Effect of ectoparasitic anchor worm ( Lernaea cyprinacea ) infection on gut microbiota and health of molly ( Poecilia sphenops )

The anchor worm, Lernaea cyprinacea, is an ectoparasitic copepod that causes severe infections, impaired growth, physiological stress, and morbidity in a wide range of freshwater fish species. However, information on acute L. cyprinacea infection in molly (Poecilia sphenops) and its potential effects on gut bacterial composition, bacterial virulence, and host health remains limited. In the present study, we investigated a case of acute L. cyprinacea infection and associated mortality in P. sphenops. The parasite was isolated and characterized using morphological and molecular approaches based on the COX1 gene. We further evaluated alterations in gut-associated bacterial communities and phenotypic characteristics of bacterial isolates, including virulence traits and antibiotic susceptibility. Six bacterial species representing the families Enterobacteriaceae, Hafniaceae, Streptococcaceae, and Aeromonadaceae were identified from infected and control fish, with parasite-infected fish showing an increased relative abundance of potentially pathogenic bacterial families. The bacterial isolates exhibited pronounced hemolytic activity, biofilm-forming ability, and swimming motility, while antibiogram analysis revealed considerable antimicrobial resistance, including multidrug-resistant phenotypes. To further assess the pathogenic potential of the recovered bacterial isolates, healthy Labeo rohita, a model fish species, were experimentally challenged with Raoultella ornithinolytica, Aeromonas dhakensis, A. caviae, A. veronii, Lactococcus garviae, and Edwardsiella tarda at different bacterial concentrations. Challenged L. rohita developed clinical signs, including hemorrhagic subcutaneous ulcers, oral lesions, and reddening at injection sites. In addition, L. cyprinacea-infected P. sphenops exhibited significant alterations in immune, stress, and antioxidant biomarkers, including increased IgM, C3, HSP70, and cortisol levels and changes in SOD and CAT activities. Overall, the findings indicate that L. cyprinacea infection is associated with alterations in gut bacterial composition and phenotype, increased pathogenic potential of bacterial isolates, and compromised host physiological status. This study provides insights into the complex interactions among ectoparasitic infection, bacterial communities, and fish health and highlights the importance of integrated parasite control, antimicrobial resistance surveillance, and biosecurity in ornamental fish culture.