The evolution of Influenza A(H5N1)

In the wide, wild world of nature, several Influenza viruses are known as ‘bird flu’s’. That simply means that wild birds and water fowl are the natural hosts for these viruses and these birds exhibit, while infected, no or only mild symptoms. Usually these infections are designated as Low Pathogenic Avian Influenza or LPAI.

But, as always, the Influenza viruses are slowly mutating and that can lead to a strain that can become High Pathogenic Avian Influenza or HPAI. Examples of these potentially lethal viruses are H9N2 (which infected two children in Hong Kong in 1999 and another in 2003), H7N7 (which infected 89 people – one fatality - in the Netherlands in 2003) H10N7 (which infected two infants in Egypt in 2004) and H10N8 (which infected two people – one fatality - in China in 2013/14).

The Influenza A(H5N1) virus was first identified in 1996 and has a staggering Case Fatality Rate of around 50%. That means that half of all patients have succumbed to the virus. Because of its success in infecting humans, you might think that the virus doesn’t have the urge to mutate any more, but nothing can be further from the truth, because it continous to mutate relentlessly.

Since 1996, the Influenza A(H5N1) virus has expanded into more than 20 different clades and subclades and various versions of the virus now circulate in different parts of the world[1]. This means the world is not at risk of just one Influenza A(H5N1) strain which has pandemic potential, but we are anxiously watching and waiting to see if one of at twenty genetically separate clades of the virus, with many minor variants of each clade, could turn out to be the next deadly pandemic.

And over time, it is expected that even more clades will emerge as the virus mutates and/or swaps genetic material with other viruses.

[1] Gutiérrez et al: A(H5N1) Virus Evolution in South East Asia in Viruses - 2009