Orthomyxovirus Genome Transcription and Replication

Viruses from the Orthomyxoviridae family infect a wide range of vertebrate hosts. Five genera are currently recognised: Influenza A, B and C, Thogotovirus (of which the homonymic viruses are the type species) and Isavirus (type species infectious salmon anaemia virus). Other so far unclassified and uncharacterised members of the family also exist (Da Silva et al., 2005). These viruses are typified by possessing a single-stranded, negative-sense RNA genome that is split into segments (between 6 and 8 depending on the genus), by replicating their genome in the nucleus and by an unusual mechanism for producing capped mRNAs (discussed in detail later); shared traits that make it likely they are descended from a common ancestor. Influenza A virus is by far the most important in terms of significance to human health and accordingly is the best studied. The World Health Organization estimates that between 5 and 15% of the population in temperate countries are infected each year, resulting in up to 500,000 deaths worldwide. In addition, periodic introduction of antigenically novel viruses from avian reservoirs into the human population causes worldwide pandemics with substantially higher attack rates and mortality levels. The current possibility that a highly pathogenic avian H5N1 strain of influenza A will make this species jump is of great concern (Peiris et al., 2007).