A cryptosporidiosis epidemic occurred among residents and visitors to Collingwood, Ontario, during March 1996. Fifty-five per cent of 36 confirmed cases were Collingwood visitors and 57% of Collingwood resident cases were under 10 years of age. The low level of reported diarrhoeal illness among adult Collingwood residents caused government officials and physicians to question whether an epidemic had occurred in Collingwood.
To better evaluate the extent of the epidemic, anonymous surplus sera from 89 adult Collingwood residents, collected for routine tests prior to, during and after the epidemic, and from 80 adult Toronto residents were tested using a Western blot assay for IgG antibody response to two Cryptosporidium antigen groups (15/17-kDa and 27-kDa).
For sera collected from 1 January 1996 to 17 June 1996, a higher fraction of Collingwood residents had a detectable serological response (P