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Avian Influenza: Virus Backgrounder (Wild Birds)

Avian influenza (AI) is a contagious viral infection that primarily affects birds and may infect other species.

Wild waterfowl are a natural reservoir of a family of viruses called "Influenza As". These viruses are not usually harmful to wild waterfowl. The highly pathogenic, Asian strain of the of the H5N1 virus is an exception - it is known to kill both wild birds and commercial poultry.

All viruses are not equal. While all part of the same family, several Influenza A viruses can exist at the same time in various mixtures in wild birds and combine in a number of ways. For example, not all H5N1 viruses are equal - different strains of H5N1 can have very different characteristics.

In order to make an assessment about the risk or relative danger that a particular virus may pose, it is important to consider three distinguishing characteristics of the virus: the H- and N-type; the pathogenicity (high vs low); and of critical importance, the strain or genetic characteristics, often based on geographic origin (ex. Asian, North American, European).

1. "H" and "N" Types

The first step in identifying a virus is to characterize its H-type and N-type. The viruses are named according to protein projections on the surface of the virus called hemagglutinin (H) and neuraminidase (N), and the combination of the two provides the basic nomenclature of the virus type (ex. H5N1 or H7N3). There are 16 different H’s and 9 different N’s, meaning that there are 144 possible combinations.

Identifying the "H"-type of the virus is of critical importance, as it provides an indication of the virulence or pathogenicity of the virus. The CFIA and international veterinary community pay particular attention to the H5 and H7 sub-types of the virus, given the ability of these two H-types to mutate from low pathogenic to highly pathogenic after they infect domestic birds. It is these two H-types that have been known to cause serious disease or mortality in domestic poultry, yet low pathogenic H5 and H7 viruses are quite common in waterfowl.

Identifying the "N"-type of the virus is an internationally-accepted practice and gives us specificity in identifying the virus and allows the tracking of the viruses in a more precise manner. It is analogous to having the first and last name of an individual to be certain of his/her identity.

2. Virus Pathogenicity

The virulence or pathogenicity of the virus is its ability to cause disease or mortality in either wild birds or domestic poultry. The world community recognizes that avian influenza viruses in birds can be classified into two categories:

1. Low pathogenic - typically cause little or no clinical signs in birds
2. Highly pathogenic - causes severe clinical signs and/or high mortality in birds Generally, Influenza A in wild waterfowl is of low pathogenicity and birds remain healthy. The exception to this is the highly pathogenic H5N1 Asian subtype, which has been proven to be deadly to both commercial and wild birds.

3. Strain Type

Of critical importance is the strain type or "fingerprint" of the virus. Different strains of the same type of virus (ex. different strains of H5N1) can exist, particularly in different parts of the world. Such strains can have very different characteristics and structure. For example, Italy recently reported a European strain of H5N1, which is low pathogenic and distinctly different from the Asian strain, which is highly pathogenic. This explains why one H5N1 virus can be present in healthy birds with no clinical signs, yet another H5N1 will be highly pathogenic and cause bird mortality.

Summary of Canadian Findings of H5 Virus in Wild, Migratory Birds

To date, two types of H5 viruses have been identified in the wild bird survey - H5N3 and H5N1. It is of critical importance to note that in all cases, these viruses have been clearly characterized by scientists as low pathogenic, North American strain. This strain is similar to other H5N1 strains identified in North America in the past.

It is not the highly pathogenic H5N1 Asian subtype, which has been associated with significant disease in wild and domestic birds that has infected and killed people.

In fact, the North American strain is very different to the Asian strain and scientists have characterized approximately 15% structural differences between the two strains. In genetic terms, this percentage difference is considered to be very substantial.

These laboratory results are confirmed by the absence of any increased mortality observed in Canadian birds. All North American samples originated from healthy birds. This can also be attributed to the low pathogenic nature of the North American strain, as opposed to the highly pathogenic Asian strain.

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