The World Health Organization declared a pandemic last month in an effort to control the spread of the flu virus, which first appeared in the United States and Mexico, but has since spread across the world and killed more than 300 people.

Genetic tests have shown the new H1N1 strain is clearly a pig virus and not a human virus, although people are catching it from other people and not from animals.

However, the Argentine case adds weight to the theory that pigs can be infected by humans.

In Canada, a farm worker was suspected of passing the virus to a herd of pigs. Blood testing later cleared him but health officials did not rule out the possibility that a human infected the animals.

"Our theory is that the pigs were infected by the farm workers who had had flu symptoms a week before the pigs started to show symptoms," the Argentine government farm spokesman said. Argentine government spokesmen generally ask not to be named.

He said 800 pigs had tested positive for H1N1 flu, but that the two workers suspected of passing the virus to the pigs had never gone to a doctor so it had not been established whether or not they had the new flu strain.

The pig farm, which is located in Buenos Aires province, was put under quarantine and the spokesman said H1N1 flu tests had proved negative since 24 June.

Argentines are increasingly worried about the new flu virus as peak flu season approaches in the Southern Hemisphere winter. The Health Ministry has confirmed 1,587 cases and 26 deaths, prompting officials to bring forward school holidays.

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