Public Health and Communicable Diseases - SA Health - SA.Gov.au

viruses are RNA viruses, and like all RNA viruses lack‘proofreading’ when they are replicating, making themgenetically unstable and variable. While each roundof RNA virus replication results in a mixed populationwith many variants, most of which are not viable, somehave potentially advantageous mutations that canbecome dominant under the right selective conditions.Reassortment of the viral genome occurs rapidly amonginfluenza viruses in nature, and is important in theappearance of pandemics in human populations.To date just over 100 people have died worldwidefrom the current H5N1 ‘bird flu’ infections which havegeographically encompassed countries housing morethan half the world’s population. Concern remainsin the medical and scientific community that H5N1may develop into the next pandemic influenza strain.Although most attention is focussed on the H5N1 virus,it is entirely possible that the next pandemic might becaused by a different influenza virus.Current human influenza viruses are believed to havearisen by genetic reassortment between previoushuman influenza viruses and nonhuman viruses, whenthere has been simultaneous infection of a host animalwith both avian and human influenza viruses. Thoughthere is no scientific proof to support it, the pig hasbeen the leading contender for the role of ‘mixingvessel’. The pig is the only mammalian species whichis domesticated, reared in abundance, and a commonhost for human influenza viruses, though mixing mayoccur in humans who are co-infected with two strains 4 .One remarkable exception to this mode of selectionof new strains was the 1918 pandemic strain, whichwas revealed by molecular sequencing in 2005 to beof purely avian origin 5 . The 1918 strain jumped species,from birds to humans, and there is concern that theH5N1 strain might do the same, or might recombinewith a human strain, either way producing a newpandemic strain. In recent years, purely avian influenzaviruses of subtypes H5, H7 and H9 have crossed thespecies barrier to directly cause disease, sometimesfatal, in humans in many Asian countries, as well asTurkey, Holland and Canada 6 .The majority of pandemic strains of human influenzafrom the 20 th century appear to have originated in China,with its large population of humans, pigs, and domesticand wild ducks. In temperate countries, influenza in pigsand humans tends to occur in winter, when free-flyingaquatic birds are absent. In tropical and subtropicalcountries such as Southern China, influenza occurs yearround in humans, pigs, and birds. There is interminglingof these species, and the opportunity for interspeciestransmission and genetic exchange among influenzaviruses 4 .Even though pandemic strains emerge only rarely,available information indicates that interspeciestransmission of influenza viruses may not be so rare,for up to 10% of persons with occupational exposureto pigs develop antibodies to swine influenza virus 7 .It remains unclear what viral and human geneticcharacteristics allow the transmission of avian influenzaviruses to humans. So far the H5N1 strain isolates fromhumans have not acquired gene segments from humaninfluenza virus strains.(Source: http://europa.eu.int/comm/health/ph_threats/com/Influenza/influenza_en.htm)