The much-feared H5N1 strain of bird flu has become resistant to one of the most effective antiviral drugs against it — and it seems that Chinese farmers' use of the compound in chickens is to blame.

This week the accusation was formally made — and formally denied. But at some point after 1997 the H5N1 strain became resistant to the amantadine family of antiviral drugs, and Chinese officials have now pledged to investigate the claim.

On 18 June, The Washington Post reported that Chinese farmers, encouraged by government officials, had been routinely using amantadine drugs in chickens. The United Nations Food and Agriculture Organization (FAO) and the World Health Organization (WHO) then questioned the Chinese government, which denied the reports, although officials did not comment on whether farmers were using the drugs.

The FAO's avian-flu surveillance network coordinator in China, Fusheng Guo, told Nature that the drugs have been widely used to combat the H9 family of viruses in chickens. Guo, who was a private consultant for Chinese farmers throughout the 1990s, says that he warned farmers not to use amantadine, because residues in the meat could make human viruses resistant. But, at the time, he did not worry about it in relation to H5N1. “Now I believe it's a serious problem,” he says.

According to Guo, the agriculture ministry was probably not aware of the use. “The men selling it knew it was illegal,” he adds.

China's health ministry told the WHO on 21 June that the problem is “worth following up in the closest way possible”, according to Roy Wadia, the WHO spokesman in Beijing. Surveillance and testing will now be required to find out how widespread the use has been and how common resistance to amantadine has become.

The first H5N1 strain that infected humans, in Hong Kong in 1997, was sensitive to the amantadines. But resistance to them, discovered in 2003, has left another — more expensive — family of antivirals, including oseltamivir (sold as Tamiflu) and zanamivir (Relenza), as the only line of defence against the virus. This leaves the poor countries of southeast Asia without a low-cost option.