Avian influenza (AI), often called "bird flu," is caused by Type A influenza viruses. Influenza viruses, including avian viruses, are classified into subtypes based on the two proteins on their surface, haemagglutinin (H) and neuraminidase (N).
These viruses occur naturally among wild aquatic birds worldwide, and can affect domestic poultry, including food-producing birds (chickens, turkeys, etc.), as well as pet and wild birds. Avian influenza is rare in humans and generally does not spread easily between people; however, limited clusters of person-to-person transmission, generally between family members, has been observed, notably for the Asian A(H5N1) and A(H7N9) subtypes.
Avian influenza viruses can be broadly classified into two types, based on the severity of the illness caused in chickens in a laboratory setting:
- Low pathogenic avian influenza (LPAI) Most avian influenza viruses are low pathogenic. These cause little or no signs of illness in infected poultry.
- Highly pathogenic avian influenza (HPAI) viruses can cause severe illness and death in poultry flocks.
However, the severity of the illness in chickens (i.e. whether the avian influenza virus is considered LPAI or HPAI) does not predict severity in humans. For instance, A(H7N9) viruses are LPAI but cause severe illness and even death in a large proportion of humans that are infected.
Wild aquatic birds, including gulls, terns, ducks, geese, and swans, are the natural reservoir (hosts) of virtually all influenza A subtypes. These viruses rarely cause deaths in the wild and are typically LPAI; however some strains can spread to domestic poultry where they can change from LPAI to HPAI.
Poultry flocks in Canada are usually free of both LPAI and HPAI viruses. However, sometimes domesticated birds become infected with avian influenza viruses through direct contact with infected waterfowl or other infected poultry, or through contact with surfaces that have been contaminated with the viruses. Both LPAI and HPAI viruses can spread rapidly and cause outbreaks in poultry. Avian influenza outbreaks have occurred in poultry in British Columbia, including the extensive outbreak of HPAI H7N3 between February and May 2004 and more recently the H5N2 outbreak in December 2014 and the H5N1 outbreak in February 2015.
Avian influenza outbreaks in poultry are of concern for several reasons:
- the potential for low pathogenic viruses to evolve into highly pathogenic viruses
- the potential for rapid spread and significant illness and death among poultry during outbreaks of highly pathogenic avian influenza
- the economic impact and trade restrictions from an avian influenza outbreak
- the possibility that novel avian influenza A viruses with pandemic potential could be transmitted to humans
During an outbreak of avian influenza in poultry, the risk to the general public is very low. Most avian influenza viruses cannot spread easily from birds to people, or from person-to-person. However, any new influenza virus in the human population is a concern because of its potential to change and adapt for more easy transmission between people. Public health officials work closely with animal health officials to ensure that poultry producers and workers who may have to come into close contact with infected birds are protected.
Avian influenza viruses usually do not infect humans and cannot spread easily from person-to-person. Rare cases have been reported, most often in people who had close unprotected contact with infected poultry or environments heavily contaminated with the virus.
The reported symptoms of people infected with avian influenza have ranged from conjunctivitis (i.e. red eyes with discharge) to influenza-like illness (i.e. fever, sore throat, muscle aches) to severe respiratory illness. Whether a virus has been characterized as highly pathogenic (HPAI) or low pathogenic (LPAI) in birds does not predict the effect it may have on people. LPAI H7N9 and HPAI Asian H5N1 have been responsible for most human illness worldwide to date, including the most serious illnesses and deaths. To date (September 2018), two human cases of H7N9 have been identified in Canada (in BC), both travel-related cases fully-recovered. Only one human case of HPAI Asian H5N1 has been identified in Canada (in Alberta), a fatal case that was also travel-related.
Human infections with avian influenza rarely spread to other people. There have been just a few instances of this reported among close contacts such as household members of someone who was ill with avian influenza. However, avian influenza is closely monitored in both birds and people due the potential for the virus to change and gain the ability to spread easily between people.
In Canada, all birds from farms affected by avian influenza outbreaks are humanely culled and disposed of and do not enter the market place. Therefore, retail poultry and poultry products (like eggs) remain safe for human consumption, but they should always be handled and cooked properly because of the risks from known food-borne illnesses, like Salmonella.