Milwaukee, 1993: The Largest Documented Waterborne Disease Outbreak in US History

An Interview with Dr. Stephen Gradus, Ph.D., MT(ASCP), D(ABMM), City of Milwaukee Health Department.

Waterborne disease outbreaks are relatively rare events in our time, but just over two decades ago, Milwaukee experienced the largest documented drinking water outbreak in US history. Caused by the chlorine-resistant parasite Cryptosporidium parvum,the outbreak affected over 400,000 people—25 percent of Milwaukee’s population in 1993—and resulted in over $96 million in combined healthcare costs and productivity losses, according to a study by the US Centers for Disease Control and Prevention. We contacted Dr. Stephen Gradus, Director of the City of Milwaukee Health Department Public Health Laboratories Division since 1990, for a look back at the event, including lessons learned, and improvements implemented.

Microphotograph of Cryptosporidium parvum oocysts
Microphotograph of Cryptosporidium parvum oocysts Image courtesy of the US Centers for Disease Control and Prevention.

What was the first sign of trouble in Milwaukee in 1993, and how did the health department respond?

On Monday morning, April 5, 1993, the laboratory’s Chief Virologist and the Commissioner of Health received calls inquiring about the nature of apparent gastrointestinal (GI) illness reports in the City. The Director of Nursing had anecdotal information that some pharmacies were selling out of anti-diarrheal medications. Unknown to the health department at that time, the Milwaukee Water Works had received some complaints regarding the aesthetic quality of tap water. After the Health Commissioner inquired about any information our Public Health Laboratory (MHDL) might have regarding GI illness in the community, we proceeded to call local hospital microbiology laboratories and ultimately emergency rooms and determined there was extreme GI illness throughout the city based on the much higher ER patient numbers and increased workload for enteric disease tests. Other staff in the department were also seeing similar indicators throughout the city. I then contacted both the North and South water treatment plants (WTP) to obtain daily water quality data from the previous 30 days, which indicated the only changing trend was the increasing turbidity readings from the south water treatment plant, yet were within federal limits. The lab also started a rapid fax-back survey of city clinical microbiology labs the following day that confirmed a dramatic increase in testing for GI pathogens citywide yet no agents were identified that would reflect such widespread illness.

How many people became sick and how many died as a result of the waterborne disease outbreak?

The initial study estimated that 403,000 residents of the five-county area around Milwaukee had watery diarrhea attributed to the outbreak. Subsequent studies suggested this was an underestimation.

The official outbreak-related attributable mortality was 69 deaths, of which 93 percent occurred in persons with AIDS.

How did your training prepare you for the challenge of responding to such a serious waterborne disease outbreak?

My training as a medical technologist and clinical microbiologist, combined with two years as a postdoctoral fellow at CDC and eight years of experience in the department, had provided preparation for addressing outbreak situations. Coincidentally I had given talks and written an article on Cryptosporidium as a waterborne pathogen prior to the outbreak. Our departmental team of dedicated public health professionals, along with a community of motivated clinical microbiologists, were key to a timely public health and laboratory response.

How did the public react to the boil water advisory and what were your greatest public communications challenges during the outbreak?

The public had many questions regarding the outbreak and the news media proved to be a critical source of information to the public, providing a steady flow of information daily for many weeks. Initially a phone bank was set up at a local TV station to field questions from the public as well. The boil water advisory affected food production and recalls, certain industry operations, medical and pet care, and the food establishment industry to name a few impacts of daily life of Milwaukeeans and surrounding communities.

Regularly scheduled press conferences by the Health Department and other officials worked well in providing information to the public.

Even though all water quality indicators were within federal guidelines, there was quite a bit of anger in the community regarding the outbreak, as well as loss of trust in the use of potable water.

As tragic as the Milwaukee episode was, were there any significant positive outcomes from the 1993 outbreak?

Since 1993, Milwaukee Water Works (MWW), with the endorsement of the Mayor and Common Council, in ongoing investment has committed $417 million in its infrastructure to ensure high-quality water (as reported to the Public Service Commission of Wisconsin). The capital budget is based on long-term planning to replace or upgrade existing infrastructure, and to install new infrastructure as needed. The Capital Improvements Program prioritizes projects based on results of water-related research, new technology and condition assessments of existing systems.

The immediate response was a renovation of facilities from 1993-1998 to strengthen the barriers related to source water protection, disinfection and filtration. The detailed improvements that Milwaukee Water Works has put forth are available at These improvements have led Milwaukee to be a leader in water quality and water testing.

A key effort that also came out of the experience has been a collaboration of MWW with the Milwaukee Health Department that we call our Interagency Clean Water Advisory Council (IACWAC). IACWAC tracks and can respond to public health issues that may be related to water. Groundbreaking in the 1990s, the ongoing partnership is now recognized nationally for its effectiveness in protecting public health. The utility relays critical information about emerging contaminants, water treatment and water quality monitoring via communications with news media and customer service representatives, and at

At the national level to address public health and regulatory issues prompted by the concern of waterborne cryptosporidiosis and in particular the Milwaukee Cryptosporidium outbreak, the CDC and EPA called a meeting at CDC in September 1994. Represented at this meeting were experts from water industry and utilities, the US Department of Agriculture, the Food and Drug Administration, local (including several of us from Milwaukee), state and federal public health agencies, laboratorians, and advocacy groups, totaling more than 300 individuals from 40 states. From the two-day meeting The Working Group on Waterborne Cryptosporidiosis was created which included 17 task forces to address specific topics related to waterborne cryptosporidiosis. CDC maintains a variety of online resources on Cryptosporidium at: .

What is Milwaukee doing today to help guard against another waterborne disease outbreak?

We are very proud in Milwaukee that our water is some of the highest quality in the nation. We have an effective, multiple-barrier process of source water protection, ozone disinfection, chlorine disinfection, biologically active filtration, and continuous water quality monitoring. Milwaukee’s drinking water quality meets or exceeds all Wisconsin Department of Natural Resources (DNR) and EPA standards. The water utility’s water quality monitoring program tests for many more illness-causing pathogens and contaminants than are required by the EPA. In fact, the utility now tests source and treated water for more than 500 contaminants. Our inter-agency collaboration also continues, and will continue, in order to promote collaboration in protecting public health.

Have the lessons learned in Milwaukee been shared with other water systems?

The outbreak has been well-documented and written about both by news media and academic researchers – and we at the City of Milwaukee Health Department continue to get calls from media and those in academics for the historical perspective.
The events led to improvements worldwide in water quality treatment processes, water quality monitoring and regulations to protect public health. In particular, the ongoing partnership between MWW and the Milwaukee Health Department for water quality monitoring and public health surveillance, ground- breaking at the time, is now recognized nationally for its effectiveness in protecting public health.

The Milwaukee Health Department Public Health Laboratory (MHDL),, was one of the original labs to participate in the validation study of the EPA Method 1622 for the detection and identification of Cryptosporidium and Giardia in water and has been testing for these parasites as well as culturable viruses since 1994. Visitors locally and worldwide have visited MHDL to observe and learn the EPA methods. More recently, as we continuously update our testing methodologies, MHDL is implementing the new EPA Method 1615 for virus detection by culture and quantitative molecular assays for enterovirus and norovirus genogroups GI and GII. The qPCR analysis may be completed within 24-48 hours. This allows MHDL to provide valuable information to our WTPs in a more timely manner while offering excess capacity to other utilities. Method 1615 is part of the third Unregulated Contaminant Monitoring Regulation which will occur during 2013-2015 to monitor 30 contaminants (28 chemicals and two viruses), and will provide a basis for future regulatory actions to protect public health.

Milwaukee is now recognized as a national leader in water quality, and we are proud to be a part of it.