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A mysterious virus that has decimated tilapia fish stocks in Ecuador and Israel has finally been identified by an international team of scientists.

The virus, dubbed tilapia lake virus, was long suspected to be behind mass die offs of valuable farmed tilapia in Ecuador and Israel since 2009. In a paper published in the journal mBio last month (5 April), the researchers say that nine of the ten genome segments they have identified are unrelated to any other known virus.

“Our findings suggest that tilapia lake virus represents a novel virus, and confirm that it poses a global threat to tilapia aquaculture,” says joint lead author Eran Bacharach, a virologist at Tel Aviv University in Israel.

The virus causes lethargy, skin flaking and kidney problems in infected fish, which ultimately kills them.

Tilapia lake virus caused stocks of farmed fish to plummet by 85 per cent in Israel, making the disease a concern for scientists and fish farmers alike. The research effort that resulted in the paper was down to a collaboration between institutes in Israel, the United Kingdom, the United States and the West Indies.

Tilapia fish aquaculture became popular (see graph) as fish stocks in the oceans dwindled in the 1990s through overfishing. The global tilapia trade is worth an estimated US$7.5 billion a year, according to the paper. The largest producers are China, Ecuador, Egypt, Indonesia and Thailand, while the largest importer is the United States.
Before the virus emerged, tilapia farms only had to protect their stock from bacteria and fungi, says Margy Villanueva, the leader of the National Sanitary Program for Aquaculture Species at the Colombian Agricultural Institute, who has worked with the international consortium. Colombia is still waiting for scientific results to confirm that its mass tilapia die off in 2009 was caused by the tilapia lake virus.

“Now we need a better understanding of the virus epidemiology, its biological properties, infectious capacity and routes of contagion,” Villanueva says. “But, above all, farmers must report any cases and establish biosecurity measures”, she says, such as disinfecting new fish and destroying affected fish stocks. Biosecurity can be an issue in countries with weak reporting systems, says Francisco Forero, a fish researcher at Jorge Tadeo Lozano University in Bogota, Colombia.

“The problem in many countries, as in Colombia, is that there are no controls when [new fish stocks] are introduced,” he says. “And producers break the biosecurity rules everywhere.”