Stressor interactions affect myxozoan abundance in a 42-year dataset from the Pearl River, Louisiana, USA

Environmental change can impact host-parasite interactions, but the effects of multiple stressors on parasites are rarely measured. Considering stressor interactions may allow parasitologists to evaluate how parasite burdens change in nature, where stressors rarely occur in isolation. This study aimed to understand how combined stressors such as warming, nutrients and pollution (i.e. metal concentrations) influence myxozoan prevalence and abundance in the Pearl River, Louisiana, USA. Fish were seined between 1963 and 2005 upstream and downstream of a pulp-mill outfall and were then preserved and accessioned into the Royal D. Suttkus Fish Collection of the Tulane University Biodiversity Research Institute. In 2024, we dissected 1188 fish individuals across 7 host species, and we identified myxozoans in 6 species. Six myxozoan genera were detected, including <i>Chloromyxum, Henneguya, Myxidium, Myxobolus, Thelohanellus</i> and <i>Unicauda</i>, with some novel host-parasite combinations. The abundance of <i>Myxobolus</i> infecting <i>Carpiodes velifer</i> gills declined by 86% over the study period, while the abundance of <i>Myxobolus</i> infecting <i>Pimephales vigilax</i> gills was significantly lower downstream of the pulp mill outfall. Among the drivers analyzed, temperature had a significant negative effect on this parasite's abundance, metal concentrations had a positive effect, and these 2 drivers interacted. Our results highlight the differential susceptibility of wild fishes to myxozoan infections and the usefulness of museum collections for understanding historical change in myxozoan burdens in fish. Since stressor-driven changes in myxozoan abundance do not follow a single pattern across species, we expect a shift in freshwater myxozoan communities with progressing climate change and pollution.