The 1993 Milwaukee Cryptosporidium outbreak was a significant distribution of the Cryptosporidium protozoan in Milwaukee, Wisconsin. This was the largest waterborne disease outbreak in documented United States history.
The Howard Avenue Water Purification Plant was contaminated, and treated water showed turbidity levels well above normal. The root cause of epidemic was never officially identified; initially it was suspected to be caused by the cattle genotype due to runoff from pastures. MacKenzie et al. and the CDC showed that this outbreak was caused by cryptosporidium oocysts(inactive form that can survive numerous environmental conditions including chlorine) that passed through the filtration system of one of the city's water-treatment plants.
Over aproximately two weeks, about 400,000 of an estimated 1.61 million residents in the Milwaukee area (880,000 were served by the malfunctioning treatment plant) became ill. At least 104 deaths have been attributed to this outbreak, mostly among the elderly and immunocompromised people, such as AIDS patients.
Milwaukee Health Department has opened a new lab “Crypto Lab” and the tests will provide information for lone-term use. The experiments helped to reduce the number of Cryptosporidium-sized particles by emphasizing the turbidity.