Public Health: Milwaukee Public Drinking Water Disaster 1993



Life cycle of Cryptosporidium spp. Sources: CDC/Alexander J. da Silva, PhD/Melanie Moser (PHIL #3386), 2002 – CDC Public Health Image Library, Public Domain



In April 1993, Milwaukee, Wisconsin suffered the largest waterborne disease outbreak in U.S. history – 100 people died and 403,000 were sickened. At that time, Milwaukee was served by two water treatment plants that used raw water from Lake Michigan. When people started complaining about the odor and taste of their tap water, calls began flooding the Milwaukee Health Department. The water treatment plants had just passed inspection and tests for bacteria and viruses came up blank. Before the outbreak, severe spring storms, flooding caused the lake’s turbidity and bacterial counts to rise dramatically. During the outbreak, effluent produced by one plant had a turbidity approaching 2.5 ntu, a high reading that indicated an increase in particulates passing through the plant. The increase may have also meant an increase in passage of Cryptosporidium oocysts.


On April 5, 1993, the Wisconsin Division of Health was contacted by the Milwaukee Department of Health after reports of numerous cases of gastrointestinal illness that had resulted in widespread absenteeism among hospital employees, students, and schoolteachers. Little information was available about the nature of the illness or the results of laboratory tests of stool specimens from those who were ill. On April 7, two laboratories identified cryptosporidium oocysts in stool samples from seven adult residents of the Milwaukee area; none of the laboratories surveyed had found evidence of increased or unusual patterns of isolation of any other enteric pathogen.


The Milwaukee Water Works (MWW), which obtains water from Lake Michigan, supplies treated water to residences and businesses in the City of Milwaukee and nine surrounding municipalities in Milwaukee County. Either of two water-treatment plants, one located in the northern part of the city, and the other in the southern part, can supply water to the entire district; however, when both plants are in operation, the southern plant predominantly serves the southern portion of the district. Examination of the two plants’ records on the quality of untreated water (intake) and treated water (that supplied to customers) revealed an increase in the turbidity of treated water from the southern plant, beginning approximately on March 21, with increases to unprecedented levels of flooding from March 23 through April 5. These findings pointed to the water supply as the likely source of infection and led to the institution, on the evening of April 7, of an advisory to MWW customers to boil their water. The southern plant was temporarily closed on April 9. The policies, procedures, and physical plant of the southern MWW facility were reviewed and inspected in April 1993. Water that had been frozen and stored by a southern Milwaukee company in 213-liter blocks on March 25 and April 9, 1993, was melted and examined for cryptosporidium oocysts with an immunofluorescent technique. Medical examination for enteric pathogens was begun among 14 clinical laboratories in Milwaukee County. The laboratories reported the retrospective and prospective test results for all stool specimens submitted for bacterial or viral culture and examination for ova and parasites and for bacterial culture. Bacteria for salmonella, shigella, campylobacter, and cryptosporidium were found. Specimens were examined by electron microscopy. Serum samples obtained during the acute and convalescent phases of illness in residents were tested for antibody to the Norwalk virus. Bacterial cultures for yersinia and aeromonas were positive. During the same medical evaluation, specimens examined for ova and parasites were found to have giardia, and specimens cultured for enteric viruses were positive, including rotavirus.


To collect data, telephone calls and surveys were made to patients and questionnaires (long and short) were distributed to collect information on demographic characteristics and clinical illness. Questions were also asked about preexisting chronic diseases, weight loss, recurrent diarrhea, and length of hospital stay. People were considered to be immunocompromised if they reported having had a positive test for the human immunodeficiency virus or if they were being treated with immunosuppressive drugs, cancer chemotherapy, radiation therapy, or renal dialysis. Telephone surveys were also done to determine the extent of the outbreak of disease. At the time of the outbreak, both of Milwaukee’s water treatment plants treated water by adding chlorine and polyaluminum chloride (a coagulant to enhance the formation of larger particulates), rapid mixing, mechanical flocculation (which promotes the aggregation of particulates to form floc), sedimentation, and rapid sand filtration. After filtration, the effluent (treated water) was stored in a large clear well until it was supplied to customers. Filters were cleaned by backwashing them with water, which was then recycled through the treatment process. Water obtained by melting ice blocks contained cryptosporidium. When samples were sent to the Centers for Disease Control and Prevention, oocysts examined by the CDC were 4 to 6 micrometers in diameter and were positive for cryptosporidium with monoclonal-antibody staining. In general, the frequencies of signs and symptoms of illness were similar in immunocompromised and immunocompetent patients. However, the immunocompromised patients had more diarrhea per day. By limiting the case definition to watery diarrhea in surveys taken, the size of the affected population (403,000), was probably underestimated.


Despite communitywide increases in diarrheal illness in Milwaukee, the recognition of cryptosporidium infection as the cause of this outbreak was delayed for several reasons. The constellation of gastrointestinal symptoms (e.g., diarrhea, abdominal cramping, and nausea) and constitutional signs and symptoms (e.g., fatigue, low-grade fever, muscle aches, and headaches) reported by Milwaukee-area residents led many physicians to diagnose viral gastroenteritis or “intestinal flu,“ without further investigation. Our findings suggest that people with diarrhea seek health care infrequently, do so only when the illness is severe or prolonged, and are unlikely to be tested for cryptosporidium infection. Unlike the detection of other intestinal parasites, which are identified by means of a standard examination for ova and parasites, the detection of cryptosporidium requires special testing. Infrequent testing for cryptosporidium in patients with diarrhea may be due to misconceptions about the incidence and severity of this infection among immunocompetent patients. In the Milwaukee outbreak, cryptosporidium oocysts in untreated water from Lake Michigan apparently entered the southern water-treatment plant and were then inadequately removed by the coagulation and filtration process. Cryptosporidium oocysts have often been found in untreated surface water used for public water supplies in the United States. The sources of the oocysts leading to the outbreak in Milwaukee and the timing of their entrance into Lake Michigan include, cattle along two rivers that flow into the Milwaukee harbor, slaughterhouses, and human sewage. Rivers that were swelled by spring rains and snow runoff, causing flooding, may have transported oocysts into Lake Michigan and from there to the intake of the water treatment plants.


Because some visitors to the MWW service area who drank very small amounts of water (<240 ml [8 oz]) had laboratory-confirmed cryptosporidiosis, the peak concentration of oocysts in the water probably far exceeded one oocyst per liter. Thus, the concentration of cryptosporidium oocysts found in the tested ice vastly underestimates the peak level in water. The number of both laboratory-confirmed and clinically defined cases of cryptosporidium infection with an onset of illness was higher than expected, suggesting that cryptosporidium oocysts had entered the water supply before the increase in turbidity was apparent. Cryptosporidiosis is an underdiagnosed condition, and outbreaks are likely to be under recognized. 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 (e.g., a goal of turbidity <0.1 NTU). It has been recommended that clinicians and laboratories consider performing routine tests for cryptosporidium in people with watery diarrhea and that public health officials make cryptosporidium infection a reportable condition. In the United Kingdom, water and health officials have already developed an extensive strategy to investigate the clinical importance of cryptosporidium found in water supplies. Intensive efforts and cooperation between the medical community and those who provide and regulate drinking water in the United States will be required to prevent future waterborne outbreaks caused by this emerging pathogen and ensure the safety of drinking water for all citizens.


Sources: NEJM,,,; Division of Parasitic Diseases, Center for Infectious Diseases; Roger Glass, M.D., M.P.H., Ph.D., Stephan S. Monroe, Ph.D., Charles Humphries, Ph.D., and Sara Stine, Centers for Disease Control and Prevention (CDC) Viral Gastroenterology Laboratory; Margaret Hurd and the staff of the CDC Parasitology Laboratory; the staff of the Wisconsin State Laboratory of Hygiene; the staff of the Survey Research Laboratory, University of Wisconsin Extension; Darren Lytle, P.E., U.S. Environmental Protection Agency; and Ava Navin, Epidemiology Program Office, CDC.


Milwaukee Water Disaster 1993



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