February 18, 2009

A new study by US University of Colorado at Boulder (CU) showed the resistance of the bird flu virus to a major class of antiviral drugs is increasing through positive evolutionary selection.

Researchers have documented the trend in more than 30 percent of the samples tested.

Researcher Andrew Hill said the bird flu virus is evolving a resistance to a group of antiviral drugs known as adamantanes, one of two classes of antiviral drugs used to prevent and treat flu symptoms.

Hill said the rise of resistance to adamantanes, which include the non-prescription drugs amantadine and rimantadane, appears to be linked to Chinese farmers adding the drugs to chicken feed as a flu preventative, according to a 2008 paper by researchers from China Agricultural University.

He also said that in contrast, resistance of the bird flu virus to the second, newer class of antiviral drugs that includes oseltamivir, a prescription drug marketed under the brand name Tamiflu, is present but is not yet prevalent or under positive genetic selection.

He added that the research on the mutations, combined with molecular evolution tests and a geographic visualisation technique using Google Earth, provides a framework for analysis of globally distributed data to monitor the evolution of drug resistance.

He explained as these adamantanes have gotten into nonhuman vectors like birds, the positive selection for resistance to bird flu is rising and if Tamiflu is ever used in the manner of adamantanes, a similar resistance developing through positive selection could conceivably be seen.

For the study, the researchers analysed 676 whole genomes of Influenza A/H5N1 from National Institutes of Health databases of viruses isolated between 1996 and 2007.

Hill said the team is comparing how often amino acid sequence changes in genes lead to mutations that affect drug resistance in the H5N1 virus and how often such changes evolve into random mutations that do not affect resistance.