Parasitological survey of farmed Murray Cod (Maccullochella peelii) in an aquaculture setting.

The Murray cod (Maccullochella peelii) is Australia’s largest freshwater fish and an iconic species of cultural, recreational, and economic importance. While parasites have historically been reported as significant threats in Murray cod aquaculture, recent data on their occurrence remain limited. This study investigated the parasites of farmed Murray cod and co-occurring species in an aquaculture facility, with the aim of identifying parasite burdens and assessing potential risks to fish health and food safety. A total of 50 Murray cod and 47 co-occurring species, including common carp (Cyprinus carpio; n = 3), mosquitofish (Gambusia affinis; n = 7), long-arm shrimp (Macrobrachium sp.; n = 9), and snails (Physella acuta; n = 28), were examined using necropsy, histopathology, and molecular methods. Only nematodes belonging to the genus Procamallanus (Spirocamallanus) were detected, infecting 54% of Murray cod and 57% of mosquitofish. No significant differences in prevalence were observed between collection sites. Histological examination revealed granulomas in the kidney and liver of one Murray cod, and incidental findings of epitheliocystis and ciliates in gills, though these were not considered clinically significant. Other species examined showed no evidence of parasite-associated pathology. The detection of a single parasite species, with no zoonotic risk and limited clinical impact, contrasts with the broader range of parasites historically reported in Murray cod farms. These findings suggest that effective farm management and biosecurity protocols are successfully limiting parasite diversity and impact in the studied system. However, the presence of reservoir species such as mosquitofish highlights the importance of continued monitoring to prevent persistence and transmission.