Identification and pathogenetic study of tilapia lake virus (TiLV) isolated from naturally diseased tilapia

As an emerging pathogen, tilapia lake virus (TiLV) has caused severe socio-economic impacts and remains a devastating factor in wild and farmed tilapia. Early diagnosis and timely reporting of TiLV are very important and necessary. In the study, a TiLV infection event in a tilapia farm is studied and reported. Naturally diseased tilapia with clinical signs such as anorexia, exophthalmia, skin abrasion and hemorrhage are collected. Multiple tissue lesions of liver, spleen and kidney are observed by histopathological analysis, such as syncytial formation, intracytoplasmic inclusion body and necrosis. No bacterial pathogens are identified from liver, spleen and kidney of the diseased tilapia, while all of them show TiLV-positive. E-11 cells are employed for TiLV isolation, and cytopathic effects are developed following incubation with supernatants of the homogenized tissues collected from the diseased tilapia. Electron micrographs of the infected cells and purified TiLV show that nearly round-shaped, enveloped viral particles (60–80 nm) can be identified. Segment 1 of the isolated TiLV has a high sequence similarity to other isolates, but phylogenetic analysis shows that it is placed in a unique cluster. Experimental infection with the isolated TiLV can cause high mortality (> 90%) in healthy Nile tilapia, with the similar clinical signs and tissue lesions to those found in naturally infected tilapia. Moreover, vacuolation and necrosis in brain, inclusion body and necrosis in heart, megalocytes and necrosis in intestine are observed in experimentally infected tilapia. Muscle, spleen, liver, intestine, brain, heart, kidney and gill of the re-infected tilapia show TiLV-positive, and liver, spleen and brain are the main target tissues. The data so far confirm a TiLV-associated disease outbreak in China, enriching the relevant information of TiLV.