Epidemic and endemic seroprevalence of antibodies to Cryptosporidium and Giardia in residents of three communities with different drinking water supplies

This study was carried out to compare cryptosporidiosis and giardiasis sero prevalence rates in residents of three communities. Community (Com 1) uses drinking water from deep wells, community 2 (Com 2) uses surface water from a protected watershed, and community 3 (Com 3) uses surface water frequent ly containing Cryptosporidium oocysts and Giardia cysts. Unfiltered drinkin g water from each community was collected at the tap and tested for Cryptos poridium oocysts and Giardia cysts during the 12 months in which sera were collected for testing. No oocysts or cysts were detected in the water from the Com 1 deep wells; oocysts and cysts were detected intermittently in the drinking water from the other two communities. A waterborne outbreak of cr yptosporidiosis occurred in a municipality adjacent to Com 3 six months int o this 12-month study. Sera from residents of each of the communities were collected proportionately by month and by population size. Coded sera were tested for IgG to Cryptosporidium using a previously developed Western blot ting method. The presence or absence of bands at 15-17 kD and/or 27 kD was recorded for the 1,944 sera tested. Definite bands at 15-17 kD and/or 27 kD were detected in 981 (50.5%) of the sera. A total of 33.2% of sera from Co m 1 (community using deep wells) were positive using the same criteria comp ared with 53.5% (Com 2) and 52.5% (Com 3) of sera from the two communities using surface drinking water Both bands (15-17 kD plus 27 kD) were detected in 582 sera (29.9%) from the three communities: 14.1% of sera from Com 1 c ompared with 32.7% from Com 2 and 31.5% from Corn 3, These findings are con sistent with a lower risk of exposure to Cryptosporidium from drinking wate r obtained from deep well sources. However, analysis of results by calendar quarter showed a significant (P < 0.001) increase in the number of Com 3 p ositive sera (compared with Com I) following the waterborne outbreak. Witho ut this outbreak-related observation, a significant overall difference in s eropositivity would not have been seen. We also observed that in sera from the community affected by the outbreak, the presence on immunoblots of both Cryptosporidium bands appeared to be the best indicator of recent infectio n. Seroprevalence rates using an ELISA to detect IgG to Giardia were estima ted using the same sera. Overall 30.3% (590 of 1,944) of sera were positive by the ELISA. A total of 19.1% of sera from Com 1, 34.7% from Com 2 and 16 .0% from Com 3 were seropositive. Rates for both Com 3 and Corn 1 did not c hange significantly over time. In Com 2, rates decreased significantly (P < 0.001) during the last half of the study period (third and fourth calendar quarters). The reasons for the decrease in seroprevalence in Com 2 sera ar e presently not known. These studies show intriguing associations between s eroprevalence, outbreak-related laboratory serologic data, and patterns of parasite contamination of drinking water. Further studies are required to v alidate the serologic approach to risk assessment of waterborne parasitic i nfections at a community level.