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 (<i>Nerodia harteri</i> 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 <i>Eustrongylides</i> sp. infections in three watersnake species (<i>N. harteri</i> ssp., <i>N. erythrogaster</i>, and <i>N. rhombifer</i>) 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 <i>c</i> oxidase subunit I and 18S rRNA genes of excised nematodes confirmed the identity as <i>Eustrongylides</i> sp. and resolved our samples into a single well supported clade divergent to previously accessioned lineages from Asia. The imperiled <i>N. harteri</i> had a higher risk of infection than sympatric watersnakes. Snakes were more frequently infected in Lake Granbury, a stronghold for <i>N. harteri</i>. Though observed infection rates were low, <i>Eustrongylides</i> 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 <i>N. harteri</i> to inform management.