Background: Pigs are considered susceptible to influenza A virus infections from different host origins because
earlier studies have shown that they have receptors for both avian (sialic acid-alpha-2,3-terminal saccharides (SAalpha-
2,3)) and swine/human (SA-alpha-2,6) influenza viruses in the upper respiratory tract. Furthermore,
experimental and natural infections in pigs have been reported with influenza A virus from avian and human
Methods: This study investigated the receptor distribution in the entire respiratory tract of pigs using specific
lectins Maackia Amurensis (MAA) I, and II, and Sambucus Nigra (SNA). Furthermore, the predilection sites of swine
influenza virus (SIV) subtypes H1N1 and H1N2 as well as avian influenza virus (AIV) subtype H4N6 were investigated
in the respiratory tract of experimentally infected pigs using immunohistochemical methods.
Results: SIV antigen was widely distributed in bronchi, but was also present in epithelial cells of the nose, trachea,
bronchioles, and alveolar type I and II epithelial cells in severely affected animals. AIV was found in the lower
respiratory tract, especially in alveolar type II epithelial cells and occasionally in bronchiolar epithelial cells. SA-alpha-
2,6 was the predominant receptor in all areas of the respiratory tract with an average of 80-100% lining at the
epithelial cells. On the contrary, the SA-alpha-2,3 was not present (0%) at epithelial cells of nose, trachea, and most
bronchi, but was found in small amounts in bronchioles, and in alveoli reaching an average of 20-40% at the
epithelial cells. Interestingly, the receptor expression of both SA-alpha-2,3 and 2,6 was markedly diminished in
influenza infected areas compared to non-infected areas.
Conclusions: A difference in predilection sites between SIV and AIV virus was found, and this difference was in
accordance with the distribution of the SA-alpha-2,6 and SA-alpha-2,3 receptor, respectively. The results indicated
that the distribution of influenza A virus receptors in pigs are similar to that of humans and therefore challenge
the theory that the pig acts as a mixing vessel between human and avian influenza viruses. Furthermore, it was
shown that AIV prefers to infect alveolar type II epithelial cells in pigs. This corresponds with findings in humans
emphasising the resemblance between the two species.
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