Isolation and characterization of Saprolegnia parasitica from cage-reared Pangasianodon hypophthalmus and its sensitivity to different antifungal compounds.

Saprolegniasis is one of the most dangerous fungal diseases of fish, causing significant mortality in fish hatcheries and young ones. The present study aimed to isolate and characterize the causative fungus from fingerlings of Pangasianodon hypophthalmus cultured intensively in freshwater cages in Indian reservoirs and to determine minimum inhibitory concentrations of different antifungal compounds against the fungal hyphae and zoospores. The fungal isolates grown on potato dextrose agar showed an abundance of gemmae, elongated mycelia, non-septate hyphae, primary zoospores, mature zoosporangia with numerous zoospores, cysts with bundles of long hairs and were further identified as Saprolegnia parasitica following PCR amplification and sequencing of internal transcribed spacer region. S. parasitica showed temperature-sensitive optimum growth in a narrow window of 12-24 ℃, which might drive its experimental pathogenesis as well as natural infections in the winter months. In vitro sensitivity testing established negligible inhibitory activity of fluconazole, boric acid, sodium thiosulfate, and potassium permanganate while clotrimazole arrested the spore and hyphal growths at 2 mgL concentration suggesting potential of the imidazole antifungal in treating S. parasitica infection in fish. The present study will serve as the baseline information for developing therapeutic and management strategies for controlling saprolegniasis in the economically significant iridescent catfish.