Ducks also spread bird flu, increasing the risk to humans from a virus that has killed 32 people in Thailand and Vietnam this year, the World Health Organisation (WHO) said on Friday.

In U.S. laboratory trials, ducks transmitted similar quantities of the virus as chickens, said Klaus Stohr, head of WHO’s influenza programme.

“We must assume that ducks could be a source of infection for humans,” he told a news conference.

Although no such cases of transmission had yet been detected, the confirmation that ducks are potential carriers increased the difficulty of stamping out the virus.

Unlike chickens with bird flu, the ducks in the trial stayed perfectly healthy, making it harder to detect the presence of the virus, Stohr added.

“We have said from the very beginning that controlling H5N1 (virus) in poultry is a long-term undertaking. We have never seen so many countries in such a wide geographical area infected with highly pathogenic avian influenza,” he said.

The research confirmed something that health officials in Thailand and other countries badly affected by bird flu had already suspected.

Millions of poultry, mainly chickens, have been slaughtered across Southeast Asia as governments battle to contain a virus that WHO officials fear could - if unchecked - eventually mutate and trigger a human flu pandemic.

So far 12 people have died in Thailand and 20 in Vietnam, including a couple of cases in which the infection seems to have been passed from one person to another.

But other laboratory work, also done in the United States, on a specimen of the virus taken from one of the victims in the suspected human-to-human transmission appeared to confirm that no mutation had yet taken place, Stohr said.

“Based on very limited data, it appears to have been an animal virus,” he said.

The WHO fears the virus, which is proving fatal in some 70 percent of cases, could lead to an influenza pandemic similar to that of 1918 when over 20 million people died worldwide.

The virus appears to be growing more resistant, Stohr said, noting that in laboratories it was surviving three times as long - six days as against two last year - when exposed to higher temperatures.

This could explain why it had continued to spread in the hot summer months in Asia, when normally it would be less active, waiting for the cooler, winter weather, he added.

The WHO has repeatedly warned that the world is not doing enough to develop a vaccine to fight any eventual avian flu epidemic.

In an effort to speed the search, Stohr said the WHO would host a meeting with chief executives from pharmaceutical firms and government officials at its Geneva headquarters on Nov. 11.