Landscape influences on host parasitism: differences between host taxa
Environmental influences on infectious diseases are of increasing interest and importance given the recent and projected abiotic and biotic changes for many habitats. These include anthropogenic activities such as agriculture and urbanization that can affect terrestrial landscapes and the presence of contaminants in aquatic ecosystems. Such modifications can in turn influence the susceptibility and tolerance of hosts to pathogens through various means such as encounter rate, behavior, and immunocompetence. We compared the prevalence of trematode parasites in two different host taxa (larval amphibians and odonates) often used as second intermediate hosts for these flatworms with complex life cycles, focusing on agricultural habitats and associated landscape features. Previous studies have found greater infection levels in larval amphibians from habitats characterized by eutrophic conditions and the presence of particular agricultural pesticides, but it is unclear whether these effects are taxon-specific. Landscape elements such as forest cover and road density are also known to influence infectious disease patterns, primarily through host/vector habitat suitability, thereby affecting pathogen encounter rate.
There was a significant difference between host types (larval amphibian or odonate) with respect to overall prevalence of trematode parasite infection, and an interaction with site agricultural activity. Overall, a greater proportion of our sampled amphibians harboured trematode infections compared to odonates, with an even stronger difference between host types from agriculture-associated ponds, whereas there was no effect of site type for odonates. In addition, infection in larval odonates was not associated with measures related to forests and roads (mean and shortest distance, respectively, to each feature). In contrast, tadpole infections were significantly related to these landscape features, and agricultural and non-agricultural ponds generally differed with respect to their associations with forest and road distance. Our results indicate that parasitism in different host taxa is not necessarily affected in the same way by anthropogenic activities, suggesting the need to further study the mechanisms involved, such as pathogen encounter and host resistance or tolerance.