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Groundwater as a vehicle for disease transmission in Southeastern Idaho: a case study.


On June 21, 1994 the Southeastern Idaho District Health Department (SEDHD) was notified by the local hospital that three construction workers had become ill apparently after consuming water obtained from a private well while re-siding a local duplex. The initial report confirmed two of the three men had tested positive for Shigella flexneri. Shigellosis, or bacillary dysentery, is an acute bacterial disease characterized mainly by diarrhea, fever, nausea, vomiting and cramps with transmission occurring through a direct or indirect fecal-oral route. Shigellosis is endemic in both developed and undeveloped countries with S. sonnei isolated most commonly in the United States. Shigella flexneri is more typically isolated in developing countries; however, this bacterium has had an historic if not present day focus in the U.S., primarily in the southeastern and western regions (1). Additionally, most communities may have more than one serotype present and mixed infections with other intestinal pathogens also occur (2). The reported disease symptoms and associated etiologic agent hoisted a large red flag for SEDHD environmental health personnel and led to the epidemiological and groundwater investigations described in the following pages.

Epidemiological Investigation

Eighteen duplex residents and 15 nonduplex residents were identified as primary contacts through an intense interview process. The interviewing process was complicated because many contacts spoke very little, if any, English. A case for the outbreak of gastroenteritis was defined as any individual who was experiencing or had experienced diarrhea, with or without blood, abdominal cramps, and had contact with food or water either prepared at or obtained from the duplex. All 18 residents and 12 of the 15 nonresidents had or were experiencing illness symptoms that met the case definition (see Table 1).

Initial onset dates for contacts who were ill varied from one to 90 days prior to SEDHD involvement. A number of the resident contacts ill at the time of the interview indicated they had been ill off and on for two or three months. Dates for each case were placed in 30-day frequency classes because exact onset dates could not be obtained. Those patients with more recent and accurately reported onset dates were placed in 15-day frequency classes [ILLUSTRATION FOR FIGURE 1 OMITTED]. The five confirmed cases of shigellosis were from individuals not residing in the duplex (Table 1). Onset of symptoms for all five cases (two construction workers and three other nonresident contacts) was within 48 to 72 hours of initial exposure to water/food at the duplexes, which correlates to the incubation period of Shigella.

Figure 1 represents the course of the epidemic. This curve mirrors a theoretical continual source epidemic in which cases increase along a normal curve as individuals are exposed and re-exposed, reaching a peak and eventually maintaining some level of disease in the population. No additional cases were reported after SEDHD was notified, identified the source, and managed the contacts.

Groundwater Investigation

Onsite investigation of the property revealed an unused, 8 ft deep, well pit that was receiving effluent of unknown quality from a 1 1/2 inch pipe that entered the pit about 6 ft below the ground surface. Samples of this effluent revealed fecal coliform and fecal streptococci bacteria in numbers indicative of domestic wastewater. With an estimated flow of approximately 1/2 gal/min, nearly 720 gal of wastewater were injected into the open bore-hole each day.

The domestic well serving the affected duplexes was 40 ft from the well pit with a neighboring well within 35 ft [ILLUSTRATION FOR FIGURE 2 OMITTED]. Subsequent sampling revealed contamination with total and fecal coliform bacteria, and Escherichia coli. Drinking water samples tested negative for Shigella.

Duplex resident and nonresident contacts involved in a
groundwater-related continuous source gastroenteritis

 ILL Not ILL + for Shigella TOTALS

Resident contacts 18 0 0 18
Nonresident contacts 12 3 (*)5 15
TOTALS 30 3 33

* Five individuals positive for Shigella are included in
illness count.

Subsurface geology was confirmed based on a drilling log from the neighboring contaminated domestic well. The lithologic log indicated coarse sand and gravel from 1-27 ft; groundwater in fractured basalt was encountered from 27-70 ft; and steel well casing was terminated at the gravel/basalt juncture (see [ILLUSTRATION FOR FIGURE 3 OMITTED] for other important features). To confirm a hydraulic connection between the "injection well" and domestic wells, a fluorescein dye trace was initiated. Fluorescein dye flushed down the "injection well" was recovered 15 days later from activated charcoal dye traps placed in two toilet tanks of the affected home served by the neighboring contaminated domestic well; charcoal packets were not placed in the affected duplex because the dwelling was vacated shortly after SEDHD became involved.

Excavation verified the effluent pipe in the well pit as a sewage overflow from the existing seepage pit serving the duplexes. Evidence suggested the pipe was an old water line previously serving two dwelling units on adjacent property to the west, one of which had burned down and one which was vacant. Possibly this line was inadvertently severed during installation of the seepage pit, as no evidence indicated this pipe had been intentionally constructed as an overflow. The pipe was, however, on an approximate 2% grade toward the unused well pit, resulting in its functioning to direct effluent into the well pit, regardless of intentionality.


Attributing outbreaks to particular disease-causing agents is often the most difficult part of any epidemiological investigation. Two major criteria must be met to implicate a specific organism. First, the agent should be found in ill persons significantly more often than in well persons from the affected population or an unaffected control community. Second, the agent should be found in the incriminated source (water), or there should have been an appropriately timed event which would have allowed the presumed agent to bypass the treatment system (3).

Data gathered from this investigation meet both criteria. Five cases of confirmed shigellosis in a ten-day period accounted for 63% (5 of 8) of the reported shigellosis cases in our eight-county health district for 1994 (4). Add the additional 25 cases of gastroenteritis (those not positive for Shigella but meeting case definition) and the first criterion has been met. Second, drinking water samples tested positive for fecal coliforms and Escherichia coli. Though Shigella was not isolated from drinking water samples, epidemiologic evidence supports a mixed infection scenario. Since all five Shigella-positive cases were nonresident contacts with symptom onset dates highly correlated to time of exposure and known incubation periods, and given the low infective dose of Shigella (10-100 organisms), we feel confident that sufficient pathogenic organisms were present in the drinking water system to initiate disease. In addition, between 1946 and 1970, 358 waterborne disease outbreaks were reported to federal agencies in the United States, 33 (9%) were caused by Shigella organisms. Most outbreaks involved private water supplies and were caused by direct fecal contamination, back siphonage from a nonpotable source into a potable system, or cross-connections between such systems. As demonstrated in this outbreak, and a larger scale outbreak in 1973 in Bucks County, Pennsylvania, despite strong epidemiologic evidence and the isolation of Shigella from affected individuals, shigellae are rarely isolated from implicated water (5). Further, spread of the disease was no doubt enhanced through indirect transmission that may have included handling food items with contaminated hands and/or muscid flies acting as mechanical vectors.


Direct intervention by SEDHD, which included educating contacts and secondary contacts on proper hygiene, finding and removing the infection source, and encouraging contacts to seek medical attention, was the key to halting spread of the outbreak. Twenty-seven of the 30 ill contacts received medication within three days of the first case report. Follow-up interviews eight days later indicated that symptoms in all patients had subsided; no additional cases were reported.

Disease prevention is the main goal of public health. However, economic benefits of preventing outbreaks often go unnoticed. In man-hours alone, this gastroenteritis outbreak cost an estimated $5,000 not including labtests, medical costs, and time lost from work.

This case study serves to remind us that outbreaks of diseases often associated with distant geographical areas lacking modern sanitation facilities and adequate health care can erupt close to home. This further emphasizes that quality of life in the United States cannot be taken for granted and brings to light three main points: (1) need for periodic sampling of private water supplies and continued monitoring of public water systems; (2) continued diligence in management of onsite sewage disposal and well construction and abandonment; and (3) need for increased awareness of environmental health hazards and the integrated roles public health agencies and the general public play in their prevention.


The authors would like to thank Dr. Delane Kritsky for kindly providing presubmission reviews that greatly improved this manuscript and Linda Archuleta for help with the graphics.


1. Weil, A.J., and I. Saphra, (1953), Salmonellae and Shigellae-Laboratory Diagnosis Correlated with Clinical Manifestations and Epidemiology, Charles C. Thomas, Springfield, IL, p. 198.

2. Benenson, Abram S. (ed.) (1990), Control of Communicable Diseases in Man, American Public Health Association, Fifteenth Edition, Washington D.C., pp. 391-394.

3. Hopkins, R.S., P. Shillam, B. Gaspard, L. Eisnach, and R.J. Karlin, (1985), "Waterborne disease in Colorado: Three years surveillance and 18 outbreaks," Am J Public Health, 75:254-257.

4. Idaho Department of Health and Welfare (1995), Idaho Disease Bulletin, Volume 2, Number 3.

5. Centers for Disease Control, (1974), "Shigella Surveillance Report No. 35, Third and Fourth Quarters 1973," issued Nov. 1974.
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Article Details
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Author:Dawson, S. Dee
Publication:Journal of Environmental Health
Date:Dec 1, 1995
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