Tilapia Lake Virus (TiLV), Aeromonas caviae and aquatic environmental variation- the triad stressors lead mass mortality in Oreochromis niloticus in wetlands of West Bengal, India.

Tilapia (Oreochromis niloticus) is a vital candidate species of aquaculture in the world. Better ability to thrive adverse water quality and stressors than other commercial fish species made it the most accepted candidate species. Sometimes, multiple infections by bacteria and virus cause massive losses due to higher morbidity, reduced production, mortality, and substantial economic losses. The present investigation reported the natural disease outbreak in O. niloticus of polyculture-based farms caused by co-infection of emerging virus Tilapia Lake Virus (TiLV) and bacterial pathogen. The clinical signs of septicemia followed by mortality were noticed. Higher organic carbon (1.8 to 3.5%) in sediments and calculated Water Quality Index (WQI) based on water parameters (106.33-154.71) beyond the recommended value (100) were also recorded. Using nested PCR, screening for TiLV revealed that 73.33% of the tested fishes were positive for the viral infection. The molecular identification of bacteria using 16S rRNA gene amplification revealed the simultaneous presence of Aeromonas caviae in TiLV infected fishes. In the pathogencity study, the bacteria proved its virulence with LD50 value of 3.07 × 106 CFU/fish. The mass mortality in tilapia was associated with the concurrent infection of pathogens, A. caviae, TiLV and adverse environmental conditions. Unlike previous study, this study also draws attention towards unfavorable water quality parameters perhaps, an additional aggravating factor in the mass mortality reported. Hence, successful management of TiLV disease of tilapia must include simultaneous preventive and therapeutic strategies for both the virus and bacteria along with corrective measures for the deviated water quality.