Genetic characterization of oseltamivir-resistant seasonal influenza A (H1N1) virus circulating during 2009 pandemic influenza in Mumbai

Background: Oseltamivir is an important antiviral agent for controlling the transmission and dissemination of influenza viruses. The emergence and widespread occurrence of oseltamivir-resistant A (H1N1) viruses are of major concern. Information on drug susceptibility of seasonal influenza A viruses in India is limited. To understand the genetic background of antiviral drug-resistance, we performed sequence analysis of seasonal influenza A viruses circulating during 2009 pandemic influenza in Mumbai. Methods: Nasopharyngealswabs positive for seasonal influenza A virus by real time reverse transcriptase polymerase chain reaction were inoculated on Madin-Darby canine kidney (MDCK) cell line for virus isolation. Molecular analysis of hemagglutinin (HA), neuraminidase (NA) and matrix (M2) genes was used to detect known mutations contributing to resistance. Resistance to neuraminidase was assayed using a commercially available chemiluminescence based NA-Star Influenza Neuraminidase Inhibitor Resistance Detection kit. Results: Investigations showed that oseltamivir-resistant virus was obtained from an untreated person with H275Y mutation in the NA gene. The dominant variant also acquired additional substitutions, including G189S, A193T and H196N in receptor binding site of HA1 domain and D354G in NA gene. Phylogenetic analysis revealed that HA and NA gene of oseltamivir-resistant isolate from Mumbai exhibited close homology to human A (H1N1) A/Brisbane/59/2007 vaccine strain. Conclusion: Resistance to oseltamivir was observed in seasonal influenza A (H1N1) isolate co-circulating in Mumbai during the 2009 pandemic influenza. The study highlights the importance of continual surveillance on influenza in India for early detection and identification of mutations conferring antiviral resistance.