Define antigenic shift code#

amantadine and rimantadine) drug resistance in H3N2 human influenza during 2004-2006 is attributable to the expansion of an amino acid mutation (S31N) in the viral M2 matrix protein which has propagated via gene segment reassortment involving two distinct lineages of the H3N2 influenza virus ( 174). For example, the abrupt rise of adamantane (i.e. Studies have shown that human influenza viruses frequently undergo gene reassortments ( 92, 136), some of which might be advantageous to viral fitness. Reassortment, which recombines genome segments from two influenza strains (when simultaneously infecting the same host cell) into a new progeny virus, is another important molecular mechanism driving the evolution and emergence of novel influenza viruses. Such ‘antigenic drifting’ has resulted in new epidemics of influenza A H1N1 and H3N2 in human population about every 3 years, with a similar process in influenza B causing epidemics nearly every 5 years ( 66). This generates further novel antigenicity allowing the virus to continually escape from the existing host immunity developed from previous infections or vaccination ( 88). Being the major targets of the human immune response, they are known to evolve rapidly under host immunological selection pressure. The viral surface proteins (especially HA and NA) play a prominent role in viral entry and escape from the human respiratory epithelial cells within which the virus replicates. In addition, both influenza A and B do exist as several genetically distinct lineages that co-circulate in humans, with the different influenza A strains, in particular, varying sufficiently in the HA component to require different HA antigens to be present in the seasonal influenza vaccine to ensure the induction of adequate specific immunity. Of the 16 HA subtypes of influenza A virus, six have been found in a significant number of human infections (H1, H2, H3, H5, H7 and H9) however, so far only three subtypes of HA (H1, H2, H3) and two subtypes of NA (N1, N2) have caused pandemics in humans ( Figure 2).

Define antigenic shift code#

Whereas there is only one type of influenza B virus, influenza A has multiple subtypes, characterized by a combination of the 16 known hemagglutinin (HA) and 9 neuraminidase (NA) genes that code for the viral envelope or surface proteins ( 205). All subtypes of influenza A have been found in waterfowls, and their infections are usually asymptomatic Figure 1A & Figure 1B. Waterfowl are the natural reservoir of influenza A virus ( 205). Whilst influenza B and C are only found in humans, influenza A virus infects various mammals (including pigs and horses, and more rarely, other animals such as dogs) and numerous species of birds (both aquatic and terrestrial). They are lipid-enveloped, single-stranded, negative-sense, segmented RNA viruses ( Figure 1) that exist in three recognized serotypes: influenza (A, B and C).

Influenza viruses are members of the Orthomyxoviridae family.