Migratory waterfowl play an important role in the maintenance and spread of zoonotic diseases worldwide. An example is cercarial dermatitis, caused when larval stages of schistosomes that normally develop in birds penetrate human skin. Members of the genus Trichobilharzia (Schistosomatidae), transmitted mainly by ducks, are considered to be major etiological agents of cercarial dermatitis globally. To better understand the diversity and distribution of Trichobilharzia spp., we surveyed ducks from the United States, eastern Canada, Argentina, South Africa and New Zealand. To aid in species identification of the Trichobilharzia worms recovered, regions of the Cox1, ND4 and ITS1 were sequenced. Furthermore, we provide molecular phylogenetic evidence for the cosmopolitan distribution and trans-hemispheric gene flow for one species, Trichobilharzia querquedulae, previously thought to be restricted to North America. These new samples from endemic non-migratory duck species indicate that T. querquedulae transmission occurs within each of the regions we sampled and that it is specific to the blue-winged+silver teal duck clade. Prevalence within this host group is >95% across the known range of T. querquedulae, indicating that transmission is common. Genetic divergence is evenly distributed among continents, and no phylogenetic structure associated with geography was observed. The results provide strong support for the global distribution and transmission of T. querquedulae and represent, to our knowledge, the first report of a cosmopolitan schistosome confirmed by genetic data. These data are the first known to support trans-hemispheric genetic exchange in a species responsible for causing cercarial dermatitis, indicating that the epidemiology of this group of poorly known zoonotic parasites is more complex than previously expected.

Multi-host helminth systems are difficult to study at a population level due to inherent spatial and temporal sampling challenges. Consequently, our understanding of the factors affecting gene flow, genetic drift and effective population size is limited. Population genetic parameters (Ne, Θ, π) are necessary in understanding fundamental processes in host-parasite evolution such as co-evolutionary dynamics, spread of resistance alleles and local adaptation. This study used museum specimens collected over 20-years of three congeneric trematode (Schistosomatidae) species: Trichobilharzia querquedulae, T. physellae, and Trichobilharzia species A . All contribute to the worldwide zoonotic disease cercarial dermatitis (i.e. swimmers Itch). Populations of each species were sampled for two mitochondrial (cox1 and nad4) and one nuclear loci (ITS1) to estimate population genetic structure, genetic diversity, effective size and population history. Significant differences in these measures were revealed among the three congeners. Trichobilharzia querquedulae maintained a well-connected globally diverse metapopulation, with an effective size approximately three times that of the other two species, which were characterized by lower overall genetic diversity and greater population structure, mediated by the definitive duck host. We hypothesize that the species-specific patterns are due to distinctive ecological preferences and migratory behaviors of their respective definitive hosts. This study demonstrates the value of natural history collections to facilitate population genetic studies that would otherwise be infeasible. Applying population genetic data within this comparative congeneric framework allows us to tease apart particular aspects of host-parasite natural history and its influence on microevolutionary patterns within complex helminth systems, including contributions to zoonotic disease.