After more than 400,000 people were infected with a waterborne disease in Milwaukee in 1993, a new federal regulation was introduced to improve water treatment standards.
Cryptosporidiosis, a waterborne disease caused by the parasite Cryptosporidium, is spread by infected faeces, and causes watery diarrhoea and stomach cramps.
A widespread outbreak of cryptosporidiosis infected at least 400,000 people in Milwaukee, Wisconsin, in April 1993 after being transmitted through the public water supply. Residents reported a range of symptoms including watery diarrhoea, abdominal cramps, fever and vomiting.
An estimated 69 people, predominantly people living with pre-existing health conditions, died as a result of the cryptosporidiosis outbreak. It had a substantial health care and social impact, costing an estimated $31.7 million in total medical costs and $64.6 million in lost productivity.
Turbidity trouble
Initially, the reasons for the outbreak remained unclear. Theories abounded, including contaminated melting water from nearby Lake Michigan entering the water supply.
In an article published in The New England Journal of Medicine, Mac Kenzie et al. identify the cause as cryptosporidium oocysts, which carries the infection between hosts, entering the filtration system of Howard Avenue Water Purification Plant, a large plant in the city’s south.
Water from Lake Michigan was obtained by Milwaukee Water Works (MWW) and supplied to businesses and homes around the city. Treated water, later supplied to customers, was investigated by Mac Kenzie et al., and found to have an increased level of turbidity (which describes the haziness of water). Turbidity levels became “unprecedented” for the period of 23 March to 5 April.
As shown in Figure 1, turbidity levels rose in the second half of March before peaking multiple times at or above 1.5 nephelometric turbidity units.
“Water-quality measurements at the MWW southern plant were within the required limits,” Mac Kenzie et al. wrote. “However, unlike the plants involved in the previous outbreaks [of cryptosporidiosis in the US], the MWW plant had no evident mechanical breakdown of its flocculators or filters.
“Inspection of the southern plant revealed that a streaming-current monitor, which can aid plant operators in adjusting the dose of coagulant, had been incorrectly installed and thus was not in use. In addition, monitors designed for continuous measurement of the turbidity of filtered water were not in operation.”
Water at both MWW plants was treated with a range of methods. This included polyaluminum chloride, a coagulant designed to enhance the formation of larger particulates; mechanical flocculation, which encourages the particulate matter to clump together; sedimentation and rapid sand filtration. The filters were cleaned by backwashing them with water that was later recycled through the treatment process.
Turbidity peaks of 1.7 NTU on 28 and 30 March 1993 were in spite of an adjustment of the polyaluminum chloride dose at the Howard Avenue Water Purification Plant.
Mac Kenzie et al. concluded that “water-quality standards and the testing of patients for cryptosporidium were not adequate to detect this outbreak”.
“The number of both laboratory-confirmed and clinically defined cases of cryptosporidium infection with an onset of illness before [23 March], when the turbidity of treated water increased, was higher than expected, suggesting that cryptosporidium oocysts had entered the water supply before the increase in turbidity was apparent.
“This occurrence would not be without precedent. In England, a waterborne outbreak of cryptosporidium infection associated with a filtered water supply occurred while the turbidity of treated water remained less than 0.5 NTU.”
Changed regulations
One of the contributing factors in the failure to prevent high turbidity, Mac Kenzie et al. wrote, was the difficulty in identifying the right amount of polyaluminum chloride in the water. To address this, the MWW reinstalled the streaming current monitor after it became clear contamination had occurred.
“The recycling of filter backwash water may increase the concentration of oocysts in water passing through filters. Therefore, this practice has been discontinued.”
Further, the MWW installed continuous turbidity monitors on each filter bed, complete with an alarm system and an automatic shutdown if the filtered water’s turbidity exceeded 0.3 NTU. More frequent monitoring of the particulars in both untreated and treated water was also instituted.
Mac Kenzie et al. suggested improved monitoring processes are, unsurprisingly, critical to ensuring higher standards of water quality.
“Until an inexpensive, rapid and sensitive means of detecting and quantifying cryptosporidium in treated water is available, we believe that water-treatment plants should consider instituting continuous monitoring of treated water for turbidity, particularly of filter effluent, and particle size.
“Plant design and water-treatment procedures should be improved to maintain the quality of treated water at a level that will make the presence of oocysts unlikely.”
The crisis precipitated a new federal regulation that led to extensive water testing. The Interim Enhanced Surface Water Treatment Rule (IESWTR) was introduced to enhance the removal of particulate matter from drinking water treatment systems.
Critically, expanded reporting requirements were instituted. The rule mandated that:
- The combined filtered water effluent turbidity must be less than or equal to 0.3 NTU in 95 per cent or more of the measurements taken each month, while not exceeding 1.0 NTU at any time.
- Existing requirements for the removal of Giardia lamblia and viruses should be continued. This included a 3-log removal or inactivation of Giardia and a 4-log removal or inactivation of viruses.
- All surface water systems serving 10,000 people or more must achieve at least a 2-log removal of Cryptosporidium.
- New microbial profiling and benchmarking requirements should be followed.
Local parallel
Cryptosporidium and Giardia pathogens were inadvertently released into the water supply in Sydney in July-September 1998, causing a water crisis. Residents across the city were told to boil their water to minimise the chance of infection.
An inquiry commissioned by the NSW Government mandated a clearer delineation of duties for Sydney Water and the Sydney Catchment Authority, and improved monitoring of water treatment facilities.
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This story was originally published in the August 2025 issue of create with the headline “Outbreak”.
Sources
The Milwaukee Cryptosporidium outbreak: Its impact on drinking water standards, laboratory diagnosis, and public health surveillance, M.Stephen Gradus, Ajaib Singh, Gerald V. Sedmak, Clinical Microbiology Newsletter, 1994, https://doi.org/10.1016/0196-4399(94)90073-6
A Massive Outbreak in Milwaukee of Cryptosporidium Infection Transmitted through the Public Water Supply, Mac Kenzie et al., The New England Journal of Medicine, 1994, https://www.nejm.org/doi/full/10.1056/NEJM199407213310304
Interim Enhanced Surface Water Treatment Rule Documents, Environmental Protection Agency, 1999, https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=2000229L.txt
What is unclear in the article is that the “new Federal regulation” is related to America only. The relevant Australian drinking water standard is the NHRMC’s Australian Drinking Water Guidelines (ADWG). Incidentally, regarding the alleged “more than 400,000 people”, in “Ensuring Safe Drinking Water: Learning From Frontline Experience With Contamination” (Steve E & Elizabeth J Hrudey, AWA 2014) state “there are valid reasons to question the magnitude of the 403,000 cases estimate” (p19), while noting that it was a massive waterborne disease outbreak .