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Avian Influenza - Virus Subtypes

What is avian influenza?

Avian influenza (AI) is a contagious viral infection caused by the influenza virus Type "A", which can affect several species of food producing birds (chickens, turkeys, quails, guinea fowl, etc.), as well as pet birds and wild birds. All subtypes of influenza A viruses are naturally hosted by wild waterfowl.

What’s the difference between low and high pathogenicity?

Avian influenza viruses can be classified into two categories: low pathogenic (LPAI) and high pathogenic (HPAI) forms based on the severity of the illness caused in domestic birds. The first causes mild illness, including ruffled feathers or reduced egg production. The second form, known as "high pathogenic avian influenza" is of greater concern. This form is extremely contagious in birds and rapidly fatal.

How many variations of the virus exist?

Influenza A viruses can be divided into subtypes based on two proteins on the surface of the virus - hemagglutinin (HA) and neuraminidase (NA). There are 16 known HA subtypes (H1-H16) and nine NA subtypes (N1-N9). Any number of combinations are possible, as each virus has one HA and one NA antigen.

Are all avian influenza viruses the same?

Most avian influenza viruses are low pathogenic and typically result in little or no clinical signs in infected birds.

Only the H5 and H7 subtypes are of concern in domestic birds. Over the past 30 years only the H5 and H7 subtypes have been known to mutate into high pathogenic forms, therefore affecting birds at a higher rate. All other subtypes (H1 through H4, H6 and H8 through H16) may cause disease and death in domestic birds. However, the illness spreads at a much slower rate than H5 and H7. Depopulation of birds infected with the H5 or H7 virus contributes significantly to reducing the amount of virus in the environment, therefore preventing the spread of the virus.

How many variations have been associated with illness in humans?

Some variations of the H5, H7 and H9 subtypes have also been associated with illness and disease in humans. Specifically H5N1 (most recently in Asia), H7N7 (previously in the Netherlands) and H9N2 (previously in Southern China and Hong Kong) have been known to cause illness in people.

For more information, visit the Public Health Agency of Canada Web site at: http://www.phac-aspc.gc.ca/influenza/avian_e.html

Which subtypes are routinely tested for?

The Canadian Food Inspection Agency has the capability to test for all sixteen subtypes of avian influenza. In the case of an epizootic (animal epidemic), the focus of testing would become the H5 and the H7 and their subtypes, which have caused disease in domestic birds in the past and which have historically been known to change from low to high pathogenic.

Is there historical data in existence to which we can compare these new survey results?

A number of scientific studies, carried out by American scientists, have been published over the last 30 years. These studies were much narrower in scope than Canada’s wildlife survey. They focussed on two migratory pathways and had only limited sampling in Canada.

The survey undertaken by the Canadian Cooperative Wildlife Health Centre is broader in scope and more comprehensive in nature thereby providing the information necessary to provide a consistent and national benchmark for Avian Influenza in wild birds.

The table below demonstrates that the findings in Quebec at 7 % and in Manitoba at 4 % are both within the literature reported range of 0 to 7.4 %.

North American Findings of Avian Influenza in Wild Birds in the Last 30 Years

The following table outlines the occurrence of Avian Influenza virus subtypes in wild birds that have tested positive for AI.

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