66 MARCH 2023 • WORLD AQUACULTURE • WWW.WAS.ORG chloroform) because of their lipid membrane (Eyngor et al. 2014). The duration of survival of TiLV outside the host has not been determined; however, studies have demonstrated horizontal, waterborne spread under experimental conditions (Eyngor et al. 2014). TiLV replicates and transcribes at the sites of pathology, specifically the liver in samples with liver lesions and the central nervous system in samples with central nervous system lesions, as indicated by results from in situ hybridization (Bacharach et al. 2016). Susceptible Tilapia Host Species Tilapias susceptible to TiLV includes wild tilapia (Sarotherodon galilaeus, Oreochromis aureus and Tristamellasimonis intermedia), farmed tilapia (Oreochromis niloticus) (Fig. 2), and hybrid tilapia (O. niloticus × O. aureus, O. niloticus × O. mossambicus) (Eyngor et al. 2014, Ferguson et al. 2014, Amal et al. 2018, Bahera et al. 2018). Other tilapia species in the genus Oreochromis such as red tilapia and wild black tilapia have also been reported to be affected (OIE 2018, Dong et al. 2017). Other fish species that were co-cultivated with tilapia at the time of disease outbreak remained unaffected (Eyngor et al. 2014, Fathi et al. 2017, Bahera et al. 2018), thus suggesting that only tilapias were susceptible to this pathogen. Other species may be vulnerable but it has yet to be determined. Susceptible Life Stages The fingerling and juvenile stages of tilapia are reported to be the most susceptible stages, with high mortality rates (Ferguson et al. 2014, Dong et al. 2017). Approximately 90 percent of mortality in red tilapia fingerlings occurs within one month of stocking (Dong Tilapia is currently the second most important group of farmed fish in aquaculture after carps. Global production was estimated at 6.4 million tonnes in 2015, with an estimated market value of US$ 9.8 billion, increasing annually (FAO 2017). Because of their high protein content, hardy nature, large size, rapid growth, prolific breeding, tolerance of high stocking density and palatability, several tilapia species, particularly in the genus Oreochromis, are being focused on as a major aquaculture fish species. Tilapia lake virus (TiLV) or Tilapia tilapinevirus is an emerging infectious agent that affects both wild and farmed populations of tilapia and threatens the global tilapia industry. The virus was first reported in 2014, when the Sea of Galilee (Lake Kinneret) in Israel experienced a major noticeable decline in tilapia catch quantities (Eyngor et al. 2014). So far, it has been reported in various regions across Asia, Africa and South America (Fig. 1) (Jansen et al. 2019). The disease related with TiLV infection is currently known under three different names: Tilapia lake virus disease (TiLVD), syncytial hepatitis of tilapia (SHT) and one-month mortality syndrome (OIE 2018, Ferguson et al. 2014, Tattiyapong et al. 2017). It typically affects the fingerling and juvenile stages of tilapia and can cause mass mortalities up to 90 percent (Dong et al. 2017). Characteristics of Tilapia Lake Virus (TiLV) TiLV is a novel enveloped, negative-sense, single-stranded RNA virus with ten segments encoding ten proteins with a diameter between 55 to 100 nm (Ferguson et al. 2014, Eyngor et al. 2014, Bacharach et al. 2016, Surachetpong et al. 2017). It belongs to Group V of the Baltimore classification system of viruses. The viral particles of TiLV are sensitive to organic solvents (ether and Tilapia Lake Virus: A Serious Concern for the Global Tilapia Industry Soibam Ngasotter, Soibam Khogen Singh and Pradyut Biswas FIGURE 1. World map indicating the geographical regions with reported tilapia lake virus (TiLV) cases. FIGURE 2. Nile tilapia Oreochromis niloticus susceptible to tilapia lake virus.
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