Avian influenza (AI) is an infectious disease of birds caused by type A strains of the influenza virus. The disease occurs worldwide.

All species of birds are believed to be susceptible to avian influenza, though some species are more resistant to infections than others.

AI infections can cause a wide spectrum of symptoms in birds, ranging from mild illness to a highly contagious and rapidly fatal disease resulting in severe epidemics.

severe cyanosis (right bird)

The latter is known as “highly pathogenic avian influenza”. This form is characterised by sudden onset, severe illness, and rapid death, with a mortality that can approach 100%.

16 subtypes of H antigen in combination with 9 subtypes of N antigen of influenza virus are known to infect birds, thus providing an extensive reservoir of influenza viruses potentially circulating in bird populations. To date, all outbreaks of the highly pathogenic form have been caused by influenza viruses of subtypes H5 and H7. Viruses of low pathogenicity can, after circulation for sometimes short periods in a poultry population, mutate into highly pathogenic viruses.

An additional hazard concerning AI is the potential spread to humans. Vaccines can induce immunity against AI, preventing or minimising disease and replication of the virus, and additionally reducing or preventing the further spreading of the disease. The level of protection by vaccines depends on the potency of the vaccines used.