Detection of zoonotic nematodes in brown trout, Salmo trutta, an introduced popular edible freshwater fish in Australia.

Introduced freshwater fish species in Australia, such as brown trout (Salmo trutta), are commonly consumed and play a significant role in recreational fisheries. However, little is known about their potential to act as carriers of zoonotic parasites. This study investigated the presence of zoonotic nematodes in brown trout collected from above and below Winburndale Dam in New South Wales, Australia. Large nematodes were observed in the musculature during visual inspection. Additional nematodes were recovered following overnight incubation of the fish tissue and the gastrointestinal tract. Morphological and molecular analyses identified the large larvae in the muscle tissue as Eustrongylides excisus. In contrast, the gastrointestinal nematodes were identified as Contracaecum bancrofti, Contracaecum rudolphii D and one Contracaecum larva that may represent a previously undescribed species. Based on our findings, we recommend analysing the ITS-1 and ITS-2 regions separately when conducting BLAST analyses for species-level identification of Contracaecum larvae, as many early GenBank submissions contain only partial sequences. Of parasites found, Eustrongylides larvae were recovered from muscle and body cavity tissues, posing a direct food safety risk, while Contracaecum larvae were confined to the gastrointestinal tract and emerged only during post-mortem incubation, highlighting the value of supplementary observational techniques in detecting parasites that may otherwise be overlooked during routine dissection. The differences in parasite occurrence between upstream and downstream populations may be attributed to variations in diet, and water flow dynamics, particularly in relation to Eustrongylides infection. Contracaecum rudolphii D larvae, which had previously only been reported from marine fish, were found in a freshwater system for the first time in Australia. This study provides the first molecular confirmation of co-infection with multiple potentially zoonotic nematodes in brown trout in Australia, underscoring the need for parasite monitoring and proper food safety practices in freshwater fish species consumed by humans and their pets, such as cats and dogs.