CASE STUDY

In 1993, residents of the area in and around Milwaukee, Wisconsin were the unlucky victims of the world's worst outbreak of cryptosporidiosis. In late March of that year, residents began to notice that their water seemed cloudy and had an off taste and a noticeable odor. By early April, the local media reported increasing numbers of cases of diarrhea in the city, and the pharmacies had sold out of antidiarrheal medicines. Stool specimens of the people who had gotten sick showed the protozoan Cryptosporidium, a common cause of diarrhea in calves. Health officials found high levels of the organism in one of the city's two water-treatment plants and issued an advisory notice to boil all water.

Over 400,000 people became ill in the five-country Milwaukee area, more than 4000 were hospitalized for the disease, and 54 died. Officials suspect that runoff from dairy farms into the Milwaukee river was responsible for the high levels of Cryptosporidium in the city's water supply. The water-treatment plant was using a new type of coagulant, which should have removed most of the organisms, but lack of familiarity with the coagulant may have led to its misuse. The tiny protozoans and their cysts could have slipped through the filtration system and, because they are resistant to chlorine, made their way to the city's water taps. Because public water- treatment plants are not required to test for protozoans such as Cryptosporidium, their presence in the water supply went unnoticed.