Susceptibility of important warm water fish species to tilapia lake virus (TiLV) infection

Tilapia lake virus (TiLV) is a novel orthomyxo-like virus that causes high mortality in tilapia and its hybrid species. Currently, no clinical infection of TiLV in other fish has been reported. In this study, a laboratory controlled TiLV infection was investigated in 10 warm water fish species consisting of giant gourami (Osphronemus goramy), snakeskin gourami (Trichogaster pectoralis), iridescent shark (Pangasianodon hypophtthalmus), walking catfish (Clarias macrocephalus), striped snake-head fish (Channa striata), climbing perch (Anabas testudineus), common carp (Cyprinus carpio), silver barb (Barbodes gonionotus), Asian sea bass (Lates calcarifer), and red hybrid tilapia (Oreochromis spp.). Our results revealed that only red hybrid tilapia and giant gourami developed clinical signs of infection including skin erosion, pale skin, exophthalmos and skin haemorrhage with a cumulative mortality rate of 60-100%. Amplification of TiLV using RT-qPCR confirmed the presence of the virus in the livers of challenged fish at 7 and 21 days post infection (dpi) with threshold cycle (Ct) values of 19.57 to 31.18, and 27.78 to ND (not detected) for red hybrid tilapia and giant gourami, respectively. Histopathological lesions were observed including hepatocellular necrosis and syncytial formation in the liver and lymphoid depletion in the anterior kidney of giant gourami and red hybrid tilapia. Moreover, the presence of TiLV in the brain and liver of giant gourami was further confirmed using in situ hybridization. A cohabitation challenge study indicated that the virus could be transmitted from infected red hybrid tilapia to naïve giant gourami with low mortality compared to an intraperitoneal injection. As polyculture systems are common practice in large parts of the aquaculture sector and multiple fish species share natural water resources, it is likely that direct and indirect contacts of infected fish could spread the disease in a farm with polyculture of multiple species. Therefore, the awareness of the spread of TiLV should be given. As such, biosecurity measures should be implemented if rearing tilapia with other fish species. Nevertheless, our study highlighted that most warm water fish species except giant gourami are resilient to TiLV infection.