The scaling of dose with host body mass and the determinants of success in experimental cercarial infections

Experimental studies of parasite transmission can help to elucidate life cycles, measure the success of infective stages under different conditions, or test the efficacy of vaccination or other forms of protection against parasitic infection. By combining the results of experiments on a particular parasite taxon, one may also answer questions such as how experimental infection doses are chosen, or what determines infection success. Here, focusing on trematodes, analyses are conducted on data compiled from a total of 145 cercarial infection experiments (62 on non-schistosomes, 83 on schistosomes) obtained from 115 studies. All of these involved experimental exposure of individual hosts to a single known dose of cercariae under controlled laboratory conditions. Across these studies, the cercarial dose used showed a strong positive relationship with the body mass of the target host, independently of the taxonomic identity of that host or of the method of infection used. Although justification for the chosen dose was rarely given, the larger the target host, the more cercariae it was exposed to. Across all experiments, there was also evidence for a weak but significant dose-dependent effect on infection success: the higher the dose used in an experiment, the smaller the proportion of cercariae recovered from the host. This effect was mitigated by either host body mass (for schistosomes) or host taxonomic identity (for non-schistosomes), with infection being lower in fish than in other host types. Experimental procedures also impacted significantly on infection success, namely the infection method used (for schistosomes) and the time between infection and recovery of parasites (for non-schistosomes). Overall, this analysis of published experimental results provides evidence of both biological processes and confounding methodological effects, and it provides strong arguments for greater rationale in the design of experimental infection studies.