks associated with recreational water that were

not included in the previous surveillance summary — United States, 1992 (N=4)*

State Month Class† Illness Etiologic agent

No. of

cases Source Setting

California Aug I Gastroenteritis AGI§ 100 Pool Water park

New Jersey Jun I Gastroenteritis

Shigella sonnei

 54 Lake Campground

New Jersey Jul III Dermatitis Schistosoma

sp.¶

 40 Lake Lake

Virginia Jul III Gastroenteritis S. sonnei 9 Lake Camp

*Refer to the Methods section for a description of the reporting variables.

†Refer to Table 1 for information concerning the classification of outbreaks.

§AGI=acute gastrointestinal illness of unknown etiology.

¶The clinical syndrome was consistent with that caused by a Schistosoma sp.

20 MMWR April 12, 1996
than sporadic cases or small outbreaks in which ill persons may consult different

physicians. Outbreaks associated with community water systems are more likely to be

recognized than those associated with noncommunity systems because the latter

serve nonresidential areas and transient populations. Outbreaks associated with individual systems are the most likely to be underreported because they generally involve

few persons. Outbreaks of acute diseases, particularly those characterized by a short

incubation period, are more readily identified than those associated with disease from

chronic, low-level exposure to an agent (e.g., a chemical).

The identification of the etiologic agent of a WBDO is dependent on the timely recognition of the outbreak so that appropriate clinical and environmental samples can

be obtained. The interests and expertise of investigators and the routine practices of

local laboratories also influence whether the etiologic agent is identified. For example,

diarrheal stool specimens generally are examined for bacterial pathogens but not for

viruses. In most laboratories, testing for C. parvum is done only on request and is not

included in routine stool examinations for ova and parasites. The water-quality data

that are collected vary widely among outbreak investigations, depending on such factors as available fiscal, investigative, and laboratory resources. Furthermore, a few

large outbreaks may substantially alter the relative proportion of cases of waterborne

disease attributed to a particular agent. The number of reported cases is generally an

approximate figure, and the method and accuracy of the approximation vary among

outbreaks. 

1993–1994 Outbreaks Associated With Drinking Water

The numbers of outbreaks reported for 1993 (18) and 1994 (12) are comparable with

those reported for recent years (except for 1992). With the addition of previously unreported outbreaks, 27 WBDOs have been reported for 1992 (the highest number since

1984), 15 for 1991, 14 for 1990, and 13 for 1989 (2,3 ). WBDO reports peaked during

1979–1983 (Figures 4, 5). The increase and subsequent decrease in the number of reports may reflect, at least in part, changes in surveillance activities rather than a

deterioration or an improvement in water systems (24 ). Having only 16.7% of the

WBDOs reported for 1993–1994 of unknown etiology (i.e., AGI outbreaks) is unprecedented for the last 24 years (Figure 4). All AGI outbreaks occurred in noncommunity

systems, which underscores the difficulty of investigating outbreaks affecting the

transient populations that typically use the water from these systems. The massive

outbreak in Milwaukee highlights the need for surveillance methods that facilitate the

early detection of outbreaks and the prompt institution of preventive measures (e.g.,

boil-water advisories). This outbreak also demonstrates that WBDOs are not restricted

to small communities, in which WBDOs are often attributed to a lack of proper equipment or inadequately trained operators.

As in previous years, protozoan parasites were the most frequently identified etiologic agents; for the period 1993–1994, C. parvum and G. lamblia were each identified

for five outbreaks. The increased number of reported outbreaks of cryptosporidiosis in

recent years may reflect the growing awareness that C. parvum can be transmitted by

the waterborne route. For the period 1976–1994 as a whole, G. lamblia was the most

frequently identified etiologic agent for outbreaks associated with drinking water. The

occurrence of the two outbreaks of giardiasis in New Hampshire (1994) underscores

the importance of requiring water systems that use unfiltered surface water to provide

Vol. 45 / No. SS-1 MMWR 21
an adequate chlorine concentration and contact time (as specified by the SWTR) to

inactivate relatively chlorine-resistant organisms such as G. lamblia (25 ).

C. parvum oocysts have been detected in 65%–97% of the surface-water supplies

throughout the United States that have been tested recently (26–28 ). Because C. parvum is highly resistant to the chemical disinfectants typically used to treat drinking

water,* the physical removal of the parasite by filtration (e.g., rapid-granular, slow

sand, or membrane filtration) is an important component of the municipal water-treatment process. Rapid-granular filtration with sand or some other media, alone or in

combination, is used by many municipalities as part of their conventional treatment of

surface water. To be effective in removing oocysts, rapid-granular filtration must be

preceded by chemical coagulation and the treatment must be optimized to remove

particles. Even when done appropriately, rapid-granular filtration cannot guarantee

the removal of all the oocysts.

Some major cities use surface-water sources but do not filter their water (e.g., Boston, New York City, Portland, San Francisco, and Seattle). They are, however, either

proceeding with plans to filter their water, or, based on criteria set forth in the SWTR,

are not required to filter it.

From 1984 through 1992, all reported U.S. outbreaks of cryptosporidiosis associated with drinking water occurred in communities whose water utilities met the

then-current state and federal standards for acceptable drinking-water quality. The

surface-water supplies that were implicated in these outbreaks had been filtered

1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993

0

10

20

30

40

50

60

Year

Number of outbreaks

AGI*

Parasitic

Bacterial

Viral

Chemical

N=647

*Acute gastrointestinal illness of unknown etiology.

FIGURE 4. Number of waterborne-disease outbreaks associated with drinking water,

by year and etiologic agent — United States, 1971–1994

*For example, at 77 F (25 C) and pH 7, the chlorine contact time to inactivate 99% of C. parvum

oocysts is 7,200 (mg/L) x min (29 ).

22 MMWR April 12, 1996
(2,30–32 ). In 1993 and 1994, the water systems implicated in the three surface waterassociated outbreaks of cryptosporidiosis also met federal and state standards. The

occurrence of such outbreaks indicates that compliance with EPA’s water- treatment

standards (e.g., for turbidity and coliform counts) did not adequately protect against

waterborne cryptosporidiosis. For example, C. parvum can be present in chlorinated

water despite the absence of coliforms. That coliforms were detected for only 55.6%

of the protozoan outbreaks (vs. for 91.6% of the outbreaks of bacterial or unknown

etiology) suggests that the use of coliforms as indicators of protozoan contamination

is not reliable. Efforts to reduce the risk for WBDOs of cryptosporidiosis should focus

on the protection of source water, the removal of oocysts by filtration, and research to

identify effective and affordable disinfection procedures.

Decreased filtration effectiveness, combined with deterioration in raw-water quality, contributed to the outbreak of cryptosporidiosis in Milwaukee. Optimal filtration

requires frequent, if not continuous, monitoring of the water’s turbidity both before

filtration (i.e., after coagulation, flocculation, and/or settling) and after filtration. In addition, the quality of the source water should be frequently assessed and the amount

of added coagulant appropriately adjusted.

More stringent EPA standards for acceptable turbidity values, which serve as an

index of the effectiveness of filtration, have become effective in all states since the

Milwaukee outbreak (6 ). The water utility in Milwaukee, at the time of the outbreak,

would not have met these new standards. Recent investigations, however, have demonstrated small numbers of C. parvum oocysts in fully treated (i.e., filtered and

1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993

0

10

20

30

40

50

60

Year

Number of outbreaks

Individual

Noncommunity

Community

N=647

FIGURE 5. Number of waterborne-disease outbreaks associated with drinking water,

by year and type of water system — United States, 1971–1994

Vol. 45 / No. SS-1 MMWR 23
disinfected) water from 27%–54% of the municipal treatment plants studied, most of

which met the new standards (33,34 ). 

A workshop sponsored by CDC and EPA on waterborne cryptosporidiosis was held

in Atlanta in September 1994* to discuss newly proposed EPA regulations and the

health risks associated with drinking tap water contaminated with small numbers of

oocysts (35,36 ). Those attending the workshop concluded that the magnitude of the

risk posed by these oocysts could not yet be assessed because of several factors, including uncertainty about the viability or infectivity of the oocysts, the infectious dose

for immunocompetent and immunocompromised persons, and the possibility of

strain variability in the infectious dose and in the ability to cause disease. The results

of a study that used a C. parvum strain derived from calves suggested that the infectious dose of oocysts for healthy persons is small (i.e., a median of 132 oocysts) (37 ).

Information based on mathematical modeling suggests that some persons might become infected if exposed to even one oocyst (38 ). Until the magnitude of the risk for

waterborne cryptosporidiosis is better characterized or until effective procedures for

removing oocysts from water are identified, severely immunocompromised persons

should be provided with educational material about the potential risk for waterborne

disease and the strategies for prevention (39,40 ).

During 1993–1994, five of the seven outbreaks caused by bacterial agents occurred

in systems that used untreated well water. During 1991–1992, all three outbreaks

caused by bacteria and the two caused by hepatitis A virus were associated with untreated well or cistern water. Adequate, continuous disinfection of groundwater used

for drinking water should reduce the occurrence of WBDOs, particularly for small systems in which intermittent contamination of wells and springs is difficult to detect or

prevent. In addition, wells and springs should be protected from sources of contamination such as surface runoff, septic-tank drainage, and sewage discharges.

The two outbreaks in Minnesota and Missouri in 1993 associated with contaminated storage towers are reminders that stored water should be protected from

contamination (e.g., by feces from birds and small mammals). They also demonstrate

that, to prevent flushing of sediment or stagnant water from storage towers into the

distribution system, caution should be exercised when such towers are cleaned.

The three cases of lead poisoning in infants were included in this summary because blood-lead levels at least as low as 10 μg/dL are associated with adverse health

effects (15,41 ). Lead is particularly harmful to the developing brain and nervous system of fetuses and young children; elevated blood-lead levels are associated with

decreased intelligence and impaired neurobehavioral development. The three cases

of lead poisoning and the one outbreak of copper poisoning underscore the importance of using appropriate materials in household plumbing systems. The two

outbreaks of fluoride poisoning highlight the need for appropriate design (e.g., the

incorporation of antisiphon devices) and maintenance of water-fluoridation systems,

as well as for monitoring fluoride levels in water (42 ).

The two episodes of an association between miscarriages and elevated nitrate levels in drinking water are the first such episodes to be reported to the WBDO

surveillance system. Although ingestion of water with high nitrate levels is known to

cause methemoglobinemia in infants (43 ), little information is currently available

about the effects of nitrate on fetal development in humans. A recent, geographically

*The workshop was attended by more than 300 persons from 26 organizations (e.g., the water

industry; state and local health departments; AIDS interest groups; and the U.S. Department

of Agriculture, the Food and Drug Administration, and other federal agencies).

24 MMWR April 12, 1996
based survey of 5,536 private wells serving individual households in nine Midwestern

states demonstrated that 13% of the wells had nitrate levels in excess of EPA’s

maximum-contaminant level (CDC, unpublished data). The two episodes reported in

this surveillance summary raise questions about the importance of nitrate as a cause

of miscarriages and the need to provide both educational information regarding nitrate in drinking water and affordable access to water-testing services.

During 1993–1994, only eight outbreaks/cases of chemical poisoning were reported

to CDC. For 1993 alone, however, the American Association of Poison Control Centers

reported 791 nitrate and nitrite poisonings, 887 copper poisonings, and 5,675 poisonings associated with swimming pools (44 ). Although their report does not provide

information as to whether these poisonings were waterborne, some of the nitrate and

many of the copper poisonings may have been associated with drinking water. 

Several reasons may help explain why waterborne chemical poisonings are rarely

reported to CDC: a) most poisonings of this nature (e.g., those associated with the

leaching of lead or copper from plumbing systems) probably occur in private residences, affect relatively few persons and, thus, might not come to the attention of

public health officials; b) exposure to chemicals via drinking water may cause illness

that is difficult to attribute to chemical intoxication, or it may cause nonspecific symptoms that are difficult to link to a specific agent; and c) the mechanisms for detecting

waterborne chemical poisonings and reporting them to the WBDO surveillance system are not as well established as they are for WBDOs caused by infectious agents.

CDC, in collaboration with EPA and the U.S. Geological Survey, is attempting to

establish mechanisms for triggering epidemiologic investigations of suspected acute

exposures to pollutants and for evaluating the deleterious effects of short- and longterm exposures to hazardous substances. Currently, water at treatment plants is

monitored for various organic and inorganic substances, and household tap water is

monitored for lead and copper. Future monitoring efforts should be tailored to improve a) the sensitivity of surveillance activities, b) the detection of associations

between environmental releases or exposure incidents and individual health events,

and c) the assessment of the public health burden associated with water-related

chemical exposures.

Although definitive conclusions cannot be reached about some of the WBDOs reported for 1993–1994, most of them probably could have been prevented by using

available water-treatment technologies and standard operating/monitoring procedures. Measures that could have been used to prevent some of the outbreaks of

chemical etiology include the application of existing knowledge about how to avoid

the siphonage of fluoridation chemicals into water, the use of appropriate materials in

water pipes and storage tanks, and the education of the public regarding the effects of

corrosive water on plumbing materials. Most of the WBDOs with a known or suspected infectious etiology could have been prevented if the water utilities had met

current regulations and followed appropriate treatment practices and operational procedures, source and treated water had been adequately monitored, and periodic

sanitary surveys had been conducted to identify and correct sources of contamination. However, the outbreak of cryptosporidiosis in Nevada demonstrates that WBDOs

sometimes can occur despite excellent source-water quality and state-of-the-art water

treatment.

Vol. 45 / No. SS-1 MMWR 25
1993–1994 Outbreaks Associated With Recreational Water

The most frequently reported WBDOs caused by exposure to recreational water

were outbreaks of gastroenteritis. Swimming and other recreational activities in

which the unintentional ingestion of water can occur are known to increase the risk for

gastrointestinal illness, even in nonoutbreak settings (45,46 ). The number of outbreaks of gastroenteritis for 1993–1994 (i.e., 14) was similar to those previously

reported (i.e., 14 for 1991–1992 and 13 for 1989–1990). For 1993–1994, however,

10 (71.4%) of the 14 gastroenteritis outbreaks were attributed to protozoan parasites,

compared with six (42.9%) of 14 for 1991–1992, none of 13 for 1989–1990, and two

(18.2%) of 11 for 1987–1988. Previous reports of cryptosporidiosis and giardiasis attributed to a waterborne source may have led to an increased awareness that these

diseases can result from waterborne transmission. For example, four outbreaks of

cryptosporidiosis associated with the use of recreational water were identified in Wisconsin in 1993 after the large drinking-water outbreak in Milwaukee.

The failure to identify deficiencies in filtration or chlorination for four of the six

outbreaks of pool-associated gastroenteritis caused by protozoa indicates that current

methods of water treatment may not always ensure protection against protozoan infections in these settings. The inactivation of G. lamblia cysts by the routine

chlorination of swimming pools may require >15 minutes, depending on such factors

as temperature, pH, and chlorine concentration. Under normal operating conditions

for pools, at least several days would be needed to inactivate C. parvum oocysts. Several types of filters are used in swimming pools. Their effectiveness for removing

oocysts is not known; and their filtration rates are generally slow, requiring up to

6 hours for a complete turnover of pool water (47 ).

During the outbreak of recreational lake water-associated cryptosporidiosis in New

Jersey, water temperatures ranged from 86 F to 95 F (30 C to 35 C). Preliminary data,

based on an ongoing study funded by the American Water Works Association Research Foundation, indicate that the number of viable oocysts, determined by

excystation, in surface water decreases by 90%–99% after 28 days at water temperatures of 86 F (30 C) (M.W. LeChevallier, personal communication, 1995). Other factors,

such as the effects of sunlight on oocysts in shallow bodies of water and the density

and composition of heterotrophic microflora, might further decrease the survival time

of oocysts in water. Although the outbreak, which lasted >4 weeks, may have resulted

from a single contamination event, the long period of transmission also may have

been caused by the reintroduction of C. parvum into the lake water by infected

persons.

Outbreaks of swimming-associated shigellosis continue to occur. The probable

source of the pathogen for the three outbreaks in 1993–1994, as for previous outbreaks, was fecal contamination of lake water by persons in the water. In 1994, the

second-ever reported outbreak that was caused by E. coli O157:H7 and associated

with exposure to recreational water occurred. E. coli O157:H7, like Shigella spp., apparently has a low infectious dose (48,49 ). Thus, infection may be acquired without

swallowing large volumes of water. 

EPA has published criteria for evaluating the quality of both marine and fresh water

used for recreation (50,51 ). Microbial monitoring has been recommended for recreational areas potentially contaminated by sewage. However, the utility of routinely

monitoring untreated water (e.g., lakes) for fecal contamination caused by bathers has

not been established. Efforts at prevention have focused on providing adequate toilet

26 MMWR April 12, 1996
and diaper-changing facilities at recreational areas and limiting the number of bathers

per unit area. An additional, important measure, although difficult to enforce, is to

prevent persons (especially young, nontoilet-trained children) from entering recreational waters if they are either experiencing or convalescing from a diarrheal illness. 

For the period 1993–1994, most of the reported outbreaks of dermatitis associated

with hot tubs, whirlpools, and swimming pools were directly related to inadequate

operation and maintenance procedures. Outbreaks of pseudomonas dermatitis associated with hot tubs are preventable if water is maintained at a pH of 7.2–7.8 with free,

residual chlorine levels in the range of 2.0–5.0 mg/L (52 ). Similarly, outbreaks of

pseudomonas dermatitis associated with swimming pools are preventable if the pH

and chlorine levels are maintained at 7.2–8.2 and 0.4–1.0 mg/L, respectively (47 ). A

person’s susceptibility and immersion time, along with the number of bathers per unit

area, also may influence the risk for infection (53 ).

The one death associated with the use of recreational water was caused by primary

amebic meningoencephalitis, a rarely reported disease in the United States. N. fowleri

infections are generally acquired during the summer months, when the temperature

of fresh water is favorable for the multiplication of the organism (54,55 ).

CONCLUSIONS

Information from the nationwide surveillance of WBDOs is used to characterize the

epidemiology of waterborne diseases in the United States. Data about the types of

water systems and deficiencies associated with outbreaks are needed to evaluate the

adequacy of current regulations for water treatment and monitoring of water quality.

The identification of the etiologic agents of outbreaks is particularly critical because

agents newly associated with WBDOs may require new methods of control. 

For agents that are recognized as important waterborne pathogens, rapid recognition and control of WBDOs are facilitated by surveillance at the local and state level.

Close communication between local health departments and water utilities is crucial.

For example, if epidemiologic evidence suggests the possibility of waterborne transmission, water utilities should be contacted promptly and asked about such factors as

recent treatment deficiencies and changes in source-water quality. Similarly, local

policies should be developed that specify the thresholds for reporting various waterquality data to health departments. Timely water testing and environmental

investigations can facilitate the identification of an outbreak’s etiologic agent and the

correctable source(s) of water contamination, as well as establish whether control

measures (e.g., boil-water advisories) are indicated.

Means of improving the surveillance system for WBDOs should be explored.

The review of information that has been gathered through other mechanisms (e.g.,

reports to poison-control centers, issuances of boil-water advisories, and computerized data on water quality) may facilitate the detection of more WBDOs. Special

epidemiologic studies may be needed that supplement the findings of this surveillance system by addressing such issues as the public health importance of newly

identified agents of waterborne disease and the effectiveness of prevention strategies

in nonoutbreak settings.

State health departments can request epidemiologic assistance from CDC for

the investigation of WBDOs. In addition, CDC and EPA can be consulted about the

Vol. 45 / No. SS-1 MMWR 27
engineering and environmental aspects of water treatment and about collecting largevolume water samples to identify pathogenic viruses and parasites.

Additional information is available from EPA’s Safe Drinking Water Hotline (telephone [800] 426-4791; e-mail sdwa@epamail.epa.gov), CDC’s Cryptosporidiosis

Information Line of the Parasitic Diseases Information Line (voice-fax telephone

system [404] 330-1242), and CDC/National Center for Infectious Diseases’ home page

on the Internet World Wide Web (http://www.cdc.gov/ncidod/diseases/crypto/

crypto.htm). Waterborne-disease outbreaks should be reported to CDC’s Division of

Parasitic Diseases; telephone (770) 488-7760.

Acknowledgments

The authors thank the state waterborne-disease surveillance coordinators; the state

epidemiologists; the state drinking-water administrators; the Office of Ground Water

and Drinking Water, U.S. Environmental Protection Agency; the Division of Bacterial

and Mycotic Diseases, NCID, CDC; and the Division of Environmental Hazards and

Health Effects, NCEH, CDC, for contributing to the waterborne-disease surveillance

summary.

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48. DuPont HL, Levine MM, Hornick RB, Formal SB. Inoculum size in shigellosis and implications

for expected mode of transmission. J Infect Dis 1989;159:1126–8.

49. Griffin PM, Tauxe RV. The epidemiology of infections caused by Escherichia coli O157:H7,

other enterohemorrhagic E. coli, and the associated hemolytic uremic syndrome. Epidemiol

Rev 1991;13:60–98.

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Carolina: Environmental Protection Agency, Office of Research and Development, Health Effects Research Laboratory, 1983; EPA publication no. 600/1-80-031.

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no. 99-960.

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54. Visvesvara GS, Stehr-Green JK. Epidemiology of free-living ameba infections. J Protozool

1990;37:25S–33S.

55. John DT, Howard MJ. Seasonal distribution of pathogenic free-living amebae in Oklahoma

waters. Parasitol Res 1995;81:193–201.

30 MMWR April 12, 1996
Glossary

In this glossary, italicized terms that are not names of microorganisms are defined

elsewhere in the glossary.

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.

Boil-water advisory: A statement to the public advising persons to boil tap water before drinking it.

Cistern: A storage tank for collected rain water.

Class: Refer to the Classification of Outbreaks section in the text and to Table 1 for a

comprehensive definition.

Coagulation: The process of adding chemicals to water to destabilize charges on naturally occurring particles to facilitate their subsequent aggregation and removal by

flocculation and/or filtration.

Coliforms: A group of gram-negative, rod-shaped, nonspore-forming aerobic and facultative anaerobic bacteria that ferment lactose, with acid and gas formation within

48 hours at 95 F (35 C). These bacteria are commonly found in soil and in the gut

and feces of warm-blooded animals. Their presence in water may indicate contamination with human and/or animal feces.

Community water system: A public water system that serves year-round residents of

a community, subdivision, or mobile-home park that has ≥15 service connections

or an average of ≥25 residents.

Contact time: The length of time water is exposed to a disinfectant (e.g., chlorine contact time).

Cross-connection: Any actual or potential connection between a drinking-water supply and a possible source of contamination or pollution (e.g., a waste-water line).

Cyst: The infectious stage of Giardia lamblia and some other protozoan parasites that

has a protective wall, which facilitates survival in water and other environments.

Disinfection by-products: Chemicals formed in water through reactions between organic matter and disinfectants.

Distribution system: Water pipes, storage reservoirs, tanks, and other means used to

deliver drinking water to consumers or to store it before delivery.

Excystation: The release of the internal (i.e., encysted) contents (e.g., trophozoites or

sporozoites) from cysts or oocysts.

Vol. 45 / No. SS-1 MMWR 31
Fecal coliforms: A subset of the coliform group of bacteria that includes such bacteria

as Escherichia coli that produce gas from lactose at 112.1±0.36 F (44.5±0.2 C) and

whose presence in water indicates contamination with human and/or animal feces.

Filter backwash: The water containing the material obtained by reversing the flow of

water through a filter to dislodge the particles that have been retained on it.

Filtration: The process of removing suspended particles from water by passing it

through one or more permeable membranes or media of small diameter (e.g.,

sand, anthracite, or diatomaceus earth).

Finished water: The water (i.e., drinking water) delivered to the distribution system

after treatment, if any.

Flocculation: The water-treatment process after coagulation that uses gentle stirring

to cause suspended particles to form larger, aggregated masses (floc). The aggregates are removed from the water by a separation process (e.g., sedimentation,

flotation, or filtration).

Free, residual chlorine level: The concentration of chlorine in water that is not combined with other constituents and thus serves as an effective disinfectant.

Groundwater system: A system that uses water extracted from the ground (i.e., a well

or spring).

Heterotrophic microflora: Microorganisms that utilize organic material for energy and

growth.

Indicator organism: A microorganism that indicates, by its presence in water, possible

or actual contamination by fecal material.

Individual water system: A small water system, not owned or operated by a water

utility, that serves <15 residences or farms that do not have access to a public water

system.

Maximum-contaminant level: The maximum permissible concentration (level) of a

contaminant in water supplied to any user of a public water system.

Nephelometric turbidity units: The units in which the turbidity of a sample of water is

measured when the degree to which light is scattered is assessed with a

nephelometric turbidimeter.

Noncommunity water system: A public water system that a) serves an institution,

industry, camp, park, hotel, or business that is used by the public for ≥60 days per

year; b) has ≥15 service connections or serves an average of ≥25 persons; and c) is

not a community water system.

Oocyst: The infectious stage of Cryptosporidium parvum and some other coccidian

parasites that has a protective wall, which facilitates survival in water and other

environments.

32 MMWR April 12, 1996
Public water system: A system, classified as either a community or a noncommunity

water system, that provides piped water to the public for human consumption and

is regulated under the Safe Drinking Water Act.

Raw water: Untreated water.

Reverse osmosis: A filtration process that removes dissolved salts and metallic ions

from water by forcing it through a semipermeable membrane. This process is also

highly effective in removing microbes from water.

Siphonage: A reversal of the normal flow of water or other liquid caused by a

negative-pressure gradient (e.g., within a water system).

Source water: Untreated water (i.e., raw water) used to produce drinking water.

Surface water: The water in lakes, rivers, reservoirs, and oceans.

Total coliforms: Nonfecal and fecal coliforms.

Turbidity: The quality (e.g., of water) of having suspended matter (e.g., clay, silt, or

plankton), which results in loss of clarity or transparency.

Untreated water: Refer to raw water.

Water utility: A water provider that distributes drinking water to a community through

a network of pipes.

Watershed: An area from which water drains to a single point; in a natural basin, the

area contributing flow (i.e., water) to a given place or a given point on a stream.

Watershed-control program: A program to protect a watershed from sources of contamination or pollution.

Vol. 45 / No. SS-1 MMWR 33
State and Territorial Epidemiologists and Laboratory Directors

State and Territorial Epidemiologists and Laboratory Directors are acknowledged for their

contributions to CDC Surveillance Summaries. The epidemiologists listed below were in the positions shown as of February 1996, and the laboratory directors listed below were in the positions

shown as of February 1996.

State/Territory Epidemiologist Laboratory Director

Alabama John P. Lofgren, MD William J. Callan, PhD

Alaska John P. Middaugh, MD Gregory V. Hayes, DrPH

Arizona Norman J. Petersen, SM (Acting) Barbara J. Erickson, PhD

Arkansas Thomas C. McChesney, DVM Michael G. Foreman

California Stephen H. Waterman, MD, MPH Michael G. Volz, PhD

Colorado Richard E. Hoffman, MD, MPH Ronald L. Cada, DrPH

Connecticut James L. Hadler, MD, MPH Sanders F. Hawkins, PhD 

Delaware A. LeRoy Hathcock, PhD Mahadeo P. Verma, PhD

District of Columbia Martin E. Levy, MD, MPH James B. Thomas, ScD

Florida Richard S. Hopkins, MD, MSPH E. Charles Hartwig, ScD

Georgia Kathleen E. Toomey, MD, MPH Elizabeth A. Franko, DrPH

Hawaii Richard L. Vogt, MD, MPH Vernon K. Miyamoto, PhD

Idaho Jesse F. Greenblatt, MD, MPH Richard H. Hudson, PhD

Illinois Byron J. Francis, MD, MPH David F. Carpenter, PhD

Indiana Gregory K. Steele, DrPH, MPH David E. Nauth (Acting)

Iowa M. Patricia Quinlisk, MD, MPH Mary J. R. Gilchrist, PhD

Kansas Gianfranco Pezzino, MD, MPH Roger H. Carlson, PhD

Kentucky Reginald Finger, MD, MPH Thomas E. Maxson, DrPH

Louisiana Louise McFarland, DrPH Henry B. Bradford, Jr, PhD

Maine Kathleen F. Gensheimer, MD, MPH John A. Krueger (Acting)

Maryland Diane M. Dwyer, MD, MPH J. Mehsen Joseph, PhD

Massachusetts Alfred DeMaria, Jr, MD Ralph J. Timperi, MPH

Michigan Kenneth R. Wilcox, Jr, MD, DrPH Robert Martin, DrPH

Minnesota Michael T. Osterholm, PhD, MPH Pauline Bouchard, JD, MPH

Mississippi Mary Currier, MD, MPH Joe O. Graves, PhD

Missouri H. Denny Donnell, Jr, MD, MPH Eric C. Blank, DrPH

Montana Todd A. Damrow, PhD, MPH Douglas O. Abbott, PhD

Nebraska Thomas J. Safranek, MD John D. Blosser

Nevada Randall L. Todd, DrPH Arthur F. DiSalvo, MD

New Hampshire M. Geoffrey Smith, MD, MPH Veronica C. Malmberg, MSN

New Jersey Lyn Finelli, DrPH (Acting) Marion Pierce

New Mexico C. Mack Sewell, DrPH, MS Loris W. Hughes, PhD

New York City Benjamin A. Mojica, MD, MPH Stanley Reimer

New York State Dale L. Morse, MD, MS Ann Wiley, PhD

North Carolina J. Michael Moser, MD, MPH Lou F. Turner, DrPH

North Dakota Larry A. Shireley, MS, MPH James D. Anders, MPH

Ohio Thomas J. Halpin, MD, MPH Kathleen L. Meckstroth, DrPH

Oklahoma J. Michael Crutcher, MD, MPH (Acting) Garry L. McKee, PhD

Oregon David W. Fleming, MD Michael R. Skeels, PhD, MPH

Pennsylvania James T. Rankin, Jr, DVM, PhD, MPH Bruce Kieger, DrPH

Rhode Island Utpala Bandy, MD, MPH Walter S. Combs, PhD

South Carolina James J. Gibson, MD, MPH Harold Dowda, PhD

South Dakota Susan E. Lance, DVM, PhD, MPH Richard S. Steece, PhD

Tennessee William L. Moore, MD Michael W. Kimberly, DrPH

Texas Diane M. Simpson, MD, PhD David L. Maserang, PhD

Utah Craig R. Nichols, MPA Charles D. Brokopp, DrPH

Vermont Vacant Burton W. Wilke, Jr, PhD

Virginia Grayson B. Miller, Jr, MD, MPH James L. Pearson, DrPH

Washington Paul Stehr-Green, DrPH, MPH Jon M. Counts, DrPH

West Virginia Loretta E. Haddy, MA, MS Frank W. Lambert, Jr, DrPH

Wisconsin Jeffrey P. Davis, MD Ronald H. Laessig, PhD

Wyoming Gayle L. Miller, DVM, MPH Roy J. Almeida, DrPH

American Samoa Edgar C. Reid, MO, DSM, MPH —

Federated States of

 Micronesia Vacant —

Guam Robert L. Haddock, DVM, MPH Florencia Nocon (Acting)

Marshall Islands Tom D. Kijner —

Northern Mariana Islands Jose L. Chong, MD Isamu J. Abraham, DrPH

Palau Jill McCready, MS, MPH —

Puerto Rico Carmen C. Deseda, MD, MPH Jose Luis Miranda Arroyo, MD

Virgin Islands Donna M. Green, MD Norbert Mantor, PhD

Vol. 45 / No. SS-1
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