Community Outbreak of Norwalk Gastroenteritis -- Georgia

An outbreak of gastroenteritis caused by the Norwalk virus recently occurred in Tate, a rural community in north Georgia. An investigation implicated the community water system as the source of infection.

On January 2-3, 1982, Tate received approximately 4.5 inches of rainfall. On January 4, residents in homes served by the municipal water system noted their tap water appeared turbid. That evening, residents reported the first cases of gastrointestinal illness. A subsequent investigation by the Office of Epidemiology, Georgia Department of Human Resources, and the Environmental Protection Division, Georgia Department of Natural Resources, included interviews of affected persons, a community telephone survey, testing of serum and stool samples from patients, and evaluation of the water system. Twenty-seven typically-ill patients identified by local physicians were interviewed. Illness was generally mild and was characterized by abrupt onset of nausea, abdominal cramps, diarrhea and/or vomiting, headache, myalgias, and low-grade fever. Duration of illness for most persons ranged from 1 to 3 days.

Fifty-seven households, systematically selected from a local directory, were surveyed by telephone. An adult member of each household provided the following information: age and sex of each household member and the household's water source, as well as information on the occurrence of gastrointestinal illness. Cases of gastrointestinal illness (defined as vomiting or diarrhea in the period January 1-9) were reported for 59/193 (30.6%) persons in the 57 households. However, 48/76 (63.2%) persons living in 25 households served by the municipal water supply were ill, as compared with 11/117 (9.4%) persons living in 32 households served by wells and other water sources (p0.001) (Figure 1). Attack rates did not vary among persons in different age groups. One or more cases of illness occurred in 20/25 (80.0%) households served by the municipal water supply, as compared with 5/32 (15.6%) households served by other sources (p0.001). Based on an estimated 800 persons served by the water system, approximately 500 people may have been ill during the outbreak.

Stool cultures obtained from three acutely ill patients were negative for bacterial pathogens. Paired serum specimens obtained from 22 patients were tested by radioimmunoassay (RIA) for antibody to the Norwalk virus; a greater than or equal to 4-fold rise in antibody titer was found for 20 (90.9%) persons. Virus particles were not detected by electron microscopic examination of the three stool specimens.

The municipal water system, constructed in 1920, was using water from four sources at the time of the outbreak--one well and three springs. The well casing was below ground level and had been flooded during the heavy rains. One of the springs was located inside a fenced hog lot behind several houses with septic tanks; moreover, the spring was unprotected. Testing of a water specimen collected on January 8 from this spring revealed a fecal coliform count of 16 MPN/100 ml. Tap water from a church, one of five sites sampled on January 8, had an elevated fecal coliform count (5.1 MPN/100 ml) and undetectable residual free chlorine. However, fecal coliforms were not detected at four sites, where residual free chlorine levels ranged from 0.3 to 10.0 mg/L. RIA testing of serum specimens obtained from three pigs that had access to the contaminated spring did not detect antibody to Norwalk virus.

On January 7, 1982, use of the Tate water sources was discontinued, and water entering the distribution system was superchlorinated. Later that day, the system was flushed with water from the nearby community, which has since served as Tate's water source. For a limited time, Tate residents were advised to boil drinking water. There are no plans to reutilize the Tate water sources until the observed deficiencies are corrected. Reported by TC Boswell, MD, DT Darnell, MD, Tate, JL Ledbetter, Georgia Dept of Natural Resources, J Benson, MD, TW McKinley, MPH, JD Smith, BS, RK Sikes, DVM, MPH, State Epidemiologist, Georgia Dept of Human Resources; HB Greenberg, MD, NIAID, NIH; Field Svcs Div, Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: The findings of this investigation strongly implicated the Tate water system as the source for Norwalk virus infection. Fecal contamination of tap water sampled on Friday, January 8, was observed at one of five sites tested, a church, where weekday water use was infrequent. Although data concerning water consumption by residents were not obtained, household residents served by the Tate system probably became infected after drinking water contaminated with Norwalk virus. Inspection of the four water sources for the system revealed two possible sites of contamination, including the spring in which fecal coliforms were detected. That spring was subject to contamination for several reasons: the cover was damaged, and the spring was open to ground-water runoff from nearby homes with septic tanks and penned animals and to the excreta of pigs that had direct access to the spring. In addition, the casing of the single well was recessed below ground level and had been exposed to flooding by surface water during the unusually heavy rainfall. The possibility cannot be dismissed that inadequate chlorination of the water system contributed to the outbreak.

A variety of etiologic agents have been implicated in water-related disease outbreaks, including bacterial, viral, and parasitic pathogens and chemical toxins (1). The clinical and epidemiologic features of this outbreak were characteristic of Norwalk infection and resembled patterns observed in previous waterborne Norwalk-associated outbreaks involving general population groups (2-5). This episode illustrates the importance of promptly reporting and investigating community outbreaks of gastroenteritis. A prompt response to this outbreak made possible identification of the causative agent and implementation of measures designed to prevent future outbreaks of waterborne illness in this community.

References

1. CDC. Water-related disease outbreaks surveillance. Annual summary, 1980. Atlanta:CDC, 1982.

2. Kaplan JE, Gary GW, Baron RC, et al. Epidemiology of Norwalk gastroenteritis and the role of Norwalk virus in outbreaks of acute nonbacterial gastroenteritis. Ann Intern Med 1982;96:756-61.

3. Gunn RA, Terranova WA, Greenberg HB, et al. Norwalk virus gastroenteritis aboard a cruise ship: an outbreak on five consecutive cruises. Am J Epidemiol 1980:112:820-7.

4. Griffin MR, Surowiec JJ, McCloskey DI, et al. Foodborne Norwalk virus. Am J Epidemiol 1982;115:178-84.

5. Baron RC, Murphy FD, Greenberg HB, et al. Norwalk gastrointestinal illness: an outbreak associated with swimming in a recreational lake and secondary person-to-person transmission. Am J Epidemiol 1982;115:163-72.

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