H7N9 influenza strains are resistant to antivirals, but tests fail to identify resistance

July 19, 2013

Some strains of avian H7N9 influenza have developed resistance to the only antiviral drugs available to treat the infection, but testing for resistance can give misleading results, helping to hasten the spread of resistant strains.

The authors of a study published in mBio, the online journal of the American Society for Microbiology, characterized viruses taken from the first person known to be stricken with H7N9 influenza and found that 35% of them are resistant to oseltamivir and zanamivir. However, lab testing of the viruses, which detects the activity of a viral enzyme, fails to detect that these strains are resistant, so monitoring for the development of resistance using this technique would prove futile. Resistant strains of H7N9 can flourish in patients treated with the drugs, inadvertently leading to the spread of resistant infections.

Researchers tested antiviral susceptibility of an H7N9 strain isolated from the first confirmed human case using a method that tests the activity of the neuraminidase enzyme. The reassuring results were misleading, however; the enzyme-based test indicated that the flu strain was susceptible to NA-inhibiting antiviral drugs, but it is not.

A closer look at the viral isolate revealed it is actually made up of two distinct types of H7N9 viruses. Roughly 35% of the viruses carry the R292K mutation, making them resistant to NA inhibitors, and 65% are sensitive to these same drugs. The enzyme-based testing gave misleading results because the functioning wild-type enzymes masked the presence of the non-functioning mutant enzymes. Researchers say it is thus crucial to use a gene-based surveillance technique that can detect resistant influenza strains in a mixed infection. Read the study.