Emerging Infectious Diseases
[Volume 5 No.6 / November - December 1999]

   Dispatches

   Computer-Generated Dot Maps as an Epidemiologic Tool:
   Investigating an Outbreak of Toxoplasmosis

   Steven B. Eng,* Denise H. Werker, Arlene S. King,
   Stephen A. Marion, Alison Bell, Judith L.
   Issac-Renton, G. Stewart Irwin,# and William R.
   Bowie

   *Capital Regional District Health Department, Victoria,
   B.C., Canada; Health Canada, Ottawa, Ontario, Canada;
   British Columbia Centre for Disease Control,
   Vancouver, B.C., Canada; University of British
   Columbia, Vancouver, B.C., Canada; Centers for Disease
   Control and Prevention, Atlanta, Georgia, USA; and
   #Greater Victoria Water District, Victoria, B.C.,
   Canada


    We used computer-generated dot maps to
    examine the spatial distribution of 94
    Toxoplasma gondii infections associated
    with an outbreak in British Columbia,
    Canada. The incidence among patients served
    by one water distribution system was 3.52
    times that of patients served by other
    sources. Acute T. gondii infection among
    3,812 pregnant women was associated with
    the incriminated distribution system.

   Epidemiologists have traditionally used maps to examine
   the spatial distribution of disease incidence.
   Computer-generated dot maps have facilitated
   identification of case clusters (1), formulation of
   hypotheses about the source of infection or spatially
   distributed risk factors (2,3), and analysis of data.
   Because most populations are served by identifiable
   water systems, waterborne disease outbreaks lend
   themselves to being plotted on dot maps (1,4-6). We
   describe an automated address-matching and base map
   system in a geographic information system structure
   used to assess information related to an outbreak of
   toxoplasmosis associated with a municipal water system.

   The Capital Regional District (population 321,585) is
   located on Vancouver Island, British Columbia, Canada.
   The district includes the City of Victoria, surrounding
   municipalities and districts, and the Gulf Islands.

   In March 1995, an outbreak of toxoplasmosis was
   suspected when 15 residents of this district were
   identified as having acute infection with Toxoplasma
   gondii. Investigation of these and subsequent cases
   confirmed an outbreak but identified no common food,
   beverage, or event source. A hand-drawn dot map of the
   initial 47 cases showed clustering in the Greater
   Victoria area. The municipal water supply system was
   considered a possible explanation for this spatial
   distribution. To examine this hypothesis,
   computer-based geographic mapping was used to study the
   distribution of all outbreak-related acute cases and
   data collected from a population-based serologic
   screening program to detect T. gondii infection in
   women who were or had been pregnant (7).

   Classification of Cases and the Water Distribution
   Systems

   Patients were classified as having acute, equivocal, or
   nonacute cases or as never infected on the basis of
   serologic tests performed at the Provincial Laboratory
   (British Columbia Centre for Disease Control) and the
   Toxoplasma Serology Laboratory, Research Institute,
   Palo Alto Medical Foundation (8-11). Cases were further
   classified on the basis of clinical symptoms, outbreak
   relatedness, patient's residence, and pregnancy status
   (pregnant, nonpregnant) (7).

   At the time of the outbreak, the Greater Victoria Water
   District operated two disinfection plants supplying
   unfiltered, chloraminated surface water to
   approximately 292,000 residents of the Capital Regional
   District. The higher-pressure Japan Gulch distribution
   system supplied water from the Japan Gulch Reservoir to
   73,000 residents, as well as a one-way transfer of
   water into the other distribution system. The Humpback
   distribution system supplied 219,000 residents (12).
   District residents were further classified as receiving
   high, intermediate, or no exposure to water from the
   Humpback Reservoir.

   Geographic Mapping

   Geographic mapping analysis was performed by using
   MapInfo Version 3.0 for Windows (MapInfo Corporation,
   Troy, NY, 1992-94). MapInfo, which enables data
   containing geographic information to be placed on a
   map, was used for geocoding (i.e., placing markers into
   the database, like pins onto a map). Street address
   data for each record in the datafile were matched
   against an electronic street map of the Capital
   Regional District. Geographic coordinates were taken
   from the electronic street map of the district, which
   is based on the Transportation Centerline Network of
   the British Columbia Ministry of Transportation and
   Highways (13). Lotus Approach 3.0 for Windows (Applied
   Software Corporation, subsidiary of Lotus Development
   Corporation) was used to prepare and validate the data
   before mapping. On the basis of information from the
   Greater Victoria Water District, a street-level scale
   map was drawn in MapInfo to delineate the geographic
   area in the Capital Regional District that receives
   municipal drinking water from the Humpback Reservoir.



Geographic Distribution of Acute Cases

All 94 persons with outbreak-related acute cases
who lived in the Capital Regional District were
grouped by geographic area as being served by the
Humpback Reservoir or other water sources. 
Incidence rates were calculated by using estimated   
population figures (1994). Information on 
residential addresses was obtained from laboratory
requisitions, physicians' offices, hospital
records, the Medical Services Plan of British
Columbia, and direct communication with patients.
For each case, the residential street address at
the time the first specimen was drawn for testing
for T. gondii antibodies was used to produce a
computer-generated dot map. Of the 94 persons with outbreak-related acute cases who lived in the Capital Regional District, 83(88%) lived in the area served by the Humpback  
Reservoir. The incidence rate of acute infection
among persons residing in the area served by the Humpback Reservoir was more than three times that for areas served by other sources (RR = 3.53; 95% confidence interval [CI]: 1.88-6.63; p = 0.0003) (Figure 1).

Figure 1. Geographic distribution of
outbreak-related acute cases of toxoplasmosis
in the Capital Regional District, Vancouver
Island, British Columbia, 1995 (n = 94).

Geographic Distribution of Women Screened during Pregnancy

Data from a population-based screening program were used to determine whether residents served by the Humpback Reservoir were more likely to have acute infection with T. gondii. Serologic screening was offered to an estimated 4,500 women living in the capital regional district who were pregnant between October 1, 1994, and April 30, 1995. To offer
screening to as many of these women as possible, information regarding the screening program was extensively distributed to women, physicians, and the public.

Serologic results were available from the Provincial Laboratory database at the British Columbia Centre for Disease Control. Residential street addresses were obtained by linking the Provincial Laboratory database with the Medical Services Plan database, using the unique personal
health number assigned by the Province. A computer datafile containing the serologic results of screened pregnant women and their addresses was provided to the Capital Regional District Health Department. The data were then prepared and validated, geocoded and mapped, and statistically analyzed.

Three dot maps were generated by MapInfo, showing
the geographic distribution of the screened
population according to their laboratory
classification of never infected (i.e., no
serologic evidence of immunoglobulin [Ig] G or IgM
antibody to T. gondii), nonacute (i.e., typically 
IgG but not IgM antibody to T. gondii), and acute 
(i.e., serologic evidence of acute infection by a
battery of tests) (7-11). Several data subsets were
generated on the basis of two variables: 1)the
residence in the area served by the Humpback 
Reservoir and 2) municipality of residence. Odds
ratios were then calculated by StatCalc in Epi Info
(version 6.03) to test the hypothesis that living
in the area served by the Humpback Reservoir was
associated with infection with T. gondii.
Of 3,982 laboratory records for screened women,
3,962 records were available for coding. The 3,812
women with successfully coded addresses comprise
85% of the estimated 4,500 women eligible for
screening. Of these women, 36 (0.9%) were
classified according to their laboratory results as
having acute infection, 216 (5.7%) as having a
nonacute infection, 3,558 (93.3%) as never having
been infected, and 2 (0.1%) as having equivocal
cases. The age distribution of the women (15 to 45
years [mean 29 years]) did not differ by infection
status (women with equivocal status were excluded).  
Women acutely infected with T. gondii were more the 
than three times as likely as uninfected women 
live in the area served by the Humpback distribution system (odds ratio [OR] 3.05, and exact 95% CI: 1.08-11.91). In contrast, distribution of women with serologic evidence of nonacute infection with T. gondii was not associated with the municipal water distribution system (OR 0.91, exact 95% CI: 0.67-1.25).

Figure 2. Geographic distribution of the residences of women screened during pregnancy and classified as never
infected with toxoplasmosis (n = 3558).

Figure 3. Geographic distribution of the residences of women
screened during pregnancy and classified as having
nonacute cases of toxoplasmosis (n = 216).

Computer-generated dot maps (Figures 2, 3, 4) provided visual information, as well as data for 
statistical analysis. These maps are characteristic 
of an outbreak associated with a one-time event.
The geographic distribution of pregnant women who 
were never infected (Figure 2) indicates that the
susceptible population is similar to the
distribution of the whole population. The
geographic distribution of pregnant women who had 
nonacute cases (Figure 3) is similar to the
population distribution, indicating that before the
outbreak, residence was not associated with
antibody. Visually comparing the geographic
distribution of pregnant women with acute cases
(Figure 4) with that of pregnant women who were
never infected (Figure 2) suggests that most of the recently infected pregnant women lived in the area served by the Humpback Reservoir. The difference between the distributions seen in Figures 3 and 4 suggests that this event was new and unusual, rather than an ongoing or intermittent exposure
that had not been recognized.

Figure 4. Geographicdistribution of the
residences of women screened during pregnancy and
classified as having acute cases of toxoplasmosis (n = 36).

Well-defined geographic areas of the Capital Regional District were classified as receiving high, intermediate, and no exposure to water from the Humpback Reservoir (exposure scores 3, 2, and 1, respectively). An analysis for linear trend in proportions was performed by using StatCalc in Epi Info to demonstrate whether a dose-response relationship was present.

If water from the Humpback Reservoir was the source of infection, the rate of infection would be expected to increase with increased exposure. The Table shows the geographic distribution of women according to the estimated concentration of water from Humpback Reservoir received at
their residence. When acutely infected women were compared with seronegative women, the trend in linear proportions across the three exposure scores was significant (chi-square = 6.67; p = 0.01). In contrast, a significant linear trend in proportions was not demonstrated when women with nonacute cases were compared with seronegative women (chi-square = 1.30; p = 0.25).

Table. Linear trend analysis of acute and nonacute infections with Toxoplasma gondii among screened pregnant women, by degree of exposure to Humpback Reservoir water

Conclusions 

The conclusions drawn from geographic mapping depend on the accuracy and validity of the datasets and the availability of denominator data to avoid making false associations that are a function of population densities. For the overall population, denominators were available from census data. Firm denominator data were available from the serologic
screening study.

With respect to accurate placement on the maps, the addresses and the delineation of the water distribution systems were critical to the evaluation. Although extensive efforts were made to verify addresses, there was some potential for misclassifying exposure. However, any
misclassification resulting from the use of the Medical Services Plan database should not have systematically biased our analyses.

The Greater Victoria Water District engineers delineated the boundaries of the Humpback distribution system and ranked the zones in the Capital Regional District according to the proportion of water received from the Humpback Reservoir. As the engineers had no prior knowledge of the distribution of outbreak-related cases of toxoplasmosis, it is unlikely
that the determination of these geographic boundaries was biased. 

A limitation of the geographic mapping analysis of cases is that rates were not adjusted for age or other covariates. However, the subsequent analyses using data from the population-based screening of pregnant women were not confounded by age. The age distribution of the women in
these analyses did not differ when grouped by infection status.

Computer mapping software had the advantages of 1) facilitating the verification and correct placement of addresses, 2) reducing the time required to map the location of large datasets, 3) enabling queries and statistical analysis of the data after mapping, 4) allowing several sets
of mapped data to be analysed simultaneously for potential
relationships, and 5) generating printouts or overheads for
presentations.

Acknowledgments

We thank Timothy Johnstone, Louise Egan, Michael Aeberhardt, Kevin Kirkwood, Bill Keenan, and Jenny Cadman for invaluable technical assistance and critical information. We also thank Jack S. Remington and the Toxoplasma Serology Laboratory, Research Institute, Palo Alto Medical Foundation.

This investigation was supported by the Ministry of Health, British Columbia; the Capital Regional District Health Department, Victoria, British Columbia; and the Laboratory Centre for Disease Control, Health Canada, Ottawa, Ontario.

Mr. Eng is with the Health Protection and Environmental Program for Greater Victoria, British Columbia. His interests focus on environmental epidemiology, manipulation and analysis of computer data, and computer-based mapping.

Address for correspondence: Steven B. Eng, Health Protection and Environmental Services, Capital Health Region, #201-771 Vernon Avenue, Victoria, B.C., Canada, V8X 5A7; fax: 250-475-5130; e-mail: steven.eng@caphealth.org.

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Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

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