Swine Flu Case Count Rising, Unusual Strain People in 2 States

An unusual strain of swine flu is circulating among people in the Southwest but is not known to have caused any deaths, the Centers for Disease Control and Prevention said Thursday.

The agency, which has found only seven cases, expects to find more now that it has begun looking intensively for them.

“We don’t yet know the extent of the problem,” said Dr. Anne Schuchat, the director of respiratory diseases for the agency, “but we don’t think this is a time for major concern.”

Five of the people infected were in Imperial and San Diego Counties in California and two were in San Antonio. They were 9 to 54 years old.

None had any contact with pigs, and in two sets of cases — involving a father and daughter and two 16-year-old schoolmates — those infected had contact with each other. That convinced the authorities that the virus was being transmitted from person to person.

The seven people were apparently infected from late March to mid-April. Only one was hospitalized, and all recovered.

The A (H1N1) flu strain they had was quite unusual, said Dr. Nancy Cox, the chief of the agency’s flu division. It contained gene segments from North American swine, bird and human flu strains as well as one from Eurasian swine.

Like some human strains, it is resistant to two older flu drugs, amantadine and rimantadine. It is not resistant to Tamiflu or Relenza. However, Tamiflu resistance is common in the H1N1 human flu strain circulating this year, so the swine strain could become resistant to Tamiflu if the viruses mixed in humans or, possibly, in pigs.

Swine flus rarely infect humans. There have been about a dozen cases since 2005, but almost all were in farm workers or others in contact with pigs.

In 1976, there was a cluster of swine flu cases among soldiers at Fort Dix, in New Jersey, one of whom died. That led to a rush to make a new vaccine and administer it to 40 million Americans. No epidemic materialized, but thousands of people claimed that the vaccine had given them Guillain-Barré syndrome, which can cause lethargy or paralysis. The episode led to the resignation of the director of the disease control center, and the agency has been wary of causing panic over influenza cases ever since.

The unusual strain this year was noticed, Dr. Schuchat said, only because the agency was trying out a new diagnostic test at a Navy laboratory and doing more testing than usual through a new Border Infectious Disease Surveillance Project along the Mexican border.

Officials at the public health agency in Canada said their Mexican counterparts had warned them this week of a “relatively high” fatality rate for people in Mexico who have had respiratory illnesses this season, some of whom had tested positive for flu. Asked about that, American officials said they had no information. A spokesman said the disease control center had asked Mexican officials to send samples for testing.

The United States flu season is tailing off now. It has been relatively mild; the major surprise had been the widespread Tamiflu resistance in the circulating human H1N1 strain.

Dr. Cox of the disease control center said officials did not yet know whether the flu shot this year protected against the new swine strain.

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