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Subtypes

The main subtypes isolated so far in pigs in Germany and other European countries are H1N1, H1N2 and H3N2. However, for the prevalence within a subtype, a distinction must also be drawn between the different strains.

Susceptibility to the different haemagglutinins and neuraminidases varies among species. Poultry may act as host for almost all subtypes and can also transmit the virus to further susceptible species. Until today, subtypes H1 to H3, H5 to H7, H9 and H10 as well as N1, N2 and N6 to N9 are known to cause infection in humans. Influenza transmissions from animals to humans have been described but occur seldom.

Pigs are susceptible to subtypes N1 and N2 as well as H1 to H5 and H9. When a pig is simultaneously infected with a porcine and humane subtype reassortment might occur leading to a potential new more virulent subtype.

The table below shows the known susceptibility of different species to HA and NA subtypes:

Hemagglutinin

Subtype

Humans

Poultry

Pigs

Bats/Others

H1 X X X
H2 X X X
H3 X X X Other animals
H4 X X Other animals
H5 X X X
H6 X X
H7 X X Other animals
H8 X
H9 X X X
H10 X X
H11 X
H12 X
H13 X
H14 X
H15 X
H16 X
H17 Bats
H18 Bats

Table: List of HA subtypes that occur in humans, pigs, poultry and other animals (modified after CDC)

Neuraminidase

Subtype

Humans

Poultry

Pigs

Bats/Others

N1 X X X
N2 X X X
N3 X
N4 X
N5 X
N6 X X
N7 X X Other animals
N8 X X Other animals
N9 X X
N10 Bats
N11 Bats

Table: List of NA subtypes that occur in humans, pigs, poultry and other animals (modified after CDC)

The replication cycle of influenza A viruses starts with adsorption of the virus particle (virion) on to the host cell. This is initiated by the viral haemagglutinin (HA) on the cell membrane, which binds to specific receptors of the host cell. These receptors differ both from host to host and from organ to organ, which influences the host specificity and tropism of influenza A viruses.

Following receptor-mediated endocytosis, the virion enters the host cell. Viral nucleocapsid are released into the cytoplasm and are then transported into the cell nucleus. Here, replication of the virus genome takes place. The resulting complexes are then transported to the cell membrane, through which the now-mature virions leave the cell aidedby neuraminidase.

Replication influenza A virus

Figure: Replication of influenza A viruses (schematic)