In £ uenza : interspecies transmission and emergence of new pandemics

Influenza is a well-known yearly threat, so why is it considered to be an emerging disease? It is because the virus evolves so rapidly that every year new viruses emerge, and every century three to four emergent strains are ‘new’ enough to cause pandemics. Historically, pandemics have been both unpredictable and devastating. As we learn more about how and where pandemic strains evolve, strategies for blocking their emergence and for recognition of potentially dangerous strains will become possible [1]. The single most significant characteristic of the influenza virus that makes it such an important threat is its enormous variability. Several features of the virus contribute to its variability. The first is that the polymerase complex makes mistakes when replicating the viral genes, and, since there is no built-in proofreading mechanism for RNA viruses, a lot of variation accumulates. Many of these variant viruses will not survive, because the acquired changes will make them unable to grow, but many genetic mistakes are well tolerated. Any preparation of influenza virus is a mixture of these variants, one of which might emerge and become a dominant new strain. The second, and probably more important, feature is the segmented nature of the genome. There are eight genetic segments, each specifying a unique viral function. When individual cells are infected by two different viruses, new strains can evolve that have some of their eight genetic segments derived from each of the infecting ‘parents’. ‘Genetic reassortment’, as this is called, is a pivotal factor in emergence of pandemic strains. Adding to the potential for variability, occasionally there also can be true genetic recombination between segments. The third feature is the large number of strains possible, because of the large number of potential permutations of the two viral surface glycoproteins, the principal determinants of humoral immunity. …