Effects of sample preservation and storage times on the detection of tilapia lake virus (TiLV) RNA in tilapia tissues

Tilapines are important for the sustainability of ecological systems and serve as the second most important group of farmed fish worldwide. Significant mortality of wild and cultured tilapia has been observed recently in Israel. The etiological agent of this disease, a novel RNA virus, is described here, and procedures allowing its isolation and detection are revealed. The virus, denominated tilapia lake virus (TiLV), was propagated in primary tilapia brain cells or in an E-11 cell line, and it induced a cytopathic effect at 5 to 10 days postinfection. Electron microscopy revealed enveloped icosahedral particles of 55 to 75 nm. Low-passage TiLV, injected intraperitoneally in tilapia, induced a disease resembling the natural disease, which typically presents with lethargy, ocular alterations, and skin erosions, with >80% mortality. Histological changes included congestion of the internal organs (kidneys and brain) with foci of gliosis and perivascular cuffing of lymphocytes in the brain cortex; ocular inflammation included endophthalmitis and cataractous changes of the lens. The cohabitation of healthy and diseased fish demonstrated that the disease is contagious and that mortalities (80 to 100%) occur within a few days. Fish surviving the initial mortality were immune to further TiLV infections, suggesting the mounting of a protective immune response. Screening cDNA libraries identified a TiLV-specific sequence, allowing the design of a PCR-based diagnostic test. This test enables the specific identification of TiLV in tilapines and should help control the spread of this virus worldwide.

During 2015–2016, several outbreaks of tilapia lake virus infection occurred among tilapia in Thailand. Phylogenetic analysis showed that the virus from Thailand grouped with a tilapia virus (family Orthomyxoviridae) from Israel. This emerging virus is a threat to tilapia aquaculture in Asia and worldwide.