Emergence of tilapia lake virus in Thailand and an alternative semi-nested RT-PCR for detection

The present study reports outbreaks of tilapia lake virus (TiLV), an emerging pathogen causing syncytial hepatitis of tilapia (SHT), in farmed tilapia in Thailand. Occurrence of the virus was confirmed by RT-PCR and nucleotide sequence homology to the TiLV from Israel. Diseased fish exhibited typical histopathological features of syncytial giant cells in the liver examined through H&E and semi-thin sections. Presence of intracytoplasmic viral particles was revealed by TEM. In situ hybridization using a specific DIG-labeled probe derived from a partial genome segment 3 of TiLV genome revealed multiple tissues tropism of the virus including liver, kidney, brain, spleen, gills and connective tissue of muscle. An alternative semi-nested RT-PCR protocol has been developed in this study for disease diagnosis. Additionally, comparative genetic analysis revealed genetic variations of Thailand-originated virus to the Israel TiLV strains, sharing 96.28 to 97.52% nucleotide identity and 97.35 to 98.84% amino acid identity. Outbreaks of TiLV in different continents might signal a serious threat to tilapia aquaculture globally.