rational use of drugs may not always delay resistance to a drug. A research team led by the National Institutes of Health in the us has found that the influenza virus h3n2 were resistant to drugs belonging to the adamantane group irrespective of whether they were exposed to the drug or not. Examining an international collection of viral genomes, they found a single type of mutation was responsible for every case of resistance.

They say resistance to the drug was developing in the virus as a natural process by which the virus developed into an effective organism. The frequency of resistance to adamantane drugs has now increased from about 2 per cent to 90 per cent around the world. For example, viruses in the us, where an average of 1.5 million adamantane dosages was prescribed annually, and countries like New Zealand and Japan, where the drug is rarely used, were resistant to the drug.
Drug is not enough This resistance can be attributed to a single replacement of an amino acid. "The study shows drug resistance can evolve by unusual means,' says Edward C Holmes, Center for Infectious Disease Dynamics, Department of Biology, Pennsylvania State University.

"The resistance mutation gets linked to another beneficial mutation located elsewhere