What is the Bird Flu (Avian Influenza)?

Bird Flu (also known as avian influenza or avian flu) is a type of influenza virus that is hosted by birds, but may infect several species of mammals. It was first identified in Italy in the early 1900s and is now known to exist worldwide. Avian influenza viruses compose the Influenzavirus A genus of the Orthomyxoviridae family and are negative-stranded, segmented RNA viruses. A strain of the H5N1-type of avian influenza that emerged in 1997 has been identified as the most likely source of a future influenza pandemic.

Subtypes pathogenic to humans

All avian influenza (AI) viruses are type A influenza in the virus family of Orthomyxoviridae and all known subtypes of influenza A virus can infect birds. Influenza type A is subdivided into subtypes based on hemagglutinin (H) and neuraminidase (N) protein spikes from the central virus core. There are 16 H types, each with up to 9 N subtypes, yielding a potential for 144 different H and N combinations. In addition, all AI viruses fall into one of 2 pathotypes: low (LPAI) and high (HPAI) pathogenicity, based on their virulence in poultry populations.

It is feared that if the avian influenza virus undergoes antigenic shift to the point where it can cross the species barrier (e.g., from birds to humans), the new subtype created could be both highly contagious and highly lethal in humans. Such a subtype could cause a global pandemic similar to the Spanish Flu that killed up to 50 million people in 1918.

However, there are substantial genetic differences between the subtypes that typically infect both people and birds. Within subtypes of avian influenza viruses there also are different strains. Avian influenza H5 and H7 viruses can be found in both "low pathogenic” or “high pathogenic” forms depending on genetic features and the severity of the illness they cause in poultry; influenza H9 virus has been identified only in a “low pathogenic” form.


H1N1 was the first identified strain of Type A influenza. In early October 2005, researchers announced that they had successfully reconstructed the Spanish Flu virus. The gene sequence indicates that the 1918 epidemic was caused by H1N1, normally considered a strain of swine flu (which itself falls under avian influenza) that in this case passed directly from birds to humans. The reconstructed virus is very different from normal human viruses in that it infects lung cells which would normally be impervious to the virus.


H5N1 is a highly pathogenic form of avian influenza. Since 1997, outbreaks of H5N1 have caused the death or culling of tens