Drug resistance is a feared phenomenon especially in case of influenza as drugs to combat the viruses are scarce. In most cases drug resistance exhibited by viruses is caused by overuse or misuse of drugs. It is a natural mechanism as microorganisms adjust to the drug targeting them and mutate into forms that can bypass the drug.

Reporting in the April 4 issue of the Journal of the American Medical Association, the University of Tokyo researchers said both zanamivir (Relenza) and oseltamivir (Tamiflu) are widely prescribed for flu in Japan. Type B influenza is normally associated with less intense outbreaks when compared to those caused by type A flu viruses.

Dr. Shuji Hatakeyama, of the University of Tokyo, and colleagues collected influenza B samples after a major outbreak of the infection in 2004-2005.

For a part of the study virus samples from patients at four community hospitals in Japan were collected. Additionally samples from 74 children were taken before and after they were treated with Tamiflu. Researchers found drug resistance in one child among the samples examined.

In another part of the study researchers collected samples from 422 untreated children and adults with flu. Here they discovered seven samples showing signs of drug resistance. Previously type A influenza was also shown as developing resistance to Tamiflu.

The rate of resistance in that case was 18 percent, but in the current study it was just 2 percent.

“If drug-resistant influenza B viruses become more prevalent, we will need new drugs to treat infected patients,” said study co-author Yoshihiro Kawaoka, a virology professor at the University of Tokyo.

"Continued surveillance for the emergence or spread of neuraminidase inhibitor-resistant influenza viruses is critically important," the authors wrote. "Further evaluation of the biological properties of neuraminidase inhibitor-resistant influenza viruses is needed to fully assess their pathogenicity in humans."

An editorial in the same journal said the study raised more questions than it answered. "Influenza B mutants with reduced sensitivity to (anti-viral drugs) are circulating, and these viruses can cause infections with no difference in duration of symptoms," it added.

The World Health Organization has developed a Global Strategy for Containment of Antimicrobial Resistance. The policy targets six key areas to overcome the problem. These range "from disease prevention, to access and appropriate use of antimicrobials, to surveillance, to the need for appropriate legislation and focused research."