A study of oseltamivir-resistant influenza viruses in Thailand, 2008-2010

On 25 January 2008, WHO was notified by Norway of a high prevalence of oseltamivir (Tamiflu©) resistance in seasonal influenza A(H1N1) viruses detected through routine surveillance and testing. Information about drug resistance is now an important piece of information guiding patient treatment recommendations. The Regional Influenza Reference Laboratory of SEA Region (RIRL), Thailand established the capacity to run the fluorescence-based NA enzyme inhibition assay. Throat swabs from patients with influenza-like illness or pneumonia were collected at 11 sentinel sites across the country. All swab specimens were transported to the RIRL. Specimens were identified using the standard protocol for real time reverse transcription polymerase chain reaction (rRT-PCR) from the WHO and US-CDC to detect influenza A/B virus and then A viruses were subtyped with specific primers from US-CDC. All specimens from sentinel sites which demonstrated influenza positive by rRT-PCR during 2008-2010 were selected for virus isolation in MDCK cells. A total of 1,211 representative influenza isolates were tested for susceptibility to oseltamivir by fluorometric neuraminidase inhibition Assay (phenotypic assay). All positive results or resistant isolates and some negative results obtained from phenotypic assay were subsequently performed partial NA gene sequencing which carried the oseltamivir resistance mutation at H274Y (N2 numbering) . The study results demonstrated that in 2008-2009, a steady increase in proportion of seasonal A(H1N1) oseltamivir resistance was observed, reaching 95.6% in 2009. In 2009-2010, the H274Y mutation was found in pandemic A(H1N1) viruses and the prevalence of resistance was 1.31%. Oseltamivir resistance was not found with influenza type B or H3 viruses during 2008-2010. Continued monitoring of antiviral resistance in influenza viruses is essential for guiding patient treatment recommendations .