Saturday, January 14, 2006

More on the sequences from Turkey

The official WHO case count from Turkey has now risen to 18, matching the largest outbreak (Hong Kong 1997) to date for H5N1. A major question is whether there is something different about the Turkish virus.

The news dribbling out about the sequencing of the Turkish isolates is not encouraging but also not surprising. As I noted several days ago, the proposition that the hemagglutinin protein of the isolates is "very close" to the avian sequences is not very informative because extremely small changes can cause important changes in host range, as studies by Stevens et al. on the 1918 HA show. That paper described studies with glycan arrays (see previous post) that looked at the binding of various viral HAs to various linkages of sialic acid, the cellular receptor. Sialic acid is linked in two forms, one characteristic of bird intestinal cells, one characteristic of human lower respiratory tract cells, although we now know that humans have avian-type linkages in sialic acid in their upper respiratory tract. Most avian viruses bind well to the avian receptor, human viruses to the human linked receptor, but the HA protein from a case from New York's second wave in the 1918 pandemic showed some affinity for both humans and birds.

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