Classification (Compact)
Influenza B virus (IBV) was identified for the first time during an outbreak of an acute respiratory disease in 1940 in Northern America, in which it was found to lack antigenic cross-reactivity to influenza A virus (IAV). IBV is classified as the only member of the genus influenza virus B within the order of the Orthomyxoviridae. IBV was estimated to have diverged from IAV around 4000 years ago based upon the calculated rate of amino acid substitutions within HA proteins, but the mechanisms of replication and transcription, as well as functionality of most viral proteins appear to be largely conserved, with some unique differences. IBV has been detected in rare occasions in seals and pigs, but humans are the major host species of IBV, in which it causes typically the same spectrum of symptoms of influenza-like illness as IAV although pathologies outside the respiratory tract have been determined in a minority of IBV infections. A large recent metagenomic study detected an IBV-like virus in a fish species suggesting that the genus may have additional vertebrate host reservoirs outside mammalian species. Sentinel surveillance detects IBV in some seasons as the dominating circulating influenza virus type highlighting its significant socio-economic impact. There are currently two lineages of IBV (B/Yamagata-like and B/Victoria-like viruses) with limited antigenic cross-reactivity. Viruses of both lineages circulate to varying extents and ratios in all parts of the world. The lack of a large animal host reservoir able to foster the appearance of pandemic variants, a lower evolutionary rate and generally considered milder courses of disease in comparison to IAV infection are typical features of IBV.
Influenza C virus (ICV) was first isolated in 1947 from a human patient having mild respiratory symptoms. Since the virus showed no cross-reactivity with antisera against influenza A or B viruses, it was classified as a new genus of the Orthomyxoviridae (influenza C virus). The virus has a worldwide distribution and the majority of humans develop antibodies against ICV early in life. Humans are the main reservoir of ICV, but occasionally the virus may also infect dogs and pigs.
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In 2011 a new Influenza C-like virus was isolated from clinically ill pigs. Since this isolate is unable to reassort with ICV it is now officially named as influenza D virus (IDV). Subsequent studies showed that IDV can also infect small ruminants and cattle, the latter of which is apparently its main reservoir.
The first part of this article reviews current knowledge on type B influenza virus, whereas the second part addresses important aspects of type C and D viruses.
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