Transmission dynamics of Schistosoma japonicum within China

Abstract

Schistosoma japonicum, a multi-host parasite, remains highly endemic in China and has recently re-emerged in previously controlled regions. One reason may be, given the current human- and bovine-based control policy, due to a serious lack of knowledge of the potential role for other species of mammals, including dogs, cats and small rodents, in the transmission. This thesis aims to contribute to our understanding of transmission dynamics of S. japonicum by investigating the implications of different reservoirs in the transmission across two contrasting geographical regions/settings: the marshland with the disease persistence versus the hilly region where the disease was once controlled. Longitudinal characterization of S. japonicum infection at both definitive host and intermediate host levels was performed throughout 2006-2007, with the highest prevalence and infection intensity observed in rodents in the hilly region and in the agriculturally important domestic animals (cattle and goats) in the marshland. Three chronobiological trials of cercarial emergence were performed to identify any host (with nocturnal vs diurnal activity)-associated biological traits of the parasite. A late afternoon shedding pattern was observed in the hilly region, compatible with a nocturnal rodent reservoir, and a morning-afternoon dual shedding pattern within marshland areas, consistent with a diurnal bovine major reservoir. Characterization of the parasite population genetic diversity, using microsatellite markers, at both larval stages, also indicated cattle to be the main definitive host reservoir species in the marshland, which was further confirmed by sibling relationship analyses. In the hilly regions, however, epidemiological, biological and molecular data indicated that, in addition to the role of rodents as the main reservoirs to maintain the disease, dogs, with their higher mobility, may also play a significant role in S. japonicum transmission in these areas. The implications of these results, in terms of parasite strain sub-structuring and targeted disease control, were discussed.