![]() Influenza pandemic originates in Asia, spreading to Africa, and then Europe. High mortality in pregnant women also recorded. Highly fatal, with deaths recorded as being due to "pleurisy and fatal peripneumony". The first influenza pandemic in which global involvement and westward spread from Asia to Europe documented. Attack rates are extremely high, but fatality is low and said to be restricted to young children. Starts in Africa, proceedes northward to all of Europe and the Baltic States. The highly pathogenic strain may swap segments with the low-pathogenicity strain and voila – a highly pathogenic strain that can infect people, cue a new pandemic! ![]() This also means pigs are a common mixing pot for cases of antigenic shift.Ī highly pathogenic strain that can infect birds but is unable to infect humans may infect an intermediate host, like a pig, at the same time as a low-pathogenicity strain that can infect humans. This is because pigs are the only mammalian species to have both receptor types and are therefore susceptible to and enable productive replication of avian and human influenza viruses in one animal. Pigs are a common intermediate host for influenza strains between humans, who have α2,6 receptors in their respiratory tract, and species such as birds who have α2,3 receptors. Influenza viruses are known to transmit zoonotically as some strains can infect multiple species. Influenza virus surface protein types determine the types of receptors that the virus can interact with, and consequently, the host species which can be infected. Pandemic outbreaks have been recorded through history, the more recent of which have been attributed to specific subtypes (summarized in Table 1). Consequently, pandemics are nearly always a result of antigenic shift events. However, antigenic shift generates novel lineages to which immunity is therefore often very poor across the population. ![]() Individuals may have partial immunity, from previous infection or vaccination, to infection with influenza strains that have undergone antigenic drift. What are the consequences of antigenic drift vs antigenic shift? If they have no effect or offer benefits, such as immune evasion, they are likely to persist and may even spread through the population, depending on other associated changes. If the changes reduce virus survival, they will be selected against and lost from the population. Not all genetic mutations will result in antigenic changes depending on 1) their position in the triplet code (non-coding changes) or 2) if the change they produce does not affect the region of the protein recognized by the immune system. Consequently, they will often be surface proteins, like haemagglutinin (HA) or neuraminidase (NA) in the case of the influenza virus.Īntigenic drift is a natural process whereby mutations (mistakes) occur during replication in the genes encoding antigens that produce alterations in the way they appear to the immune system (antigenic changes) (Figure 1). ![]() Antigens are molecules that are recognized by the host immune system as foreign and induce an immune response. ![]()
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