Spatial and temporal variability in schistosome cercarial density detected by mouse bioassays in village irrigation ditches in Sichuan, China.

A mouse bioassay was used monthly over the infection season of 2001 to determine the temporal and spatial variability of schistosome cercarial density in irrigation ditches in five villages in southwestern Sichuan Province in the People's Republic of China. Analysis of variance showed that approximately half of the variability was due to the village and site within the village, with little contribution from air temperature, weekly average rainfall, or the month within the infection season in which the bioassay was performed. The location-specific variability in these data suggest that epidemiologic studies will generally have low power to detect the influence of water-contact intensity on human parasite burden without taking account of variations in cercarial density at sites of water contact.