MASSIVE OUTBREAK OF WATERBORNE CRYPTOSPORIDIUM INFECTION IN MILWAUKEE, WISCONSIN - RECURRENCE OF ILLNESS AND RISK OF SECONDARY TRANSMISSION

Contamination of the public water supply in Milwaukee during March and April 1993 resulted in a massive outbreak of cryptosporidium infectio n, We investigated the clinical and epidemiological features of visito rs to the Milwaukee area in whom cryptosporidiosis developed, and we c onducted a telephone survey of Milwaukee County households to evaluate the risk of recurrent illness and secondary transmission. Cryptospori dium infection during this outbreak generally seemed more severe than cases described in previous reports of large case series. The risk of secondary transmission within a household was low (5%) when the index case involved an adult. The recurrence of watery diarrhea after appare nt recovery was a frequent occurrence among visitors with laboratory-c onfirmed cryptosporidium infection (39%) and among visitors and Milwau kee County residents with clinical infection (21%). The interval betwe en the initial recovery and the onset of recurrence was prolonged (gre ater than or equal to 5 days) in 6%-8% of persons, This pattern of rec urrence and its impact on transmission and our understanding of the pa thophysiological mechanisms of cryptosporidium infection merit further investigation.