Study: H7N9 likely resistant to current antiviral drugs

As the newest strain of avian influenza, H7N9, continues to spread, scientists are urgently trying to figure out how to thwart transmission of the deadly virus.

A new study published in mBio, a journal of the American Society for Microbiology, found that some strains of the H7N9 avian influenza that emerged in China this year have developed resistance to the only antiviral drugs on the market intended to treat the infection. What makes treating the disease even more complicated is the tendency for antiviral resistance testing to produce misleading results, encouraging the spread of resistant H7N9 strains, the study found. No approved H7N9 vaccine exists yet.

Looking at samples from the first confirmed human case of H7N9, researchers found that 35% of the viruses were resistant to oseltamivir (Tamiflu) and zanamivir (Relenza), neuraminidase (NA) inhibitors used to treat H7N9. When the researchers ran further tests on the viruses to measure the activity of the neuraminidase enzyme, the tests failed to detect that the strains were resistant, suggesting that monitoring strains for resistance on a larger scale would be ineffective.

When scientists took a closer look at the viral isolate, they noted that it is actually composed of two distinct types of H7N9 viruses--35% that carry the R292K mutation, making them resistant to NA inhibitors, and 65% that are sensitive to these same drugs. The enzyme-based test gave false results, the researchers concluded, because the functioning normal enzymes masked the presence of the nonfunctioning mutant enzymes.

The study results suggest that using NA inhibitors like Tamiflu and Relenza to treat H7N9 strains might have the opposite of the intended effect and instead encourage the resistant virus strain to spread.

As of July 4, the World Health Organization (WHO) has been informed of a total of 133 laboratory-confirmed cases, including 43 deaths. There is no evidence yet that the disease is transmissible among people. Scientists have postulated that the disease likely emerged from contact with farm animals, especially chickens. Until the source of infection can be identified, WHO said in a recent H7N9 update that there will likely be more cases of human infection with the virus.

- here's the study abstract
- and the press release

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