Thread: Sialic Acid Evolution

Apparently pigs have both 'human' and 'bird' sialic acid, hence swine flu. But how did pigs get the bird kind and we didn't? After all we're more closely related to pigs. Do we have a non-functional 'bird' sialic acid? Or are bird and pig sialic acid different but can be infected by the same virus?

Can you provide the source where you read this Golkarian? I don't doubt you but I'd like to read more details. It's not something I've heard of before.

Actually it was based on custom notes for my microbiology course. Maybe I'll look for another source as well.

Influenza has been shown to infect all sorts of animals, from dogs to seals. Horse flu, H7N7, has been known to infect humans during outbreaks.

As I understand it the flu strains endemic in pigs happen to have HA that can bind the common mammalian sialic acid residues as well as the common avian sialic acid reservoirs. It's not a matter of structural similarity of the pig and avian sialic acid, but rather of the broad range of receptors available to those strains of influenza combined with the prime conditions of farms for inter-species infection.

Edit: Also, the HA gets a lot of focus because it is definitely necessary for infection and is used for typing influenza strains, but many other influenza genes are likely involved in achieving infection in other species.

Originally Posted by Golkarian

Apparently pigs have both 'human' and 'bird' sialic acid, hence swine flu. But how did pigs get the bird kind and we didn't? After all we're more closely related to pigs. Do we have a non-functional 'bird' sialic acid? Or are bird and pig sialic acid different but can be infected by the same virus?

I think that you may be confused about the details here. There is no such thing as 'human' or bird 'sialic acid'. Sialic acid is our name for a precisely defined monosaccharide structure.

I can take a guess at what the correct answer to your problem is: the influenza virus infects cells in the lungs of infected people. It recognises the correct cells by the specific sugar structures which are displayed at the cell surface. The precise structure of these complex structures varies between species.

The avian flu virus has evolved to recognise a particular pattern of sialic acid residues on a sugar structure present upon cells from birds. This is sufficiently different from the structure on human cells that the virus is mostly harmless to us (it cannot recognise the cells in our lungs so it cannot infect us). However, the pig sugar structure by coincidence shares some similarities with both the human and avian sugar structures. This is unfortunate for us because it means that pigs can become infected with both human and avian flu viruses. In this manner the two viruses can swap genes and we can get 'avian' flu viruses emerging which can recognise and infect human cells.