Peking duck, salted duck eggs, duck soup: China is famous for its duck delicacies, and duck farms dot the country's agricultural belt. So last spring, when Chinese farmers noticed their prized birds were producing fewer eggs than usual, they began to worry. Egg production plummeted by as much as 90% in some flocks. Ducks were waddling about awkwardly, their coordination off kilter, and eating less than usual. Some died within days.
By the end of the year, an estimated 4.4 million ducks in Fujian, Shandong, and Zhejiang provinces, the swath of eastern China where duck farming is common, had caught the mysterious illness. And the outbreak reached at least six other provinces, along with rural areas outlying Beijing.
Enter microbiologist George Gao and colleagues at the Chinese Academy of Sciences in Beijing. By analyzing the affected animals, the scientists isolated an aggressive new flavivirus, a class of viruses that includes yellow and dengue fevers—the first flavivirus ever identified in ducks.
The discovery of what the scientists have dubbed the BYD virus raises some critical concerns. In addition to potentially devastating Chinese duck farming and the economy that depends on it, the flavivirus could put humans at risk. "Most flaviviruses are zoonotic," meaning they can be transmitted from animals to people, Gao says, "so infection of human beings cannot be ruled out."
When reports of the duck disease first reached the researchers last April, the symptoms farmers described—particularly a severe drop in egg production—alarmed them. Arriving at the farms, the microbiologists collected tissue and serum samples from animals in affected flocks for testing. As detailed in a paper published this month in PLoS ONE, Gao and his colleagues worked methodically to find the culprit.
First, they ruled out avian flu by testing serum samples from the animals for flu antibodies. Then, working with brain and ovary tissue from ducks that had died within 6 hours of infection, they isolated the BYD virus.
To make sure they had nailed the right culprit, they injected healthy ducks with BYD, comparing them with a control group. The BYD ducks soon fell ill.
Gao and colleagues say the BYD virus is closely related to the Tembusu virus, a flavivirus found in Southeast Asia. Like that other virus, they suggest, BYD could be spread by mosquitoes.
Transmission by mosquitoes should not be a foregone conclusion, says Ernest Gould, a virologist and visiting scholar at the Université de la Méditerranée in Aix-Marseille, France, noting that temperature changes over the period of the virus's spread are not consistent with those of other mosquito-borne flaviviruses. (Ducks started falling ill in the cool weather of early spring, when mosquito populations were presumably low, and the outbreak continued well through autumn.) "Much more detailed epidemiological studies are required," Gould wrote in an e-mail.
But, he adds, the paper "raises an important alarm with potential implications way beyond the boundaries of China." He says a rapidly spreading Chinese flavivirus could mean a global problem.
Because of the pervasiveness of duck farming in China, Gao and colleagues stress that the disease should be closely monitored, in part because it could spread to people. The next step, they say, is the development of a BYD vaccine.