Avian influenza, or ‘bird flu’, is a disease caused by an avian influenza A virus that normally infects only birds. In fact, all subtypes of the influenza A virus have their origin in wild birds, which means that influenza is a disease which cannot be eradicated. Waterfowl, such as ducks and geese, are natural reservoirs and many of such wild birds carry low pathogenic avian influenza (LPAI) viruses in their gastrointestinal tract. These birds develop no or minor clinical signs upon infection with these viruses. In domestic poultry, LPAI viruses of the H5 and H7 subtypes may develop into highly pathogenic avian influenza (HPAI) viruses by mutation of their H proteins. Such HPAI viruses cause massive disease outbreaks in domestic poultry with mortality rates of up to 100%. Avian influenza viruses are viruses of birds that may from time to time cross species barriers to infect mammals, including humans.
Low and highly pathogenic avian influenza
Pathogenicity is a measure of the extent to which a virus causes disease. Avian influenza is caused by either LPAI or HPAI viruses. LPAI viruses cause very mild disease signs, such as ruffled feathers and reduced egg production, or no signs at all. The highly pathogenic forms are far more dramatic, spreading very rapidly through poultry flocks, with mortality that can approach 100%, often within 2-3 days. Only viruses of the H5 and H7 subtypes may mutate from LPAI into HPAI viruses. LPAI viruses are introduced into poultry flocks through contacts with wild birds. In poultry, LPAI viruses can develop mutations in their HA surface glycoprotein, by which they become HPAI viruses that cause huge outbreaks of fatal disease in domestic poultry.