Risk of resistant avian influenza A virus in wild waterfowl as a result of environmental release of oseltamivir.

Oseltamivir is the best available anti-influenza drug and has therefore been stockpiled worldwide in large quantities as part of influenza pandemic preparedness planning. The active metabolite oseltamivir carboxylate (OC) is stable and is not removed by conventional sewage treatment. Active OC has been detected in river water at concentrations up to 0.86 µg/L. Although the natural reservoir hosts of influenza A virus (IAV) are wild waterfowl that reside in aquatic environments, the ecologic risks associated with environmental OC release and its potential to generate resistant viral variants among wild birds has largely been unknown. However, in recent years a number of in vivo mallard (Anas platyrhynchos) studies have been conducted regarding the potential of avian IAVs to become resistant to OC in natural reservoir birds if these are drug exposed. Development of resistance to OC was observed both in Group 1 (N1) and Group 2 (N2, N9) neuraminidase subtypes, when infected ducks were exposed to OC at concentrations between 0.95 and 12 µg/L in their water. All resistant variants maintained replication and transmission between ducks during drug exposure. In an A(H1N1)/H274Y virus, the OC resistance mutation persisted without selective drug pressure, demonstrating the potential of an IAV with a permissive genetic background to acquire and maintain OC resistance, potentially allowing circulation of the resistant variant among wild birds. The experimental studies have improved the appreciation of the risks associated with the environmental release of OC related to resistance development of avian IAVs among wild birds. Combined with knowledge of efficient methods for improved sewage treatment, the observations warrant implementation of novel efficient wastewater treatment methods, rational use of anti-influenza drugs, and improved surveillance of IAV resistance in wild birds.