(AP) -- U.S. health officials are stepping up testing of swine flu cases for Tamiflu resistance, now that an American has come down with a resistant strain.
A California teenager was diagnosed with swine flu last month after arriving in Hong Kong on June 11, and has since recovered. The 16-year-old is a San Francisco resident and likely was infected in the United States, health officials said Tuesday.
Her illness was mild, but noteworthy. She's just the third person in the world to be diagnosed with a strain resistant to Tamiflu, the primary pharmaceutical weapon against the new virus.
The other two resistant cases - patients in Denmark and Japan - had been taking Tamiflu as a preventive measure after coming into contact with someone with swine flu. The Californian girl had not taken Tamiflu, meaning she apparently was infected by an already-circulating resistant strain before she traveled to Hong Kong.
"It's a little more concerning" than the two previous cases, said Dr. Tim Uyeki of the federal Centers for Disease Control and Prevention.
Hong Kong health officials, known as aggressive about trying to detect and isolate swine flu cases, detected the resistant strain in the girl.
Until an effective vaccine is developed, the drugs Tamiflu and Relenza are considered the best defense against the swine flu virus, which has caused nearly 34,000 reported illness in the United States, including at least 170 deaths.
Health officials say they are not alarmed, and have been expecting to see some swine flu cases shrug off Tamiflu treatment. Such resistance has been seen in other types of flu. Late last year, CDC officials reported that the most common flu bug circulating at the time was overwhelmingly resistant to the drug.
They also believe resistance is not a widespread problem. No resistance was seen in the CDC's analysis of about 200 U.S. swine flu samples. California officials say they have found no resistance in their tests of about 30 other samples in that state.
Spread of a Tamiflu-resistant strain may not be ongoing, said Uyeki, a CDC flu expert. "These are likely to be sporadic cases, but it's very important to monitor" for them, he said.
Health officials acknowledge they don't have a complete understanding of what's going on. The samples tested for resistance represent just a tiny fraction of the more than 1 million swine flu cases estimated to have occurred in the United States since the virus was first detected in April.
The CDC is calling for health departments to send in more samples for testing Tamiflu resistance, Uyeki said. California is already stepping up such testing, said Ralph Montano, a spokesman for the California Department of Public Health.
Health officials are continuing to recommend Tamiflu as a treatment for swine flu, Uyeki said.
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