Water event
Milwaukee Cryptosporidium outbreak
The largest documented waterborne disease outbreak in United States history, caused by Cryptosporidium passing through a functioning but inadequately performing filtration plant.
Key facts
- Event type
- Waterborne disease outbreak
- Started
- 1993-03-23
- Ended
- 1993-04-08
Cause
Deterioration in filtration performance at the Howard Avenue plant allowed oocysts through. Chlorination, operating normally, was ineffective against them.
Impact
- An estimated 403,000 people ill
- At least 69 deaths, predominantly among immunocompromised people
- Substantial economic cost to the city
Response
A boil water advisory was issued once the cause was identified. The plant was subsequently upgraded.
Aftermath
Transformed US drinking water regulation, driving the Interim Enhanced Surface Water Treatment Rule and strict turbidity limits. Established that chlorination alone is not a sufficient barrier, and that filtration performance must be monitored continuously.
Relationships
- Located in: United States
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Sources
- Peer-reviewed scientific literature — Open-access hydrology, water chemistry and water use research. Public but restricted · Publisher terms — public access, reuse not clearly granted