The effects of Neobenedenia girellae (Monogenea) infection on host amberjack Seriola dumerili (Carangidae): Hematological and histopathological analyses

• Neobenedenia girellae is a skin monogenean.
• The infections hindered the growth of Seriola dumerili.
• The infections affected Ht, Na, TP, GLOB and BUN values of the fish.
• The damage to the epidermis by the parasite disrupted the osmotic balance.
• The infections impaired hepatic and kidney functions.

We investigated the influence of infection with the flatworm Neobenedenia girellae (Capsalidae) on the growth of the amberjack Seriola dumerili (Carangidae) in a 14-day rearing experiment. We assessed both hematological parameters and abnormalities present in histopathological sections of the skin and internal organs of infected fish compared with uninfected fish. Uninfected fish grew significantly larger than fish in the infected groups, and mortality was observed only in the severely infected groups, as defined by an infection level > 1 parasite/cm2 fish surface. Hematological parameters, hematocrit, blood urea nitrogen, and total protein and globulin values were significantly lower in the severely infected groups than in the control groups; conversely, the mean Na+ concentration recorded in the severely infected groups was significantly higher than that detected in the control groups. The epidermis of the severely infected fish was significantly thinner than that in the uninfected fish. Most hepatocytes in the livers of the uninfected fish showed glycogen deposition, whereas this was not the case for the severely infected fish. Additionally, we observed hepatocyte vacuolar degeneration and cell necrosis and liver congestion in 40% of the infected fish, and many tubular epithelial cells from the severely infected fish were necrotic and fragmented. These hematological and histopathological symptoms indicated that N. girellae infection disrupts the osmotic balance of the host fish and impairs liver and kidney function.Statement of relevanceAs background, Seriola species are the major commercially cultured fish in Japan and are cultured in floating net pens and cages. The parasite N. girellae (Capsalidae), a capsalid monogenean that infects the skin of host fishes, is problematic in these conditions. In S. dumerili (Carangidae), N. girellae feeds primarily on epithelial cells, and indeed at high numbers it causes not only hemorrhaging of the S. dumerili skin surface but also mortality. However, physiological impairment of the host by N. girellae infections has not been investigated. In this study, we provide a detailed analysis of several relevant biological parameters. Specifically, after infection by N. girellae, we examined not only growth of S. dumerili but also a wide range of hematological parameters and characterized distinctive histopathological features of the liver, kidney, spleen, and intestine. The results of these analyses are consistent with the hypothesis that N. girellae infection disrupts osmotic balance in S. dumerili. The resulting ionic disequilibrium provokes liver and kidney dysfunction that ultimately affects fish growth, and in severely infected fish this dysfunction can lead to death.