* Cases diagnosed in California, Texas

* No reason for concern yet - CDC

* Flu is unusual mixture but no U.S. deaths seen (Adds details about cases in Mexico, Canada concern)

By Maggie Fox, Health and Science Editor

WASHINGTON, April 23 (Reuters) - Seven people have been diagnosed with a new kind of swine flu in California and Texas, the U.S. Centers for Disease Control and Prevention reported on Thursday.

All seven people have recovered but the virus itself is a never-before-seen mixture of viruses typical among pigs, birds and humans, the CDC said.

"We are likely to find more cases," the CDC's Dr. Anne Schuchat told a telephone briefing. "We don't think this is time for major concern around the country."

Only one of the seven cases was sick enough to be hospitalized and all have recovered, Schuchat said.

CDC officials are unsure whether the cases are related to an unusually late and severe flu season in Mexico in which 20 people have died.

"Generally the period of infection ends during the last week of February and the first week of March, but this year there was an atypical situation where the transmission period was prolonged until April," Mexico's Ministry of Health said in a statement.

Canadian officials have asked doctors to keep an eye out for cases of respiratory illness among travelers from Mexico.

"Symptoms from those seriously ill in Mexico include high fever, headache, eye pain, shortness of breath and extreme fatigue with rapid progression of symptoms to severe respiratory distress in about five days," the British Columbia Centre for Disease Control said in a statement.

In the United States, the CDC reported the new strain of swine flu on Tuesday in a boy and a girl from California's two southernmost counties.

Now, five more cases have been found via normal surveillance for seasonal influenza. None of the patients, whose symptoms closely resembled seasonal flu, had any direct contact with pigs.

Two of the new cases were among 16-year-olds at the same school in San Antonio "and there's a father-daughter pair in California," Schuchat said. The boy whose case was reported on Tuesday had flown to Dallas, but the CDC has found no links to the other Texas cases.

The agency will issue daily updates at http://www.cdc.gov/flu/swine/investigation.htm.

HUMAN TO HUMAN

"We believe at this point that human-to-human spread is occurring," Schuchat said. "That's unusual. We don't know yet how widely it is spreading ... We are also working with international partners to understand what is occurring in other parts of the world."

The CDC's Dr. Nancy Cox said virus samples from the seven appear to carry genes from swine flu, avian flu and human flu viruses from North America, Europe and Asia.

"We haven't seen this strain before, but we hadn't been looking as intensively as we have," Schuchat said. "It's very possible that this is something new that hasn't been happening before."

Surveillance for and scrutiny of influenza has been stepped up since 2003, when H5N1 bird flu reappeared in Asia. Experts fear this strain, or another strain, could spark a pandemic that could kill millions.

The influenza A strain is an H1N1, the same subtype as one of the seasonal flu viruses now circulating. Now that the normal influenza season is waning, it may be easier to spot cases of the new swine flu, Schuchat said.

The CDC is asking doctors to think about the possibility of swine flu when patients appear with flu-like symptoms, to take a sample and send it to state health officials or the CDC for testing.

Cox said the CDC is already preparing a vaccine against the new strain, just in case.

"This is standard operating procedure," he said. (With additional reporting by Mica Rosenberg in Mexico City and Allan Dowd in Vancouver)

AlertNet news is provided by