WASHINGTON - Global health officials underestimated the risk that pig herds might be a source of new influenza strains, choosing instead to focus on the threat of bird flu, researchers in Mexico said on Thursday.

They analyzed samples from people infected with the new H1N1 swine flu virus, which has been confirmed in more than 19,000 people in 64 countries, killing about 120. U.S. health officials say this number reflects only a fraction of the true number of cases.

This virus most likely evolved from recent swine viruses, Gerardo Nava of the National Autonomous University of Mexico and colleagues wrote in their report, published in the online journal Eurosurveillance.

These findings indicate that domestic pigs in North America may have a central role in the generation and maintenance of this virus.

The global pork industry has rushed to defend pig products, saying pig meat is no danger to people. But health experts have also noted there is very little surveillance done to track influenza among pigs -- even though the virus is very common in the animals and just as transmissible as it is among people.

Flu viruses have also been shown to pass from pigs to people and from people to pigs.

These observations also reiterate the potential risk of pig populations as the source of the next influenza virus pandemic, Nava and colleagues wrote.

Although the role of swine as 'mixing vessels' for influenza A(H1N1) viruses was established more than a decade ago, it appears that the policy makers and scientific community have underestimated it.

GENETIC ANALYSIS

Nava's team looked at all available genetic sequences of H1N1 viruses circulating in North America for the last two decades. H1N1 has been around since the 1918 pandemic, infects both people and pigs, and mutates regularly.

They did not find very many samples, something Nava said reflects how little testing is done to monitor influenza in swine herds.

I think that we forgot about swine farms, Nava said in a telephone interview.

Experts began calling for better surveillance of influenza in swine in 1998, he said.

He called for stepped-up testing of swine and said farmers, producers and government officials will have to consider the expensive possibility of mass slaughters of infected swine.

Quick slaughter of entire poultry flocks has been credited with helping to control outbreaks of avian influenza, but pork farmers usually wait out outbreaks of influenza among herds, because it rarely makes pigs very sick.

The problem is not that the pig is going to die or even pass the virus to a human, Nava said. The problem is that the virus is recombining (in the pig's body) and getting new sequences, new genes.

By concentrating on avian flu, Nava and colleagues said, officials ignored a possibly bigger source of new influenza viruses -- pigs and the people who work with them.

Global health experts have been warning of a pandemic of influenza and the chief suspect has been H5N1 avian influenza, which has infected 433 people since 2003 and killed 262.

Nava said fears of damaging the pork industry may have led animal and human health officials to be too cautious.

We understand the commercial interests, Nava said.

This week the U.S. Agriculture Department said it would launch a pilot surveillance project to look for new strains of flu virus in pigs.

Mexico Medicine