A pig disease ravaging China now appears to have spilled into Vietnam. Scientists fear that a deadlier strain of a longtime foe, porcine reproductive and respiratory syndrome (PRRS), or blue-ear disease, may be on the loose.
"We're fairly confident that there has been an outbreak of PRRS here," says Andrew Speedy, who represents the U.N. Food and Agriculture Organization (FAO) in Vietnam. An FAO mission that completed a weeklong visit to Vietnam on 20 August also uncovered many secondary infections that increase the disease's death toll, Speedy says. The team concluded that PRRS can likely be controlled through vaccination and antibiotic treatment of secondary infections.
PRRS was first identified in the United States in the mid-1980s; the causative arterivirus was isolated in 1991. The virus does not infect humans. In pigs, it attacks macrophages, which ingest and remove invading bacteria. With a crippled immune response, pigs are susceptible to secondary infections. Adults usually recover and develop immunity, but the virus and secondary infections can kill piglets, whose ears often turn blue from the secondary infections.
"The evolution of the virus is really quite startling. It is probably one of the most rapidly evolving viruses that I know of," says Trevor Drew, head of virology at the Veterinary Laboratories Agency in Weybridge, U.K. He explains that, typically, a virus circulating in a new host initially causes severe disease and becomes less pathogenic over time. With PRRS there is evidence that the opposite has occurred: A nonpathogenic strain was circulating among pigs in North America before it evolved and started causing disease.
An even nastier strain seems to be on the rampage in China, killing sows as well as piglets. This strain first appeared in the summer of 2006. China has reported losing 400,000 pigs to the disease through death or culling in 2006 and another 243,000 this year, although some contend that official numbers vastly understate the losses.
In an analysis of viral samples from pigs in China published online in PLoS ONE on 13 June, a group led by George Gao, a microbiologist who heads the Institute of Microbiology in Beijing, described a genetic variation that may be responsible for the virus's increased deadliness. Drew, who finds that claim plausible, says that the possibility of a new and deadlier strain "is very worrisome."
In the pink. A veterinary worker prepares to vaccinate pigs against PRRS in China's Shandong Province.
CREDIT: REUTERS/CHINA DAILY/LANDOV
Media outlets, including an article last week in The New York Times, reported allegations that China has refused to share virus samples and cooperate with international organizations. Drew says that's not so, pointing to Gao's paper and to China's reporting on the outbreak to the Paris-based World Organisation for Animal Health. Guo Fusheng of FAO's Beijing office says that the organization has not yet asked China's Ministry of Agriculture for samples or to allow an outside team to investigate, although it expects to make a formal request shortly.
Vietnam didn't wait to be asked. The disease was first reported there in March. Last month, the government asked FAO for assistance after the outbreaks increased in June and July. Samples have been sent to U.S. labs for analysis.
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