Local media and epidemic monitoring company Vertect identify Perote, a town in Veracruz and home to a large pig farm, as the first site of any unusual influenza activity in Mexico.

The Veracruz state government rejected the accusations, saying residents in Perote often suffer from respiratory illnesses because the town is located in a mountainous area, where temperatures vary widely.

"We only have a case tested positive for the H1N1 virus, a boy of five years eight months and he is now safe," said a government official, adding that an outbreak had been detected elsewhere around April 8, while the boy's case was "around mid-April".

However, Veracruz state news agency reported in March that residents were complaining of flu-like symptoms as early as in February and March, blaming a local pig farm, Granjas Carroll, a subsidiary of the U.S. pig farming giant Smithfield Foods.

Smithfield issued its own statement on Sunday, saying it had "found no clinical signs or symptoms of the presence of swine influenza in the company's swine herd or its employees at its joint ventures in Mexico."

The official said the outbreak was not believed to be atypical and stressed that the local government had put up a sanitary cordon to prevent the spread of any virus.

He also argued that recent statements from the World Health Organization had dismissed a direct link between the deadly flu virus and pigs.

Earlier on Monday, the World Organization for Animal Health (OIE) said that the virus has characteristics of avian, human and swine virus and it is more accurate to call it the "North American Flu."