28 Influenza 2006

Figure 6. An unimpeded sneeze sends two to five thousand bacte- ria-filled droplets into the air. Image copyright by Prof. Andrew Davidhazy, Rochester Institute of Technology. Used with permission. (http://www.rit.edu/~andpph)

As influenza viruses are normally highly species specific, they only rarely spill over to cause infection in other species. This is due to differences in the use of cellular receptors. Avian influenza viruses bind to cell-surface glycoproteins containing sialyl-galactosyl residues linked by a 2-3-linkage, whereas human viruses bind to receptors that contain terminal 2-6-linked sialyl-galactosyl moieties. For an avian virus to be easily transmitted between humans, it is fundamental that it acquires the ability to bind cells that display the 2-6 receptors so that it can enter the cell and replicate in them. While single amino acid substitutions can significantly alter receptor specificity of avian H5N1 viruses (Gambaryan 2006), it is presently unknown which specific mutations are needed to make the H5N1 virus easily and sustainably transmissible among humans, but potential routes whereby H5N1 might mutate and acquire human specificity do exist (Stevens 2006).

Since 1959, human infections with avian influenza viruses have only rarely occurred. Of the hundreds of strains of avian influenza A viruses, only four are known to have caused human infection: H5N1, H7N3, H7N7, and H9N2 (WHO 200601). Apart from H5N1, human infection generally resulted in mild symptoms and rarely in severe illness (Du Ry van Beest Holle 2003, Koopmans 2004). For the H5N1 virus, close contact with dead or sick birds (i.e., slaughtering, plucking, butchering and preparation) or exposure to chicken faeces on playgrounds seem to be the principal source of human infection (WHO 200601).

H5N1: Making Progress

At the moment, H5N1 infection in humans is relatively rare, although there must have been widespread exposure to the virus through infected poultry. This in an indicator that the species barrier to the acquisition of this avian virus is still quite high for H5N1 – despite having been in circulation for nearly 10 years. However, over the past years, H5N1 strains seem to have become more pathogenic and to have expanded their range of action: