In Mexico where there is a swine flu outbreak, the viruses in 12 cases match six in the U.S., prompting U.S. federal authorities to call for intensified screening of samples from flu victims in the area and in flu victims who have recently traveled to Mexico.

U.S. researchers plan to go to Mexico for investigation, according to the CDCP.

The outbreak has caused concern because the virus appears to be spread from human to human, which is the crucial requirement for a new virus to precipitate a large-scale outbreak, Besser said.

None of the American victims has had any contact with pigs and only one of them has traveled to Mexico recently, he said.

Although it commonly causes respiratory problems in pigs, swine flu is rare in humans. Only 12 other cases of human infection have been detected since 2005, according to the CDCP. Of the 12 cases, 11 of the patients had had contact with pigs.

Symptoms include fever, lethargy, cough and lack of appetite. Documented cases of swine flu being transmitted from one human to another are rare, according to the CDCP.

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Editor:Yang Jie