Early in 1993, there was an outbreak of flu-like symptoms in many people in the Milwaukee area. After some investigation, city inspectors realized the city water supply had been contaminated by cryptosporidium, which various testing and filtering systems had failed to detect and screen out. It is estimated that over 400,000 people were sickened, and approximately 100 people died (according to some reports, concentrated among people with AIDS).
According to one source, "The 1993 Milwaukee Cryptosporidium outbreak was a significant distribution of the Cryptosporidium protozoan in Milwaukee, Wisconsin, and the largest waterborne disease outbreak in documented United States history. One of Milwaukee's two water treatment plants was contaminated. Over the span of approximately two weeks, 403,000 of an estimated 1.61 million Milwaukee residents served by the treatment plants were sickened."
The story was at the top of every local newscast for some days, and weeks later many people were still boiling water or avoiding tap water altogether. The story was tragic news for those with compromised immune systems, and was one more complication in the attempt to maintain the health and well-being of thousands of local people infected with HIV and with AIDS. For many of these, as well as others with compromised immune systems, the contamination was fatal.
The story went national, and 12 years later was still cited in national news stories and scientific studies regarding the safety of city water supplies, or of "when things go wrong". (12 years later the author still hears people say they haven't drunk Milwaukee tap water since.)
The City of Milwaukee and State of Wisconsin made numerous changes to filtration and testing procedures in an attempt to ensure such a tragedy can never happen again. One of the simplest is to incerease the frequency of testing for, and acceptable level of, turbidity (cloudiness) in drinking water. View the Wisconsin DNR's guidelines on cryptosporidium here.
A fairly concise clinical report of the event was written by the Wisconsin Division of Health, Bureau of Public Health, Madison 53703, and published in the July 1994 issue of the New England Journal of Medicine:
BACKGROUND. Early in the spring of 1993 there was a widespread outbreak of acute watery diarrhea among the residents of Milwaukee. METHODS. We investigated the two Milwaukee water-treatment plants, gathered data from clinical laboratories on the results of tests for enteric pathogens, and examined ice made during the time of the outbreak for cryptosporidium oocysts. We surveyed residents with confirmed cryptosporidium infection and a sample of those with acute watery diarrhea consistent with cryptosporidium infection. To estimate the magnitude of the outbreak, we also conducted a survey using randomly selected telephone numbers in Milwaukee and four surrounding counties. RESULTS. There were marked increases in the turbidity of treated water at the city's southern water-treatment plant from March 23 until April 9, when the plant was shut down. Cryptosporidium oocysts were identified in water from ice made in southern Milwaukee during these weeks. The rates of isolation of other enteric pathogens remained stable, but there was more than a 100-fold increase in the rate of isolation of cryptosporidium. The median duration of illness was 9 days (range, 1 to 55). The median maximal number of stools per day was 12 (range, 1 to 90). Among 285 people surveyed who had laboratory-confirmed cryptosporidiosis, the clinical manifestations included watery diarrhea (in 93 percent), abdominal cramps (in 84 percent), fever (in 57 percent), and vomiting (in 48 percent). We estimate that 403,000 people had watery diarrhea attributable to this outbreak. CONCLUSIONS. This massive outbreak of watery diarrhea was caused by cryptosporidium oocysts that passed through the filtration system of one of the city's water-treatment plants. Water-quality standards and the testing of patients for cryptosporidium were not adequate to detect this outbreak.
One concise summary written by J. Gordon Millichap, MD of Northwestern University Medical School, states:
The massive outbreak of Cryptosporidium parasitic infection in Milwaukee in 1993 was a reflection not only of the inadequate filtration system and inefficient water quality monitoring but also resulted from the slow response of physicians to recognize and diagnose the cause of the waterborne illness in patients who sought medical help. Cases were misdiagnosed as viral gastroenteritis or "intestinal flu" without further investigation, and the special testing procedures required for the detection of Cryptosporidium in stools examined for ova and parasites were not requested. The delay in diagnosis resulted in an outbreak involving nearly one half a million individuals. If the medical community had been more alert to possible contamination of water supplies with parasites, the outbreak would have been contained and the deaths of 100 patients may have been prevented.
The massive waterborne outbreak of cryptosporidiosis in 1993 in Milwaukee caused illness in approximately 403,000 persons and generated substantial healthcare costs and productivity losses. We estimate that on average, ill persons incurred approximately $79 in medical costs and $160 in productivity losses, resulting in $31.7 million in total medical costs and $64.6 million in total lost productivity. Since epidemiologic estimates of incidence contribute substantially to total cost estimates for any outbreak, information on average cost of illness by severity can be applied to any range of epidemiologic estimates to assess the sensitivity of total costs. For example, in the Milwaukee outbreak, the 95% confidence interval for epidemiologic burden (incidence or prevalence) of illness ranged from 370,000 to 435,000 persons (2,400 to 6,400 for severe cases, and 38,000 to 50,000 for moderate cases). Applying these epidemiologic burden of illness estimates to the average cost per case by severity, total medical costs and productivity losses for the Milwaukee outbreak ranged from $75 to $118 million.
Although only 1% of persons who experienced diarrheal illness associated with the outbreak were hospitalized, their medical costs accounted for 89% of the total outbreak-related medical costs. Persons with suppressed immune systems were the most severely affected, accounting for 66% of hospitalizations and 74% of the total outbreak-related direct medical costs. Persons with AIDS incurred hospital costs five times greater than persons with no underlying condition. Persons with underlying conditions other than AIDS incurred almost twice the cost of hospitalization compared with persons with no underlying condition.
Examination of 1993 outbreak
(In Step, vol. 10 no. 8; ? 1993)
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(In Step, vol. 10 no. 8; ? 1993)
Page 3- Examination of 1993 outbreak
(In Step, vol. 10 no. 8; ? 1993)
(CNN web site, September 2, 1996)
Credits: design and arrangement by Don Schwamb.
Last updated: May-2006