Mortalities in cultured Pangasianodon hypophthalmus due to oomycete Saprolegnia parasitica infection in Uttar Pradesh, India

Abstract

Mortalities were observed in several Pangasianodon hypophthalmus farms (n = 52) of Uttar Pradesh, North India during December 2020–January 2021. The affected fish were lethargic, swimming near the water surface and had white or grayish cotton-like patches over the body surface. Wet mount examination of the patches revealed presence of non-septate hyphae suggestive of oomycete infection. However, no parasites were observed in skin and gill scrapings. Histopathological examination of the affected skin tissue indicated severe destruction of the epidermis, degenerative changes and penetration of the oomycete hyphae in the dermis and underlying musculature. From the affected skin lesions, oomycete could be isolated which was identified as Saprolegnia parasitica based on morphological characteristics, and amplification and sequencing of the internal transcribed spacer region. Importantly, from the diseased P. hypophthalmus, no bacteria and viruses could be isolated from kidney, and pooled kidney and spleen tissues, respectively. Further, following experimental infection with S. parasitica zoospores by immersion method, 100% mortality was observed in P. hypophthalmus, indicating that the isolate was virulent. From the experimentally-infected striped catfish, S. parasitica could be reisolated and the histopathological lesions were similar to those observed in naturally infected fish, thereby fulfilling Koch’s postulates. Based on the gross and histopathological lesions, isolation of oomycete, molecular identification and bioassay, it was concluded that S. parasitica was responsible for mortalities in P. hypophthalmus farms. This forms the first report of mortalities of striped catfish due to oomycete S. parasitica infection and the findings will pave way for developing control measures for reducing losses in striped catfish farms due to saprolegniasis.