Milwaukee Cryptosporidium Outbreak
1993
In the spring of 1993, Milwaukee, Wisconsin experienced the largest documented waterborne disease outbreak in U.S. history. The city's water supply became contaminated with Cryptosporidium, a microscopic parasite that causes the diarrheal disease cryptosporidiosis.
The contamination was likely caused by runoff from cattle pastures that entered Lake Michigan near the water intake for one of the city's water treatment plants. The treatment process at the time was not effective at removing Cryptosporidium.
Cryptosporidium parasite under microscope
- An estimated 403,000 people (about 25% of the city's population) became ill with watery diarrhea
- Over 4,000 people were hospitalized
- At least 69 deaths were attributed to the outbreak, primarily among elderly and immunocompromised individuals
- The outbreak lasted approximately two weeks
Once the outbreak was identified, the city issued a boil water advisory and began flushing the water system. The contaminated water treatment plant was shut down, and the city relied on its other treatment plant while improvements were made.
In response to the outbreak, Milwaukee invested over $89 million to upgrade its water treatment facilities, including the addition of ozonation as a disinfection method effective against Cryptosporidium.
- The importance of protecting water sources from agricultural and urban runoff
- The need for multiple barriers in water treatment to address different types of contaminants
- The limitations of conventional water treatment methods for removing certain pathogens
- The vulnerability of immunocompromised individuals to waterborne pathogens
The Milwaukee outbreak led to significant changes in drinking water regulations, including the Enhanced Surface Water Treatment Rule and the Long Term 2 Enhanced Surface Water Treatment Rule, which specifically address Cryptosporidium and require additional treatment for surface water sources.