doi:10.1038/nindia.2012.154 Published online 15 October 2012
Researchers from the National Institute of Cholera and Enteric Diseases (NICED) in Kolkata have spotted a classical case of 'sporadic reassortment' — mixing of genetic material from swine and human origin viruses — in an influenza A viral strain with H1N2 subtype. The study supports the possibility of 'reassortment events' during influenza season when infectivity is high and two different subtypes of the virus circulate together in the same geographical location.
The mixed strain circulated during the swine flu pandemic in 2009-2010 and has been found from a 25-year-old man in Mizoram. The strain has been named A/Eastern India/N-1289/2009.
Influenza A viruses of the H1N2 subtype were isolated previously in India and Japan during 2001–02. They were reassortants of the human H1N1 and H3N2 viruses and distinct from the H1N2 swine influenza viruses. However, repeated attempts to isolate the recombinant virus had failed. The researchers say this could have been due to the 'loss of viability' as the samples came from Mizoram (about 1219 kms from Kolkata) by courier and took more than 48 hrs to reach the NICED lab.
Reassortment is responsible for some major genetic shifts in the history of the influenza virus. The 1957 and 1968 pandemic flu strains were caused by reassortment between an avian virus and a human virus. The 2009 H1N1 swine flu virus has been found to have an unusual mix of swine, avian and human influenza genetic sequences.
After sequencing the full genome, the researchers found the unique reassortment event where the N-1289 virus acquired its hemagglutinin (HA) gene from a 2009 pandemic H1N1 virus with swine origin and the other genes from H3N2-like viruses of human origin.
Co-circulation of both these influenza viruses during 2009 and complete disappearance of seasonal H3N2 and H1N1 strains in 2010 was also observed in eastern India. Such co-circulation is the prime cause of the generation of genetically reassortant viruses, the researchers say.