Serological evaluation of Cryptosporidium oocyst findings in the water supply for Sydney, Australia

From July to September, 1998, high levels of Cryptosporidium oocysts and Gi ardia cysts were detected in Sydney, Australia drinking water. To evaluate whether Sydney residents had an elevated risk of infection, serological res ponses to two Cryptosporidium antigen groups (15/17- and 27-kDa) were compa red for 104 Sydney blood donors and 104 Melbourne blood donors. Over half o f all donors had a detectable response to the 15/17-kDa antigen group (Sydn ey-56.7%, Melbourne-61.5%) and to the 2.7-kDa antigen group (Sydney-66.3%, Melbourne-77.9%). There were no statistically significant differences betwe en Sydney and Melbourne donors in the intensity of serological responses fo r either the 15/17-kDa (p = 0.81) or the 27-kDa (p = 0.45) antigen groups. These results are not consistent with an elevated risk of Cryptosporidium i nfection among either Sydney or Melbourne blood donors, despite the detecti on of oocysts in the Sydney drinking water. Given the public health concern s over detecting oocysts in drinking water, serological studies offer a rap id and inexpensive method for comparing the levels of infection in an expos ed and unexposed population.