Science - October 18, 2010

Shrimp virus gets more aggressive

Viruses usually get weaker as they spread. Not so the shrimp virus known as WSSV, which is getting more aggressive as it spreads, and poses a threat to global shrimp farming, say Wageningen virologists in the journal PLoS One.

Most viruses, such as the flu for example, gradually die out as an epidemic advances. But the white spot syndrome virus (WSSV) is an exception to the rule. This virus becomes more dangerous the longer it stays around.
The Wageningen researchers reconstructed the way the virus spreads from its starting place, and mapped the genetic changes that take place along the way. They took samples of the virus in shrimp farms in five Asian countries, and compared them with each other and with data from the literature on viruses in Taiwan, China and Thailand. By studying the virus samples in chronological order, they were able to observe a remarkable pattern. Bits of DNA disappear from the virus genome at the same time as the virus becomes more virulent. Both changes look like evolutionary adaptations to shrimp farming on the part of the virus.
The WSSV virus was discovered in shrimp populations in Taiwanese shrimp farms in 1992. It was not worryingly virulent at first, but later there were severe outbreaks of the disease elsewhere. Documentation on outbreaks in 1992 (in China) and 1999 (in Ecuador) shows that local shrimp production dropped by two thirds in the years following the outbreak. Meanwhile, the virus has spread from Taiwan all over the world and has even reached wild populations of shrimp in Europe.
The research was carried out by virologist Mark Zwart, who has moved to Spain, together with Just Vlak's Wageningen research group. Zwart, an ex-student of Vlak's, received a Rubicon grant from the NWO for this research project last year.