Wildlife Research and Management - Health
Date Published: August 20, 2007
Number of Pages: 20
Avian influenza is widely endemic in wild populations of waterfowl and many other species of birds. The emergence and spread of a Highly Pathogenic Avian Influenza (HPAI) H5N1 subtype in Asia over the past few years (hereafter called HPAI H5N1) has elevated concerns about potential expansion of this virus to North America. Apprehensions among government agencies and the public are based on a range of possibilities that include sickness and mortality in wild bird populations, introduction of a disease that could devastate the poultry industry, and potential mutation of the virus into a form that could be highly infectious and pathogenic to humans—possibly the source of a flu pandemic. Currently, public concern has been heightened by extensive media coverage about this virus in Asia, its spread to Europe, and the small number of human infections—much of it includes speculation that migratory birds are a primary vector and will bring it to North America. Thus, government agencies, particularly state and federal wildlife agencies, are being called upon to develop an early detection system to determine if and when the virus arrives in North America.
Some clarifications of terms and the current situation are warranted because the terminology of avian influenza is often confusing, and it is important that a shared understanding of this disease is accurate. For purposes of this plan, here are some key points and assumptions as of June 2007:
- Migratory aquatic birds are the natural reservoir for many of the 144 subtypes of avian influenza, named for their protein components hemagglutinin (H) and neuraminidase (N). Most avian influenza types are not very pathogenic, but the H5 and H7 types seem to be more pathogenic to domestic poultry.
- The terms “highly pathogenic” (HPAI) and “low pathogenic” (LPAI) refer specifically to pathogenicity to domestic poultry—testing for HPAI is documented by mortality rates in dosed poultry.
- Some avian influenza varieties may mutate into forms that become pathogenic to specific taxa (e.g., birds, swine, humans). The currently prominent HPAI H5N1virus is highly pathogenic to some birds, particularly domestic poultry, but is not easily transmitted to people. This is primarily a bird disease that has infected a small number of people who have been heavily exposed to infected poultry or raw poultry parts.
- The HPAI H5N1 strain has not been detected in North America. Low pathogenic H5N1 and a wide variety of other AI types have been documented in poultry and wild waterbirds.
- The degree to which migratory birds may be agents in the spread of HPAI H5N1 is not definitive. In nearly all cases of expansion in Eurasia, movement of poultry and poultry products by humans are suspected as the primary vehicle for transmission of the virus across the continents. Mortalities of wild birds have been associated with contact or shared use of habitats with domestic birds. However local distribution of HPAI H5N1 may be linked to transmission by migratory birds.
- Currently, there is inadequate information about the virulence of HPAI H5N1 in wild bird species, its persistence in wild populations, and the degree to which it can spread from bird to bird during seasonal and annual cycles. Fecal and respiratory contamination is assumed to be the primary mode of transmission, and viruses can remain viable for extensive periods in cold, fresh water.
- The onset of a major human influenza pandemic could result if some form of AI—HPAI H5N1 or any other type—adapted into a form that was infectious and virulent among humans and easily transmitted from human to human. It is not a given that HPAI H5N1 is the mostly likely threat for a global pandemic.