Pathogenesis and expression profile of selected immune genes to experimental Edwardsiella tarda infection in iridescent shark Pangasianodon hypophthalmus

Abstract

Pathogenesis and immune gene expression of the iridescent shark catfish Pangasianodon hypophthalmus to experimental Edwardsiella tarda infection was studied. Healthy Pangasianodon were inoculated intramuscularly with a sub-lethal dose of 1.77 × 107 cells/fish of E. tarda to induce edwardsiellosis. Petechial haemorrhages, inflammation at the site of injection, vertical hanging from the water surface, swollen abdomen and anorexia were the common signs observed in challenged catfish fingerlings. The morbid catfish also exhibited typical signs of septicaemia with haemorrhages on ventral parts of the body and fin bases and congested vent prior to death. The mortality was observed 48 h post injection. The average percent morality was 42 % in fish exposed to 1.77 × 107 cells of E. tarda, whereas, only6.6 % mortality was observed in placebo control (catfish received 0.1 mL of 0.85 % sterile physiological saline) and no mortality in negative control group (catfish received no injection) over an observational period of 20 days. The kidney and liver of moribund fish demonstrated different necrobiotic changes. In kidney, cellular and cloudy swelling, degeneration of renal tubules, necrosis of glomerulus, dilation of Bowman’s space, hypertrophy, fibrosis of renal tubules and increased melano-macrophage reaction were frequently observed. In liver, disorganized hepatocytes, blood congestion and nuclear pyknosis were evident. To ascertain the host response to E. tarda infection, mRNA levels of three different classes of innate immune components viz. cytokine interleukin-1β (IL-1β), acute phase protein transferrrin and complement C3 were assessed by quantitative real-time PCR. Significant up-regulation (P < 0.05) of IL-1β and C3 gene expression was observed in infected fish upto 15 days post infection (dpi) in liver, kidney, spleen and blood though the level of increase differed among organs and dpi. Significant up-regulation (P < 0.05) of transferrin gene expression was seen at 1 dpi in kidney, spleen and blood while no significant variation could be observed in liver. The tissue specific pathology and changes in immune gene expression may help to improve the knowledge of E. tarda pathogenesis and role of the innate immune genes in this tropical catfish. The innate immune response of host to E. tarda which is evident from increased expression of innate immune genes may be further enhanced through immunostimulants and nutraceuticals to prevent disease onset and use of antibiotics.