Immune-pathological response induced by Saprolegnia parasitica in Pangasianodon hypophthalmus and assessment of clinical efficacy of clotrimazole.

Saprolegnia parasitica causes significant morbidity, mortality and economic loss in fish hatcheries and growing culture systems, especially during winter. The present study examined clinical and histopathological alterations in Pangasianodon hypophthalmus experimentally infected with the oomycete and evaluated the clinical efficacy of antifungal drug clotrimazole as bath treatment. Besides development of off-white cotton wool-like fungal growth on fins and skin, invasion of hyphae into underlying musculature and muscle fibre necrosis, experimental infection also induced gill lamellar hyperplasia, hepatocyte, kidney glomerular and tubular degeneration, increase in blood glucose, creatinine, aspartate transaminase (AST) and alanine aminotransferase (ALT) enzyme activities indicating widespread systemic changes in the infected fish. Globulin levels significantly decreased (P < 0.05), suggesting compromised ability of the infected fish to mount an adaptive immune response. Further, expressions of immunity and stress-related genes such as catalase, glutathione peroxidase, superoxide dismutase, heat shock protein (HSP70), tumor necrosis factor, inducible nitric oxide synthase and MHC II were up or down regulated in tissue specific manner suggesting different tissue-specific responses to the oomycete infection. Clotrimazole bath treatment of experimentally infected fish showed more than 80% survival of infected fish proved its potential in controlling saprolegniasis in fish. The present study broadens our knowledge on pathogenesis of the oomycete infection in fish and also shows a way for effective control of the infection.