Scientists net virus behind massive tilapia die-offs

An international team of researchers has identified a new virus that attacks wild and farmed tilapia, an important source of inexpensive protein for the world’s food supply.

In work published recently in mBio, the team showed that the Tilapia Lake Virus (TiLV) was the culprit behind mass tilapia die-offs that occurred in Ecuador and Israel in recent years. The work also provides a foundation for developing a vaccine to protect fish from TiLV.

“Tilapia is one of the most important fish industries worldwide,” Eran Bacharach, a molecular virologist and one of the lead researchers on the study said in an article. “Moreover, because they eat algae, they are ecological gatekeepers for freshwater and they are an inexpensive, important source of protein in poorer countries.”

According to the article, the tilapia industry is valued at $7.5 billion USD each year. Various countries in Asia and South America are the largest tilapia producers and the U.S. is the largest importer, consuming 225,000 tons of these fish each year.

In 2009, both wild tilapia species in Kinneret Lake, also known as the Sea of Galilee, and fish in commercial ponds in Israel began suffering from an unknown disease with high rates of mortality — up to 70%.  Then, a couple of years later, fish in commercial ponds in Ecuador also suffered a mass die-off. The article explains that on first glance, the two diseases seemed unrelated because the fish in Israel showed brain and nervous system symptoms, while the fish in Ecuador suffered from liver symptoms.

Hoping for answers, researchers working on both outbreaks sent diseased fish samples to the lab of W. Ian Lipkin, an expert in hunting down new viruses, in late 2012.

“This was an atypical viral discovery project,” Lipkin said in the article. His team’s usual approach to tracking down which virus is causing a particular disease is to pursue a genetic sequence analysis of blood, feces or tissues from a diseased animal, remove all the known genetic sequences found in normal animals, and then compare what is left to known sequences in databases. “But, in this instance, what my colleague, Nischay Mishra found didn’t look like any previously entered sequences,” Lipkin added.

In this case, the team found 10 short RNA gene sequences. “The more we studied them, the more convinced we became that what we had represented an entirely new virus,” Lipkin said.

The article explains that while nine of the gene segments shared no similarities with any other known viral proteins, one segment looked weakly similar to an influenza C virus protein. The 10 segments also had similar starting and ending sequences, a feature of segmented viruses. In addition, the team showed that the virus replicates itself in the nucleus of fish cells. These characteristics led the team to classify TiLV as an orthomyxo-like virus, related to the same family of viruses as influenza.

The team also showed that the virus expresses 10 proteins that correspond to the 10 gene segments. They also sequenced the virus from tilapia from Ecuador and Israel and showed that it was the same virus causing the fatalities in two locations halfway around the globe.

Because the viruses from the two sites shared almost identical gene sequences, Bacharach believes they came from the same source. How the virus traveled between Israel and Ecuador, and in which direction, is still a mystery, however.

“Our research provides the first means of detection — knowing the genetic sequence of the virus is the first step to designing diagnostic and screening assays,” Bacharach said. Such tests will allow fish farmers to detect when the virus is present in a commercial pond and limit its spread.

The finding brings other practical applications with it, too, Lipkin added: “Building a vaccine would save billions of dollars and preserve an industry that ensures employment in the developing world and food security.”

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