A MASSIVE OUTBREAK IN MILWAUKEE OF CRYPTOSPORIDIUM INFECTION TRANSMITTED THROUGH THE PUBLIC WATER-SUPPLY

Background. Early in the spring of 1993 there was a widespread outbrea k of acute watery diarrhea among the residents of Milwaukee. Methods. We investigated the two Milwaukee water-treatment plants, gathered dat a from clinical laboratories on the results of tests for enteric patho gens, and examined ice made during the time of the outbreak for crypto sporidium oocysts. We surveyed residents with confirmed cryptosporidiu m infection and a sample of those with acute watery diarrhea consisten t with cryptosporidium infection. To estimate the magnitude of the out break, we also conducted a survey using randomly selected telephone nu mbers in Milwaukee and four surrounding counties. Results. There were marked increases in the turbidity of treated water at the city's south ern water-treatment plant from March 23 until April 9, when the plant was shut down. Cryptosporidium oocysts were identified in water from i ce made in southern Milwaukee during these weeks. The rates of isolati on of other enteric pathogens remained stable, but there was more than a 100-fold increase in the rate of isolation of cryptosporidium. The median duration of illness was 9 days (range, 1 to 55). The median max imal number of stools per day was 12 (range, 1 to 90). Among 285 peopl e surveyed who had laboratory-confirmed cryptosporidiosis, the clinica l manifestations included watery diarrhea (in 93 percent), abdominal c ramps (in 84 percent), fever (in 57 percent), and vomiting (in 48 perc ent). We estimate that 403,000 people had watery diarrhea attributable to this outbreak. Conclusions. This massive outbreak of watery diarrh ea was caused by cryptosporidium oocysts that passed through the filtr ation system of one of the city's water-treatment plants. Water-qualit y standards and the testing of patients for cryptosporidium were not a dequate to detect this outbreak.