Transmission of Bilharziasis. 2. Production of Cercariae.

While a knowledge of cercarial infection rates is essential to an understanding of the dynamics of bilharziasis transmission, rather little attention has been paid to methods of determining these rates. The author considers that the most valuable information is likely to be obtained if studies of infectivity are carried out hand in hand with studies of snail population density and structure. Different methods employed in examining snail populations for infection are discussed, together with the patterns of Schistosoma haematobium and S. mansoni cercarial production in relation to natural infections. Other important aspects considered are cercarial shedding in relation to unisexual and bisexual infections, and the use of laboratory animal exposure in natural waters as a means of determining transmission patterns. The author stresses that infection rates and worm burdens obtained from the animal exposure method have to be evaluated carefully in relation to the state of the habitat.