The WHO said its efforts to track the spread of bird flu have been complicated by the failure of China's Agriculture Ministry to share samples of a newly discovered strain of the virus.
Scientific research released this week said that the new strain, called H5N1 Fujian-like, had spread widely over the past year, being found in almost all poultry outbreaks and some human cases in China, and now becoming prevalent in Hong Kong, Laos, Malaysia and Thailand.
Despite that prevalence, the ministry has not given the WHO any samples of the new strain, said Julie Hall, an infectious disease expert at the WHO's Beijing office.
"There's a stark contrast between what we're hearing from the researchers and what the Ministry of Agriculture says," Hall said. "Unless the ministry tells us what's going on and shares viruses on a regular basis, we will be doing diagnostics on strains that are old."
While new strains of viruses emerge regularly, health experts need to know when one becomes dominant in order to develop methods to detect and fight the disease, Hall said.
The ministry's reluctance has been an ongoing source of aggravation at the WHO. International health experts have repeatedly complained about Chinese foot-dragging in cooperating on investigating emerging diseases like bird flu and the SARS virus.
Telephones at the Agriculture Ministry were not answered on Wednesday and it did not immediately respond to faxed questions.
Some countries are slow to share genetic information or samples of viruses because they fear they will be pushed aside in the global race to produce a lucrative vaccine.
"This is a new disease. Nobody knows how to tackle it, nobody in the world has all the answers," Hall said. "But if they share, then we will all gain from that."
She said the ministry has not shared bird flu virus samples from poultry since 2004, a key impediment in developing diagnostic tools and vaccines.
Released this week, the year-long study by Chinese and US scientists found that, since June last year, one out of every 30 geese and one out of every 30 ducks in live markets tested positive for H5N1 in six southern Chinese provinces.
In that same period, however, the ministry reported only three outbreaks in the same provinces, Hall said.
The study was conducted in Fujian, Guangdong, Guangxi, Guizhou, Yunnan and Hunan -- densely populated provinces where people live in close proximity to ducks, pigs and other farm animals, making the area a common breeding ground for flu viruses.
Out of 108 virus samples taken from infected poultry between April and June of this year, 103, or 95 percent, had the H5N1 Fujian-like strain, according to the results of the study reported in Tuesday's issue of Proceedings of the National Academy of Sciences.
The H5N1 flu has devastated poultry in China and several other Southeast Asian countries and has also claimed more than 150 human lives. Most of the people affected lived close to flocks of chickens or other poultry.
Public health authorities fear that the virus will mutate into a form that can spread easily among people, raising the potential for a worldwide pandemic that could kill millions.