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Resistance to swine flu antiviral drugs can develop faster than expected, research finds

The pandemic H1N1 influenza virus can develop resistance to the commonly used antiviral drugs much faster than expected, federal researchers said Friday. Previous research had suggested that it would take 24 days or longer for resistance to the drugs to develop in a patient, but a new study reported in the journal Clinical Infectious Diseases found that resistance could appear in as little as nine days. The study also found that one patient developed resistance to the experimental intravenous drug peramavir, the first time clinically significant resistance to that drug has been observed.

Dr. Matthew J. Memoli and Dr. Jeffrey K. Taubenberger of the National Institute of Allergy and Infectious Diseases reported on two immunocompromised patients who developed persistent swine flu infections. Both patients had undergone blood stem cell transplants several years earlier. The resistant form of the virus appeared in one patient after 14 days and in the second after nine days.

Both patients received oseltamivir (Tamiflu) for extended periods, but continued to shed virus -- a sign that the virus had developed resistance to the drug. When one patient's condition worsened despite 24 days of treatment, he was given peramivir for 10 days. The patient continued shedding virus, indicating that the virus had become resistant to that drug too. Laboratory tests confirmed the resistance. The patient was then given the only remaining antiviral, zanamivir (Relenza) for 10 days and fully recovered. Zanamivir must be inhaled, however, and many seriously ill patients are not able to take it for that reason.

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