CRYPTOSPORIDIOSIS IN WASHINGTON-STATE - AN OUTBREAK ASSOCIATED WITH WELL WATER

In 1994, an outbreak of cryptosporidiosis occurred in a rural communit y in Washington State where water was supplied by two deep unchlorinat ed wells. Confirmed case-patients had a stool specimen containing Cryp tosporidium parvum oocysts. Probable case-patients had diarrhea lastin g greater than or equal to 5 days. Sixty-two households (68.1% of 91) responded to a survey, Eighty-six cases (15 confirmed, 71 probable) we re identified, for an attack rate of 50.9% (86/169 residents). Drinkin g unboiled well water was associated with being a case-patient (relati ve risk, 1.84; 95% confidence interval, 0.89-3.82), and a significant dose-response relationship was found between water consumption and ill ness (P = .004). Water that was presumed to be treated wastewater from a piped irrigation system was found dripping along one well's outer c asing, which was extensively rusted. Presumptive Cryptosporidium oocys ts were found in well water and in treated wastewater. This investigat ion demonstrates that even underground water systems are vulnerable to contamination.