Comparison of oral and anal inoculation of Enteromyxum leei into olive flounder Paralichthys olivaceus

Abstract

Enteromyxum leei causes emaciation disease in fish, resulting in economic losses in the aquaculture industry. To develop control methods against E. leei infection, efficient and stable experimental infection of the parasite should be established. To determine the optimal experimental infection of E. leei, we carried out oral and anal inoculation into the olive flounder with different doses of parasite. A total of 90 fish were inoculated with approximately 500 primary cells of E. leei/μL through the oral and anal routes with volumes of 20, 50, and 100 μL. The oral inoculation group showed a higher prevalence, heavier infection intensity, and a greater decrease in relative condition factor than the anal inoculation group. However, there was no significant association between the indicators of parasite infection and the inoculation dose. The mortality in the anal inoculation group was higher than that in the oral inoculation. Dead fish showed rectal hernia and abdominal distension with ascites, and Edwardsiella piscicida was isolated from their kidneys and ascites. Statistical analysis revealed a significant relationship between mortality and E. piscicida infection. Based on the results, we suggest that oral inoculation is more efficient and safer than anal inoculation. In addition, the effect of a bacterial infection should be considered when diagnosing the mortality of cultured olive flounder infected by E. leei.