Massive Outbreak of Waterborne Cryptosporidium Infection in Milwaukee, Wisconsin: Recurrence of Illness and Risk of Secondary Transmission

TLDR

A massive cryptosporidium outbreak in Milwaukee’s public water supply during March–April 1993 highlighted severe infection and raised questions about recurrence, transmission, and underlying mechanisms. The study examined clinical and epidemiological characteristics of visitors who developed cryptosporidiosis and surveyed households to assess recurrence and secondary transmission risk. The authors performed a clinical and epidemiological assessment of affected visitors and conducted telephone surveys of households to evaluate recurrence and secondary transmission. The outbreak produced more severe cryptosporidiosis than prior large series, with a low 5 % secondary transmission rate from adult index cases, frequent recurrence (39 % in laboratory‑confirmed visitors, 21 % overall), and prolonged intervals (>5 days) in 6–8 % of cases.

Abstract

Contamination of the public water supply in Milwaukee during March and April 1993 resulted in a massive outbreak of cryptosporidium infection. We investigated the clinical and epidemiological features of visitors to the Milwaukee area in whom cryptosporidiosis developed, and we conducted a telephone survey of Milwaukee County households to evaluate the risk of recurrent illness and secondary transmission. Cryptosporidium 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 apparent recovery was a frequent occurrence among visitors with laboratory-confirmed cryptosporidium infection (39%) and among visitors and Milwaukee County residents with clinical infection (21%). The interval between the initial recovery and the onset of recurrence was prolonged (> or = 5 days) in 6%-8% of persons. This pattern of recurrence and its impact on transmission and our understanding of the pathophysiological mechanisms of cryptosporidium infection merit further investigation.