Over the last four years, a variety of different names have been used in publications to refer to emerging lineages of the highly pathogenic H5N1 avian influenza viruses. As a result, discussion and comparison of the various lineages remains difficult.

The H5N1 viruses have now appeared in at least 59 countries across three continents with continuing capacity to infect humans. The viruses appear to be evolving and diversifying and while most genes have undergone reassortment yielding many different genotypes, the haemagglutinin (HA) protein, remarkably, has not been replaced in the various isolates since 1996. Following the evolution of the HA protein provides an initial constant by which the strains may be effectively compared. It was proposed therefore to develop a clade nomenclature system based upon the evolution of the HA.

This would enable:

• a unified system to be developed to facilitate the interpretation of sequence/surveillance data from different labs;
• the stigmatizing labelling of clades by geographical reference to be replaced;
• the phylogenetic tree to be expanded in the future; and
• a starting point to be established to develop a more extensive system in the near future that is based upon antigenic variation and reassortment into multiple genotypes.

An international core group of eight scientists and their collaborators were convened to initiate this process with the encouragement and approval of three international agencies: the World Health Organization (WHO), the World Organisation for Animal Health (OIE) and the Food and Agriculture Organization (FAO). Phylogenetic analysis was performed using a variety of approaches on all of the publicly available H5 HA sequences that have evolved from the A/Goose/Guangdong/96 H5N1 isolate. The initial results support the idea that the currently circulating highly pathogenic avian influenza H5N1 viruses could be effectively grouped into numerous clades logically designated by a hierarchical numbering system. For example the so-called ´Fujian-like lineage´ within the antigenically distinct Clade 2.3 of H5N1 would be designated Clade 2.3.4, with other distinct clade 2.3 branches called 2.3.1 and 2.3.2 and so on, while the ´Qinghai lineage´ would be designated Clade 2.2.

If such a system were to become universally accepted, it might serve as a model for other influenza lineages and genes. Some of the data summarized by the Working Group were presented for evaluation at the 6th meeting on Options for the Control of Influenza held in Toronto, Canada from 17 to 23 June 2007 and it was decided that a summary of the publicly available H5N1 sequence data would be published on international websites and would be updated on an on-going basis.

It is therefore recommended that the information and clade designations posted below be used by researchers when referring to the currently circulating highly pathogenic H5N1 avian influenza viruses.

For current information and phylogenetic trees

WHO/OIE/FAO H5N1 Evolution Working Group poster, Toronto, 2007 [ppt 483kb]
Note: To easily view this poster, save it to your hard drive and then open the file in PowerPoint. You can then use the ´zoom´ function to view text and trees in detail.

Phylogenetic tree from Evolution Working Group poster [pdf 54kb]
zoomable PDF

For more information

Email: whoinfluenza@who.int