The serological detection of antibodies to Cryptosporidium has emerged as a potential tool for population level studies. Second generation ELISA methods are known to detect immunoglobulin G (IgG) to two Cryptosporidium sporozoite surface antigens (apparent molecular mass 17 and 27 kDa). During 1996, 1,292 cases of cryptosporidiosis were reported to public health in the province of British Columbia in western Canada. This rate, seven times higher than that in 1995, was due to waterborne outbreaks occurring in this province. It was recognized that collection of serum specimens for study of the host immune response would be important and sera were collected from 41 adult volunteer cases associated with these outbreaks. A total of 232 sera were collected over a 24-month period. These Kinetics Cases sera were used to assess the host immune response. Sera from residents of six communities, some of which had outbreaks of cryptosporidiosis due to contamination of their drinking water, were also tested to determine the usefulness of serology at a population level. The first objective of this project was to determine the sensitivity of the second-generation enzyme linked immunosorbent assays (ELISA) developed at the Centers for Disease Control and Prevention (CDC) and of a previously described immunoblot assay. After that, the researchers planned to address some issues related to specificity of the ELISA. Then, the goal was to analyze changes in levels of antibody reactivity over time in persons with laboratory-confirmed cryptosporidiosis. The next objective was to compare initial results of the ELISA and immunoblot at a community level and further assess the usefulness of the ELISA at a population level by testing sera of residents of six communities, some of which had waterborne outbreaks of cryptosporidiosis. Finally, the researchers planned to evaluate the usefulness of saliva specimens for the detection of antibodies to Cryptosporidium using the ELISA assay. A serological approach appeared to be a useful epidemiological tool to assess cryptosporidiosis at the population level. The host immune response to both Cryptosporidium antigens was documented. The second generation CDC ELISA was the optimal format. The 17 kDa assay, in which the IgG response diminishes within 12 months, appeared to be a useful tool for studying recent events while the 27 kDa appeared to detect cumulative events. When applied to six communities (outbreak and non-outbreak), the seroprevalence within the communities varied. Outbreaks were detectable in some communities. Originally published by AwwaRF for its subscribers in 2003