Chapter 15 – Influenza Viruses

Influenza virus (family Orthomyxoviridae) is an enveloped and negative-sense RNA virus. It possesses multiple RNA genomes (ie, eight RNA segments). Unlike other RNA viruses, viral RNA replication occurs in the nucleus. Viral mRNA transcription is primed by capped RNA snatched from cellular mRNAs, a process termed “cap-snatching.” Diverse subtypes of influenza virus are specified by the antigenicity of two envelope proteins, HA and NA (eg, H5N1). Rapid emergence of genetic variants is the salient feature of influenza virus. In addition to incremental changes in antigenicity (antigenic drift), abrupt changes (antigenic shift), which are the result of genetic recombination among eight RNA segments (genetic reassortment), occasionally occur and cause pandemic flu. A nonstructural protein NS1, the major virulence factor, potently blocks the host innate immune response. Yearly vaccination is required for effective protection from seasonal flu.