Could Fish Feeding Behaviour and Size Explain Prevalence Differences of the Nematode <i>Eustrongylides excisus</i> among Species? The Case Study of Lake Garda

The nematode <i>Eustrongylides excisus</i> is a parasite of freshwater fish- and fish-eating birds, with known differences on prevalence values among fish species. Thus, the present study aims to explore the hypothesis that the feeding behavior and the size of fish belonging to different trophic levels could explain such differences. For that, 14 sampling sites were selected to perform a fish parasitological survey on Lake Garda (Italy) during spring-summer 2020. Amplification of nuclear ribosomal internal transcribed spacer (ITS) rDNA sequences of nematodes morphologically ascribable to the genus <i>Eustrongylides</i> allowed to identify them as <i>E. excisus</i>. From the five studied fish species (<i>Perca fluviatilis</i>, <i>Lepomis gibbous</i>, <i>Coregonus lavaretus</i>, <i>Alosa fallax lacustris</i> and <i>Micropterus salmoides</i>), only three presented the parasite <i>E. excisus</i>: <i>P. fluviatilis</i>, <i>L. gibbous</i> and <i>M. salmoides</i>, with significant differences in prevalence values among species (<i>p</i> = 0.002). Additionally, there were differences in prevalence values within the same fish species captured from different sampling sites. Findings showed that mainly piscivorous fish were positive for <i>E. excisus</i> and how the prevalence was highest in <i>M. salmoides</i>. As regard the fish size, a negative correlation between body size and <i>E. excisus</i> was found in <i>P. fluviatilis</i> due to the feeding habit of juvenile perch which feed mainly zooplankton and benthic invertebrates (i.e., oligochaetes, which are the first intermediate hosts of <i>E. excisus</i>). The study findings advance novel knowledge in the field of pathogens of zoonotic importance in the aquatic environment.