Identification of the etiological agent of tilapia disease in the Lake of Galillee

Abstract

Background to the topic. Tilapines serve as the second most important group of farmed fish worldwide. Massive mortality of wild and cultured tilapia has been observed recently in Israel but the pathogen of this disease has not been identified. We proposed to identify the agent responsible for disease.  Major conclusions, solutions, achievements. We characterized the lesions in diseased fish and found that the brain was one of the affected organs. We found conditions to isolate from brains of diseased fish the etiological agent of the tilapia disease and to propagate it in cell culture. This led to the identification of the pathogen as a novel RNA virus, which we named Tilapia Lake Virus (TiLV). Electron microscopy of TiLV revealed virion-like particles and ether/chloroform-sensitivity assays demonstrated that TiLV is enveloped. Low passage TiLV, injected intra-peritoneally to tilapia, induced a disease with over 80% mortality. Cohabitation of healthy with diseased fish demonstrated that the disease is contagious, and that mortalities occur within few days. Fish surviving initial mortality were immune to further TiLV infections, suggesting the mounting of protective immune response. Screening cDNA libraries and high throughput sequencing determined the sequence of TiLV genome. This demonstrated that TiLV is indeed a novel virus and allowed the design of a PCRbased diagnostic test.  Implications, both scientific and agricultural. The characterization of a novel, emerging RNA virus that imposes major threat to the tilapia industry, enables the specific identification of the virus in tilapines. This allows prompt screening and surveillance of TiLV, epidemiological studies, and disease containment. This also potentially opens the way for the development of vaccines against TiLV.