HONG KONG, Feb 26 (Reuters) - A leading virologist said on Thursday that ducks are to blame for the resurgence of the H5N1 bird flu virus in China and Vietnam, and called for wider surveillance and vaccination of ducks to stop the problem.

The virus has infected at least 14 people in both countries since the start of this year, killing seven of them.

Experts said what was mystifying about the latest round of outbreaks was the absence of the disease in poultry in China despite the human infections. H5N1 is passed primarily from animal to human.

Robert Webster, a leading H5N1 expert, told a medical conference in Hong Kong that 'silent infections' of H5N1 in ducks may be the reason behind the human cases in China.

Most types of ducks are not sickened by the virus and in most countries in Asia, they mingle freely with chickens, providing ample opportunity for the virus to jump between species.

'The problem is in the ducks in Asia, there is no visible disease in these birds,' said Webster of the St Jude Children's Research Hospital in the United States.

The absence of visible infection in poultry makes it harder to track the disease and take preventive measures.

'In China, there is no disease (in poultry) ... I suspect there are some viruses in duck populations that they (authorities) don't know about ... Maybe there are silent infections, it's a vast country.'

Webster said it was very difficult to get farmers to vaccinate their apparently healthy ducks, especially when these exercises cost a lot of money.

'In Vietnam, the problem is again the duck because ... if you were a farmer and there is no disease on your farm, why do you want vaccines? Really it comes down to where the silent infections are,' he told a small group of reporters later.

'Farmers only respond to problems and if there is no problem, why spend the money? That's why we can't get rid of the virus.'

Since 2003, the H5N1 avian influenza virus has infected 408 people in 15 countries and killed 254 of them. It has killed or forced the culling of more than 300 million birds as it spread to 61 countries in Asia, the Middle East, Europe and Africa.

While H5N1 rarely infects people, experts fear it could mutate into a form that people could easily pass to one another, sparking a pandemic that could kill tens of millions and topple the global economy.

Webster called for better disease surveillance and culling where the virus is detected.

'You have to do massive surveillance, it will be very expensive, and compensation for farmers (in places where the H5N1 virus is detected), where there is no disease, that is difficult,' he said.