Simultaneous detection of <i>Flavobacterium columnare</i> and <i>Saprolegnia ferax </i>during disease outbreak in cultured pacu, <i>Piaractus brachypomus</i> (Cuvier, 1818) from India

The present study reports the simultaneous detection of Flavobacterium columnare and Saprolegnia ferax in cultured pacu (Piaractus brachypomus), during winter. The gross lesions in moribund pacu showed erosion of fins, cotton-wool like growth on the body surface and prominent discolouration on skin and gills. Wet mount from the gill tissue showed haystacklike appearance, presumptive of columnaris disease. F. columnare was detected in gill tissues of diseased pacu by PCR using specific primers. Phylogenetic analysis of 16s rDNA amplified from DNA of diseased pacu gills revealed that the sequence belonged to genetic group 2 of F. columnare. Histopathological findings of the diseased fish showed extensive damage in gills. In addition, wet mount of surface lesions revealed long thin aseptate hyphae presumptive of oomycete infection. Skin and fin tissue from diseased pacu inoculated on GPY agar plates showed cotton wool like growth. The isolated oomycete was purified and identified as S. ferax through amplification and sequencing of ITS region. These results attribute the mortality of pacu to infection with F. columnare and S. ferax. Considering the increase of pacu culture in many countries, and specially its polyculture with major carps in India, the findings of the present study necessitate early detection of these pathogens for health management of pacu. Keywords: Discolouration, Haystack-like, Histopathology, Oomycete infection, Molecular detection, ITS gene, Wet mount