Adamantanes have been used to treat influenza A virus infections for more than 30 years. Previous surveillance studies have identified a low incidence of resistance to amantadine and rimantadine among circulating influenza virus strains. However, it has been 10 years since the last comprehensive global study of resistance to these drugs was published.
In the largest study of adamantane resistance to date, Rick Bright (Centers for Disease Control and Prevention, Atlanta, GA, USA) and colleagues screened 7000 influenza A isolates for gene mutations known to confer drug resistance. They found that drug resistance increased from 0.4% in 1994-1995 to 12.3% in 2003-2004. They also found that 61% of resistant viruses isolated since 2003 were from people in Asia. Some Asian countries had drug resistance frequencies exceeding 70%.
Dr Bright comments: "We were alarmed to find such a dramatic increase in drug resistance in circulating human influenza viruses in recent years. Our report has broad implications for agencies and governments planning to stockpile these drugs for epidemic and pandemic strains of influenzaWith the increasing rates of resistance shown here, amantadine and rimantadine will probably no longer be effective for treatment or prophylaxis in the event of a pandemic outbreak of influenza." (Quote by e-mail; does not appear in published paper)