Divergent patterns of infection among helminths and Ophidiomyces ophidiicola in water snake communities (Nerodia spp.)

Environmental conditions and host biology create complex patterns in coinfection among parasites and pathogens. These relationships may be synergistic, where presence of one species increases susceptibility to infection by subsequent species, or antagonistic, where infection with one species limits infection by subsequent species. Therefore, understanding coinfection dynamics is important for predicting disease in wildlife. In this study, we examined coinfection patterns in three snake hosts ( Nerodia harteri, N. erythrogaster , and N. rhombifer ) that use aquatic and terrestrial habitats, which leads to exposure to diverse native parasite species and emerging fungal pathogens. Two such native parasites, oral trematodes ( Ochetosoma spp.) and nodule-forming nematodes ( Eustrongylides spp.), are known to cause skin lesions under certain conditions, which may facilitate subsequent invasion by fungal or bacterial pathogens. We used non-invasive procedures to survey oral trematodes, nodules (a potential indicator of nematode infection), and one emerging fungal pathogen ( Ophidiomyces ophidiicola ) among 371 snakes across seven artificial reservoirs in Texas in 2025. Using a combination of single-parasite models and co-occurrence analyses, we aimed to clarify the environmental and biological factors driving presence of these three parasites/pathogens. Overall prevalence was moderate for trematodes (20%) and O. ophidiicola (25%), and low for nodules (6%). In single-parasite models, the main predictors of infection were host species for trematodes (highest in N. erythrogaster ), reservoir identity for nodules (highest at Lake Granbury), and season for O. ophidiicola (highest in Spring). We found no evidence that infections occurred together more or less frequently than expected by chance, even after accounting for variation in infection associated with host species, reservoir, and season. Our results show that infection patterns in Nerodia spp. are driven largely by host and environmental factors, rather than by interactions among parasites.