Identification of a novel RNA virus, lethal to Tilapia

ABSTRACT

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 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 E-11 cell line, and induced a cytopathic effect at 5 to 10 days post-infection. Electron microscopy revealed enveloped icosahedral particles of 55 to 75 nm. Low passage TiLV, injected intra-peritoneally to tilapia, induced a disease resembling the natural disease – typically displayed by lethargy, ocular alterations and skin erosions – with over 80% mortality. Histological changes included congestion of internal organs (kidney, brain) with foci of gliosis and peri-vascular cuffing of lymphocytes in the brain cortex; ocular inflammation included endophthalmitis and cataractous changes of the lens. Cohabitation of healthy with diseased fish demonstrated that the disease is contagious, and that mortalities (80-100%) occur within few days. Fish surviving initial mortality were immune to further TiLV infections, suggesting the mounting of 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 controlling the spread of this virus worldwide.