On August 8, 1983, the Utah Department of Health was notified by the Tooele County Health Department (TCHD) of an outbreak of diarrheal illness in Tooele, Utah, possibly associated with a contaminated public water supply that resulted from flooding during Utah's spring thaw. By September 30, 1983, 1,272 individuals were identified who met the following case definition: diarrhea lasting more than 5 days or recurrent diarrhea and two or more of the following symptoms: abdominal pain or cramping, bloating, nausea, weight loss, vomiting, or fever over 37.8 C (100 F). A total of 1,230 of the patients resided in Tooele (9.8% of the population of 12,500); the remaining 42 patients resided elsewhere but had visited or worked in Tooele.

Cases were identified from two sources: 1,104 came from Tooele physicians' daily rosters, and 168 responded to announcements by the local news media. Individuals were contacted by telephone and asked standardized questions.

For comparison, individuals living in a city of similar size and sociodemographics 65 miles distant and with its own municipal water system were selected randomly and asked the same questions as the patients. Three (2.9%) of 103 comparison individuals interviewed met the case definition. The difference between the prevalence of diarrheal illness in Tooele and that in the comparison city was statistically significant (p 0.02). Statistical comparison of the patients from Tooele and the individuals from the comparison town failed to incriminate exposure to mountain stream water (a common source of giardiasis in Utah), pet ownership, food, day-care centers, or anal intercourse--all recognized modes of giardiasis exposure.

The age and sex distributions of patients were similar to those of the general population served by the water district. Besides diarrhea, the most common symptoms were abdominal pain or cramping (88%) and bloating (77%). Sixty-seven percent complained of nausea; 32%, of vomiting; and 17%, of fever over 37.8C (100 F). Of 410 individuals submitting stool specimens for bacterial and parasitic examination, 105 (26%) had Giardia lamblia. No other pathogenic parasites were observed, and no Salmonella, Shigella, Yersinia, or Campylobacter were isolated. Approximately 90% of the 1,100 persons receiving medication were treated with metronidazole (Flagyl*); the remainder were treated with quinacrine (Atabrine*) or furazolidone (Furoxone*) or were given symptomatic medications.

Because of complaints about muddy water, the municipal water system in Tooele was inspected during the last week in July, and a pipe damaged by flooding, probably during the week of July 17 when Tooele experienced several days of heavy rain, was identified. During this week, three of five routine bacteriologic samples from this source had unsatisfactory coliform counts. Diarrheal illness peaked on August 1, approximately 2 weeks after the heavy rains and the abnormal coliform counts (Figure 6). The incubation period of waterborne giardiasis has been estimated as 7-14 days (1). On August 1, in response to complaints of murky tap water, the implicated water source was disconnected from the public water system. Ten days later, the system was hyperchlorinated to inactivate G. lamblia cysts.

The number of new cases declined steadily throughout August, and continuing surveillance indicates that no new cases have been epidemiologically linked to the public water system. A detailed cost analysis estimated the direct costs of the giardiasis cases at over $116,000. Reported by DM Perrotta, PhD, CR Nichols, MPA, AP Nelson, MPH, L Scanlon, G Smith, RE Johns, Jr, MD, State Epidemiologist, Utah Dept of Health, D Forster-Burke, M Bateman, G Dalton, MS, Tooele County Health Dept, Utah; Protozoal Diseases Br, Div of Parasitic Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Flooding associated with abnormal weather patterns last year caused extensive damage in many areas in the United States, including Utah. This report illustrates a less obvious consequence of such natural disasters. A similar period of heavy water run-off associated with unseasonably warm weather and ash fall from the Mount St. Helens volcano eruption in 1980 was also linked to an outbreak of diarrhea due to G. lamblia (2).

It is unclear that this present outbreak of diarrheal illness was due solely to giardiasis, although this parasite was the only pathogenic agent identified. Because normal chlorine levels were temporarily unable to control bacterial contamination, some of the diarrhea cases may have been caused by unidentified bacteria or viruses.

Quinacrine (Atabrine) is the drug of choice for adults with giardiasis (3). Although individuals who receive quinacrine often complain of its bitter taste, the drug has not been associated with long-term adverse effects, as has metronidazole. The efficacy of quinacrine is thought to be better than that of metronidazole, and quinacrine costs considerably less (4).

References

1. CDC. Unpublished data.

2. Weniger BG, Blaser MJ, Gedrose J, Lippy EC, Juranek DD. An outbreak of waterborne giardiasis associated with heavy water runoff due to warm weather and volcanic ashfall. Am J Public Health 1983;73:868-72.

3. The Medical Letter. Drugs for parasitic diseases. 1982;24:5-12.

4. Wolfe MS. Giardiasis. New Engl J Med 1978;298:319-21.

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