TUESDAY, SEPTEMBER 27, 2016

H7N9 influenza antiviral resistance study shows resistance detection flaws

Researchers with the Memphis, Tennessee-based St. Jude Children's Research Hospital recently found that antiviral resistance testing of the H7N9 influenza virus can give misleading results, leading to the spread of resistant strains.

The study, which was published in mBio, found that 35 percent of tested H7N9 avian influenza strains were resistant to the antivirals oseltamivir and zanamivir, which are also known as Tamiflu and Relenza. Laboratory testing of the viruses, however, failed to detect that the strains were resistant, nullifying attempts to monitor for the development of antiviral resistance.

"If H7N9 does acquire human-to-human transmissibility," Robert Webster, the corresponding author of the study, said, "What do we have to treat it with until we have a vaccine? Oseltamivir. We would be in big trouble."

The authors tested the susceptibility to antivirals of an H7N9 strain isolated from the first confirmed human case of the disease. The enzyme-based test results incorrectly found the virus was susceptible to neuraminidase inhibitors.

The researchers determined the viral isolate was made up of two distinct types of H7N9 viruses. The R292K mutation made approximately 35 percent of the H7N9 virus strains resistant to NA inhibitors. Webster said the enzyme-based testing was incorrect because the functioning wild-type enzymes masked the presence of non-functioning mutant enzymes.

Using NA inhibitors to treat a patient with a resistant strain could lead to increased spread of the resistant strain.

"The great need at the moment are additional drugs aimed at additional sites in the influenza genome," Webster said. "There are some [drugs] in the pipeline, but they are still under testing at the moment. We'd better get some vaccine seed stocks up and ready. The antiviral option for controlling H7N9 isn't too good."

As of July 12, 132 people were infected with H7N9 influenza, including 43 deaths.