Wild birds are the natural hosts of all influenza type A viruses, of which there are numerous subtypes distinguished by their hemagglutinin (HA – 16 types) and neuraminidase (NA – 9 types) surface proteins. Subtypes H5, H7 and H9 can infect both birds and humans, but H7 and H9 infections in humans are uncommon. Within subtypes H5 and H7 there are both low pathogenic and highly pathogenic strains. Highly pathogenic strains can cause severe illness in both domestic birds and people, and death rates are high, although many wild birds (particularly water fowl) seem to be resistant to disease even from these more virulent viruses. Both sick and seemingly healthy birds can shed the virus in their respiratory secretions and droppings, and contact with this contaminated material can spread the virus to other birds. Transmission of the currently circulating avian influenza viruses from birds to people is uncommon. Fortunately, person-to-person transmission of avian influenza is uncommon.
The big worry with influenza viruses is that they have a high propensity to "mutate" – sometimes they change slowly (i.e. antigenic drift), and sometimes they change quickly (i.e. antigenic shift). When two different influenza viruses infect the same animal or person, the different components of each virus can recombine to make a new virus that will share some characteristics of both the original viruses. For example, if a highly pathogenic avian influenza virus (like the H5N1 virus from Asia) were to recombine with a human influenza virus, it could create a virus that causes severe disease like the avian virus, but which can be readily transmitted between people. This would create the potential for a serious outbreak of severe, even fatal illness in people.
Highly pathogenic avian influenza of either the H5 of H7 subtype is reportable in many areas of the world (including Canada), due to the potential for massive mortality in domestic bird flocks, as well as the risk of severe illness in people should such a virus start to circulate in the human population. Since 2004, avian influenza has been detected in Canada four times – highly pathogenic H7 types were found in British Columbia (2004) and Saskatchewan (2007), and low pathogenic H5N2 viruses were found twice, also in British Columbia (2005, 2009). The latest outbreak in January 2009 resulted in the destruction of 60 000 birds on the BC premises where the virus was originally detected, as well as all the birds on a second premises where the virus was found just over two weeks later. This disease is taken very seriously, and the response to these outbreaks by the Canadian Food Inspection Agency (CFIA) must always be swift and definitive.
The risk of highly pathogenic avian influenza to the general public in countries such as Canada that are (usually, though not at the moment) considered free of the disease is very low. The virus cannot survive in poultry products (i.e. meat, eggs) that are properly cooked. Contact with wild birds (dead or alive), particularly migratory birds that could potentially be from other continents, is likely the highest risk. Pet birds are also susceptible to these viruses, so its important to prevent them from coming in conatct with wild birds as well, especially because owners have such close contact with their feathered friends.
More information on influenza in animals and people is available in our archives.