Disproportionate infection by Eustrongylides sp. (Nematoda: Dioctophymatoidea: Dioctophymatidae) in the imperiled Harter's watersnake, Nerodia harteri

Describing host-parasite dynamics in natural systems is a priority for hosts of conservation concern. Harter's watersnake (Nerodia harteri ssp.) is one such species, endemic to north-central Texas and impacted by habitat alteration, low genetic diversity, and emerging infectious diseases. In this study, we report new records of Eustrongylides sp. infections in three watersnake species (N. harteri ssp., N. erythrogaster, and N. rhombifer) from five artificial reservoirs in the Brazos and Colorado River watersheds in north-central Texas. Nematodes were contained within subcutaneous nodules on the midbody of sampled snakes, and we quantified nodule presence as a proxy for infection prevalence. From seven individuals, we extracted nematodes from nodules for molecular analysis. Phylogenetic analyses on the mitochondrial cytochrome c oxidase subunit I and 18S rRNA genes of excised nematodes confirmed the identity as Eustrongylides sp. and resolved our samples into a single well supported clade divergent to previously accessioned lineages from Asia. The imperiled N. harteri had a higher risk of infection than sympatric watersnakes. Snakes were more frequently infected in Lake Granbury, a stronghold for N. harteri. Though observed infection rates were low, Eustrongylides sp. infections can induce severe pathologic responses in intermediate hosts, and our findings highlight the need for assessments of fitness implications and further parasitological surveillance in N. harteri to inform management.