Co-infection of Lactococcus garvieae and Tilapia lake virus (TiLV) in Nile tilapia Oreochromis niloticus cultured in India.

Tilapia lake virus (TiLV) and Lactococcus garvieae are 2 major pathogens of cultured Nile tilapia Oreochromis niloticus. In June-July 2018, a disease outbreak was reported in Nile tilapia cultured in brackish water floating cages in Kerala, India. Affected fish died gradually, and cumulative mortality reached ~75% within 1 mo. In the present study, TiLV and L. garvieae were isolated from the infected fish and confirmed. Nucleotide analysis of the partial sequence of segment 3 revealed that the present TiLV isolate showed 100% similarity with TiLV MF574205 and 97.65% similarity with TiLV KU552135 isolated in Israel. The partial 16S rDNA nucleotide sequence of L. garvieae shared 99% similarity with the 16S rDNA nucleotide sequence of L. garvieae isolated from Nile tilapia in Brazil. Eight virulence genes (hly1, hly2, hly3, NADH oxidase, adhPav, LPxTG-1, LPxTG-4, adhC1) were amplified in the present isolate. In the experimental challenge study, the onset of mortality started earlier in fish co-infected with TiLV and L. garvieae (3 d post-infection [dpi]) compared to other groups. Cumulative mortality (90% at 12 dpi) was significantly higher in the co-infected group than in fish infected with TiLV (60% at 12 dpi) and L. garvieae (40% at 12 dpi) alone. This study reveals that synergistic co-infection with TiLV and other bacteria may increase mortality in disease outbreaks. To the best of our knowledge, this is the first reported co-infection of L. garvieae with TiLV associated with mass mortality in Nile tilapia in India.

A natural outbreak of infectious spleen and kidney necrosis virus threatens wild pearlspot, Etroplus suratensis in Peechi Dam in the Western Ghats biodiversity hotspot, India.

A large-scale mortality of pearlspot, Etroplus suratensis was reported from Peechi Dam, an artificial tropical lake made for irrigation and drinking water supply in Kerala, India during 2018. This dam is located in the premises of Western Ghats, recognized as one of the biodiversity hotspots of the world. The objective of this study was to identify the aetiological agent of this large-scale mortality of E. suratensis by systematic diagnostic investigation and identification of the pathogen. Virus isolation was carried out on a species-specific pearlspot fin (PSF) cell line. Infected PSF cells showed cytopathic effects (CPEs) like cell shrinkage, rounding, enlargement, clustering, and subsequent detachment of cells with a high viral titre of 106⋅95 TCID50 ml-1 at 8 days post-inoculation (dpi). Histopathological examination of the fish showed the presence of numerous abnormal enlarged basophilic cells and intracytoplasmic eosinophilic inclusions in the liver. Moreover, transmission electron microscopy (TEM) analysis revealed the presence of large numbers of 125-132 nm viral particles in the spleen tissues. PCR amplification and phylogenetic analysis of the major capsid protein (MCP) gene sequence confirmed that the causative agent was infectious spleen and kidney necrosis virus (ISKNV) of the genus Megalocytivirus. The experimental infection recorded 86.7 ± 2.7% mortality in the E. suratensis (body weight 11.01 ± 2.7 g; body length 8.01 ± 2.23 cm) injected with 1 × 104⋅25 TCID50 ml-1 ISKNV per fish. Our detailed investigation provided definitive diagnosis of ISKNV in the severe mass mortality event in wild E. suratensis in Peechi Dam, India, adding one more species to expanding host range of ISKNV infection. The high mortality rate of ISKNV infection in pearlspot suggests the perilous nature of this disease, particularly among the wild fish population.