Report on Mortalities of Nile Tilapia (Oreochromis niloticus) Due to Tilapinevirus tilapiae and Co-infecting Bacteria in Winter Season

Background: Tilapinevirus tilapiae, commonly known as Tilapia lake virus, is a major viral infection of cultured tilapia, causing severe rates of fatalites and has evolved as a serious hazard to the tilapia in the world. Predisposing factors, notably temperature and the development stage of the host play a decisive role in influencing the extent of pathogenesis and the mortality rate associated with the disease. The seasonality of TiLV outbreaks has been observed in multiple countries, with a higher frequency occurring during the summer months. Methods: In the current investigation, the occurrence of TiLV disease was examined in fry and fingerlings of tilapia during the winter season (January - February, 2020) in Pune District, Maharashtra, India. The disease was observed in fish reared in open cages (Case I) and in nursery rearing farm (Case II). Infected tilapia displayed generalized gross clinical signs. TiLV was identified and confirmed in both instances using a semi-nested reverse transcriptase PCR by amplifying TiLV genome segment 3 and further confirmed by histopathogical investigation. Result: The research demonstrated the vulnerability of early developmemtal stages of O. niloticus to TiLV disease, suggesting the potential occurrence of this infection during the winter season, characterized by lower temperatures. Additionally, microbial investigations exposed the prevalence of Aeromonas spp., predominantly associated with TiLV-positive fish. These results align with previous publications, underscoring the potential role of bacterial infectious agents (Notably aeromonads), in exacerbating TiLV disease during outbreaks.