Influenza B viruses are subdivided as Yamagada or Victoria based on genetic lineage ( Xu et al., 2004). At least 16 distinct HA and 9 distinct NA surface proteins have been identified ( Treanor, 2010). This subtyping is the basis of the H#N# designations of the influenza A viruses. Influenza A viruses are further divided into subtypes based on the characteristics of the hemagglutinin (H or HA) and neuraminidase (N or NA) surface proteins ( Treanor, 2010). Influenza viruses are named based on the type of influenza, the location of initial isolation, strain designation number, and the year of isolation (e.g., A/Brisbane/59/2007). ![]() The impact of these changes depends on the extent of the change, but because viral epitopes from the variant strains that result from antigenic shifts and drifts may not be recognized by the immune system, vaccines must be altered regularly to combat the infection. Antigenic shift is usually caused by genetic recombination between two strains of influenza virus one from a strain that can infect humans and one that, prior to the genetic exchange, could not ( Han and Marasco, 2011). This change, known as “antigenic shift,” results in a new virus that is so different from previous viruses that no immunity exists in the population and can lead to pandemic influenza ( Treanor, 2010). Occasionally, influenza viruses undergo an abrupt and dramatic change in genome. Small antigenic changes, known as “antigenic drift,” occur regularly, usually as point mutations in the virus genome or through exchange of small gene segments with another strain of influenza virus ( Han and Marasco, 2011 Treanor, 2010). Influenza viruses are highly changeable viruses. ![]() Divided into three types-A, B, and C-these viruses can infect a range of hosts from humans only (influenza B) to humans and swine (influenza C) to multiple host organisms including humans, swine, equine, avian, and marine mammals (influenza A) ( Treanor, 2010). Influenza viruses are 80–120 nm enveloped viruses of the family Orthomyxoviridae ( Treanor, 2010).
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