While first reports of avian influenza (fowl plague) date back more than a century, the viruses responsible for this disease were first identified in 1955 as Type A influenza viruses. Avian influenza viruses can be classified into 2 categories: highly pathogenic avian influenza (HPAI) and low pathogenic avian influenza (LPAI), based on the severity of the illness caused in poultry. HPAI can spread rapidly, causing severe disease and high death rates whereas LPAI causes mild to no disease and few symptoms. Low pathogenicity forms of the virus are found routinely in wild waterfowl populations, and these birds are the reservoir of all avian influenza viruses.
To date, all HPAI outbreaks have been attributed to the H5 and H7 subtypes of the avian influenza virus. In a poultry population, these subtypes tend to mutate from low pathogenic forms to highly pathogenic forms as the viruses adapt to their host. Highly pathogenic H5 or H7 in poultry is very contagious, and a quick response is necessary to contain and prevent further spread.
Disease outbreaks involving HPAI viruses have been periodically reported in the Americas, Europe, Asia, Australia and Africa, and their distribution reflects domestic poultry production, migratory routes, time of year and the disease surveillance and reporting systems present.
Effect on Human Health
In general, human illness due to infection with avian influenza viruses is rare with minor clinical signs; however, in 1997 a highly pathogenic H5N1 subtype known as the Asian strain was transmitted from poultry to humans who were in close contact with sick or dead birds. Few people got sick, but of those that did, many died. The mortality rate was approximately 60%. Since then, the H5N1 Asian strain has continued to cause outbreaks of disease in wild birds and poultry flocks throughout Asia, and greater efforts have been made to control the disease in animals and prevent transmission to humans. Although the H5N1 Asian strain has never been detected in North America, by the spring of 2006, it had spread to Europe, the Middle East and Africa, possibly through the trade of infected birds or their products and the movements of migratory birds. All H5 and H7 subtypes must be reported to the World Organisation for Animal Health (OIE).
Avian Flu in Canada
Avian influenza is a reportable disease under the Health of Animals Act and Regulations in Canada, meaning that all suspected cases must be reported to the Canadian Food Inspection Agency.
Canada's first case of HPAI (H7N3) occurred in March 2004 in a commercial chicken breeder farm in British Columbia. The farm was immediately quarantined and the flock was humanely destroyed. Within weeks, birds on 42 commercial and 11 backyard premises were found to be infected with HPAI. The farm-to-farm movement of people, equipment or birds was likely the primary method for virus spread, but once the virus was introduced into a densely populated region, short-distance airborne transmission from barn to barn through dust contaminated with the virus may have infected nearby flocks.
To contain disease spread, the Government of Canada follows international disease control guidelines. Accordingly in 2004, the government initiated a depopulation program that focused on rapidly isolating, containing and eliminating detected cases of avian influenza. Due to the highly infectious nature of the virus, susceptible birds on farms in close proximity to infected premises were also destroyed.
All infected and exposed birds were humanely destroyed, primarily using carbon dioxide gas. Disposal methods included incineration, burial or composting. The outbreak ended in June 2004 and all premises on which HPAI had been detected were thoroughly cleaned and disinfected. Poultry farms were then allowed to begin restocking.
In September 2007, HPAI was reported in Saskatchewan when an H7N3 subtype was detected in a commercial operation. Fortunately in this case, the disease was contained to one farm and only the birds on that farm had to be destroyed.
Canada's preparedness for future avian influenza outbreaks includes work with industry to enhance biosecurity on Canadian poultry farms and surveillance of the wild bird population in Canada. The annual wild bird survey is designed to monitor for the presence of all AI viruses, but particularly for the H5N1 Asian virus spreading throughout the world.
See also Pandemic.