Largescale mortality of Oreochromis mossambicus in lakes and reservoirs of Karnataka due to coinfection of Tilapia Lake virus (TiLV) and multidrug-resistant Aeromonas veronii: An emerging fish disease in India

In India, culture of tilapines has been gaining importance over last few decades and it is the second widely farmed fish species in the World. There have been increased large-scale mortalities observed in last few years due to DNA and RNA virus associated with coinfection of bacteria, fungi, parasites etc., were considered as a major global threat to the tilapia and aquaculture industry. In this study, large scale mortality of tilapia (Oreochromis mossambicus) was observed in wild populations located in Krishna Raja Sagara reservoir in Mysuru, Thonnur Lake and Erode Lake in Pandavapura, Mandya. A detailed investigation was performed from these regions which reported mass mortality in these water bodies. Tissues from the fishes (brain, gill, kidney, spleen and liver) were collected for examination of parasites, histology and molecular detection (virus and bacteria). The results of polymerase chain reaction showed that all the samples were found negative for DNA viruses and for one RNA virus namely VNN but all the samples from 3 regions (reservoir and lakes) were positive for TiLV. Syncytial hepatitis lesion along with glial cells were observed in the TiLV-infected tilapia. TiLV isolated from this study (OM1 and OM2) shared 94.30% and 95.52% nucleotide identity with the TiLV isolated from West Bengal, India (MF502419.1) and Israel (KJ605629.1) respectively. NCBI-BLAST search results using gyrB gene revealed that the isolates G 1 & H 2 were identified and confirmed as Aeromonas veronii with 99.52% and 99.42% nucleotide similarity to the A. veronii isolate XX-57 (JX025939.1) from diseased Chub and A. veronii isolate JH-12 (JX025895.1) from diseased gold fish from China respectively. To the best of our knowledge, this is the first report of concurrent infection TiLV and A. veronii associated with mass mortality in Oreochromis mossambicus in Karnataka, India. In vitro study showed the cytopathic effects on OniL cell lines with initially elongated cells followed by rounding and finally detachment leading to death of the cells. •Syncytial hepatitis lesion was observed in the TiLV-infected Oreochromis mossambicus.•TiLV isolated from this study (OM1 and OM2) shared 94.30% and 95.52% nucleotide identity with West Bengal, India and Israel respectively.•A. veronii was resistant to several antibiotics with MAR index of 0.675.•Aeromonas veronii with 99.52% and 99.42% nucleotide similarity to the isolate XX-57 from diseased Chub and JH-12 goldfish respectively.•This is th