Spotting pandemic influenza viruses
The results of a new study have provided fresh insights into influenza pandemics by raising the possibility that all three pandemic influenza strains of the 20th century may have been generated through a series of multiple reassortment events and emerged over a period of years before pandemic recognition. The results indicate that each of these strains was produced by reassortment between the previously circulating human virus and at least one virus of animal origin. The novel gene segments for the H2N2/1957 and H3N2/1968 pandemics seem to have originated from avian hosts, but the zoonotic sources of the introduced viral gene segments for the 1918 pandemic remain ambiguous. However, evidence suggests that, over a number of years, avian gene virus segments have entered mammalian populations where the viruses may have undergone reassortment with the prevailing human virus. Given the frequent interspecies transmission of influenza viruses between swine and humans, it is most likely that such reassortment events occurred in pigs before pandemic emergence.
This work suggests that in the 1918 and 1957 pandemics, novel NA and internal genes may have been introduced into the prevailing human virus strains before the acquisition of the novel pandemic HA. Frequent detection of seasonal human influenza strains in pigs indicates that pandemic precursor viruses probably have circulated in either swine or human populations. The precursors to the H2N2 and H3N2 pandemics have not been detected, probably because they originated in Asia where little or no surveillance was conducted at that time.
If future pandemics arise in this manner, this interval may provide the best opportunity for health authorities to intervene to mitigate the effects of a pandemic or even to abort its emergence. However, the findings argue the need for highthroughput characterization of all 8 gene segments of human virus isolates, even those that have unremarkable HA antigens, particularly of human viruses isolated in hotspots for zoonotic infections with avian influenza viruses. At present, global influenza surveillance in humans focuses attention primarily on hemagglutinin. Although this focus will continue to be required for strain selection for seasonal influenza vaccines, our findings argue that this surveillance will not suffice for early warning of an incipient pandemic.
Dating the emergence of pandemic influenza viruses. PNAS USA July 13, 2009. doi: 10.1073/pnas.0904991106
Pandemic influenza viruses cause significant mortality in humans. In the 20th century, 3 influenza viruses caused major pandemics: the 1918 H1N1 virus, the 1957 H2N2 virus, and the 1968 H3N2 virus. These pandemics were initiated by the introduction and successful adaptation of a novel hemagglutinin subtype to humans from an animal source, resulting in antigenic shift. Despite global concern regarding a new pandemic influenza, the emergence pathway of pandemic strains remains unknown. Here we estimated the evolutionary history and inferred date of introduction to humans of each of the genes for all 20th century pandemic influenza strains. Our results indicate that genetic components of the 1918 H1N1 pandemic virus circulated in mammalian hosts, i.e., swine and humans, as early as 1911 and was not likely to be a recently introduced avian virus. Phylogenetic relationships suggest that the A/Brevig Mission/1/1918 virus (BM/1918) was generated by reassortment between mammalian viruses and a previously circulating human strain, either in swine or, possibly, in humans. Furthermore, seasonal and classic swine H1N1 viruses were not derived directly from BM/1918, but their precursors co-circulated during the pandemic. Mean estimates of the time of most recent common ancestor also suggest that the H2N2 and H3N2 pandemic strains may have been generated through reassortment events in unknown mammalian hosts and involved multiple avian viruses preceding pandemic recognition. The possible generation of pandemic strains through a series of reassortment events in mammals over a period of years before pandemic recognition suggests that appropriate surveillance strategies for detection of precursor viruses may abort future pandemics.
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Tags: Biology, Emerging disease, Health, Influenza, Medicine, Microbiology, pandemic, Science, Virology, virus
