An emerging virus is killing farmed fish, but breeders can help them fight back
About a decade ago, farm-raised tilapia in Israel began to die mysteriously. The fish had ulcerated skin and internal hemorrhages; sometimes ponds full of fish were wiped out. In 2014, researchers identified the culprit: a previously unknown virus they named tilapia lake virus. Since then, the virus has been detected on farms in Asia, Africa, and the Americas. There’s no cure and no vaccine, and the virus is likely spreading, threatening one the world’s most important farmed fish. “It’s a major global problem,” says John Benzie, a geneticist at WorldFish, an international publicly funded research center.
New findings, however, are providing hope that Nile tilapia (Oreochromis niloticus), the most common kind of farmed tilapia, could be bred to resist the virus. In a lucky break, in 2018 the virus struck a pond at WorldFish holding numerous tilapia produced for breeding experiments, and some of the fish proved completely resistant to the virus, Benzie and colleagues reported last month in Aquaculture. “It’s good news for the tilapia sector,” says Morten Rye, a fish geneticist at Benchmark Genetics, a company that breeds tilapia and other aquacultural species.
Tilapia is the second most popular fish in aquaculture, after carp, with farmers in more than 120 countries now harvesting about 6 million tons per year. It’s especially important in developing nations, many of which rely on a productive strain first developed in the 1990s at WorldFish.
The emerging virus affects several types of farmed tilapia, and was probably causing problems for at least several years before its discovery. Although researchers know some regions have been hit hard, the overall distribution and impact of the virus are not clear. Very few countries have reported data to the World Organisation for Animal Health, which requires member nations to immediately notify it of all outbreaks of tilapia lake virus and other emerging diseases. “We really need descriptive and analytical epidemiological studies to help us understand the situation in the field,” says Mona Dverdal Jansen, a veterinary epidemiologist at the Norwegian Veterinary Institute.
In the meantime, researchers at WorldFish are working with the University of Edinburgh’s Roslin Institute to breed Nile tilapia that resist the virus. The 2018 outbreak in WorldFish’s pond in Malaysia is helping them identify promising genetic variants. In all, the pond contained 1821 individually tagged tilapia belonging to 124 groups of siblings. Each group had different pairs of parents. Nearly 40% of all the fish died; that was enough to allow the researchers to pick out which groups of siblings had the highest survival rates
Just as encouraging, the WorldFish outbreak enabled researchers to show that about 50% of the variation in survival was due to genetics. “That bodes well for future breeding to improve resistance to the virus,” says co-author Ross Houston, a fish geneticist at the Roslin Institute. The researchers also found that bigger—and more valuable—fish were just as likely to resist the disease as the smaller fish. That finding suggests breeders won’t have to sacrifice yield to boost resistance, which would be a problem for growers.
Still, getting resistant tilapia into the world’s ponds could take years. Researchers need a genomic test that would allow breeders to quickly identify fish with desirable genes, and they need a reliable and realistic way to infect the fish to find out whether they really are resistant. Rye cautions that even if breeders develop a winning strain, mass-producing the fish in hatcheries and distributing them—especially in the developing world—will be a tall order. “It’s not changing everything overnight,” he says.
Some companies are pursuing a different approach: developing a vaccine against tilapia lake virus. Win Surachetpong, an immunologist at Kasetsart University, says there are promising signs; his lab has found elevated levels of antibodies in tilapia exposed to the virus, for example. But viable candidates are still far off, and even a highly effective vaccine might not be cheap enough to be economically viable, especially because tilapia is a relatively low-value fish often grown by poor farmers.
Given such issues—and the threat that the virus is continuing to spread—creating hardier tilapia breeds has become “a matter of urgency,” Benzie says. “We’re going hell for leather on this.”
*Correction, 5 March, 1:50 p.m.: This story was updated to note that countries are required to report all outbreaks of tilapia lake virus because it causes an emerging disease.