Investigation of an outbreak of cryptosporidiosis associated with treated surface water finds limits to the value of case control studies.

Fifty-two cases of cryptosporidiosis satisfied the case definition employed in investigation of an outbreak in Spring 1996 among residents of the Wirral peninsula supplied by a single water treatment plant using river water. The attack rate among those whose water was supplied solely from the plant was 1.42 per 10,000, compared with 0.42/10,000 among those having some but less than 50% supplied from it. Single oocysts were detected in treated water from this plant on four occasions during the investigation. A case control study did not demonstrate a significant association between illness and water consumption and no obvious failure in water treatment procedures occurred during the relevant period. Nevertheless, according to PHLS criteria, this outbreak was strongly associated with water, as the descriptive epidemiology was consistent and oocysts were detected, albeit in small numbers, in treated water. This paper discusses the value of detection of oocysts and case control studies in investigating waterborne outbreaks of cryptosporidiosis. Populations that normally drink surface water, particularly river water, are thought to be exposed repeatedly to cryptosporidium oocysts and so develop some protective immunity. Case control studies are therefore likely to have less power in the investigation of outbreaks in localities where the population normally drink treated surface water. Although oocysts are often isolated from treated surface waters without being associated with obvious disease in the population, their detection should still be considered in assessing the strength of association of waterborne outbreaks.