1. This study investigated the geographical distribution and growth properties of avian influenza A (H5N1) isolates with mutations that confer resistance to amantadine and rimantadine. It also explored whether naturally occurring mutations associated with resistance to oseltamivir are present at a low frequency in H5N1 isolates and if so, whether these quasi-species may be the source of the emergence of oseltamivir-resistant strains following exposure to this drug.
2. Naturally occurring avian H5N1 virus mutants resistant to the main types of antiviral drugs were noted.
3. The fitness of these mutants in avian and mammalian hosts needs further investigation.