Avian influenza, commonly known as avian flu or bird flu, is a highly contagious viral disease that can infect birds and some mammals. There are many different strains of avian influenza viruses that vary in their ability to cause disease and mortality in different bird species. Some strains, such as the highly pathogenic H5N1, can result in nearly 100% mortality for some species of birds. However, other strains may cause little to no symptoms in infected birds.
In general, domestic poultry are considered to be the birds most susceptible to highly pathogenic forms of avian influenza. Chickens and turkeys are especially vulnerable, but ducks, geese, and guinea fowl are also at high risk of infection and death. Wild waterfowl such as ducks and geese are natural reservoirs of low pathogenic avian influenza viruses and can spread the virus through migrations and interactions with domestic birds. Some wild bird species, including swans, gulls, terns, shorebirds, and raptors have also proven susceptible to highly pathogenic strains under natural conditions or experimental studies.
What types of birds are most often infected with avian flu viruses?
The main types of birds that are most vulnerable and affected by avian influenza viruses include:
- Domestic poultry – Chickens, turkeys, ducks, geese
- Wild waterfowl – Ducks, geese, swans
- Other wild birds – Gulls, terns, shorebirds, raptors
Domestic poultry like chickens and turkeys kept in high density on farms are at greatest risk of avian flu outbreaks. Migratory waterfowl like ducks and geese commonly carry low pathogenic flu strains and can transmit it to other birds. Some studies have found mortality events from highly pathogenic flu in populations of wild raptors as well.
What types of domestic poultry are most susceptible?
Among domestic poultry, studies indicate:
- Chickens are highly susceptible with near 100% mortality in some outbreak strains like H5N1.
- Turkeys are also very vulnerable with high mortality rates from avian flu.
- Ducks and geese can become infected and die from some strains, but may show lower mortality rates than chickens and turkeys especially with H5N1.
- Guinea fowl are also susceptible. One study found a mortality rate of 67% from H5N1 in guinea fowl.
The reason chickens and turkeys tend to suffer such high mortality rates from avian influenza involves factors like:
- Repeated exposure from high density in poultry houses.
- Lack of pre-existing immunity.
- Respiratory anatomy that favors viral replication.
- Difficulty limiting infection due to confinement.
Mortality Rates in Domestic Poultry
Bird Type | H5N1 Mortality Rate |
---|---|
Chickens | 80-100% |
Turkeys | 75-100% |
Ducks | 50-100% |
Guinea Fowl | 67% |
This table shows estimated H5N1 avian influenza mortality rates in common domestic poultry birds. Chickens and turkeys are especially vulnerable.
What waterfowl species are impacted by avian flu?
Wild migratory waterfowl, especially ducks and geese, are considered a natural reservoir for avian influenza viruses. Many waterfowl species can become infected and transmit low pathogenic strains that may mutate into highly pathogenic variants.
Specific types of waterfowl that have been shown to be susceptible to avian flu include:
- Mallards
- Northern pintails
- Mute swans
- Whooper swans
- Canada geese
- Snow geese
One extensive study inoculated different duck and goose species with a highly pathogenic H5N1 strain and observed the following mortality rates:
Waterfowl Species | Mortality Rate |
---|---|
Mallards | 100% |
Northern pintails | 67% |
Swan geese | 100% |
Canada geese | 44% |
This demonstrates that dabbling ducks like mallards and pintails, as well as some goose species can suffer high mortality from highly pathogenic avian influenza under experimental conditions.
What other wild birds are affected by bird flu?
In addition to waterfowl, avian influenza viruses have been detected in many other groups of wild birds. Some types of wild birds that have tested positive or died from avian flu include:
- Gulls
- Terns
- Shorebirds (ruddy turnstones, sanderlings)
- Raptors (peregrine falcons, gyrfalcons)
- Seabirds (auklets, murres)
- Scavengers (crows, magpies)
Birds that congregate at wetlands and shorelines or scavenge infected carcasses seem to be at highest risk. One 2014-15 outbreak of H5N8 flu in the United States caused mortality events in captive gyrfalcons and wild raptors. Wild raptor and crow deaths have also occurred from H5N1 overseas. Further research is still needed on susceptibility of many wild bird species to emerging avian flu strains.
Example Wild Bird Mortality Events from H5N1 and H5N8 Avian Influenza
Species | Location | Year |
---|---|---|
Gyrfalcons, peregrine falcons | United States | 2014-15 |
Common buzzards | Germany | 2006 |
Great cormorants | Japan | 2004-05 |
This table shows some examples of wild raptor and waterbird mortality events attributed to H5N1 and H5N8 highly pathogenic avian influenza viruses.
Why are poultry more vulnerable to avian flu than wild birds?
There are several key reasons why domestic poultry may suffer higher mortality rates from avian influenza compared to wild birds:
- Confinement – High density confinement provides constant exposure and transmission.
- Lack of genetic diversity – Poultry breeds lack heterogeneity unlike diverse wild populations.
- No prior immunity – Poultry populations lack previous exposure to avian flu viruses.
- Respiratory anatomy – Chickens and turkeys have respiratory tracts favorable to viral replication.
- Exposure dose – Poultry may experience higher infective doses than wild birds.
Additionally, domestic birds may experience higher viral loads and more rapid onset of illness compared to wild birds. Evolutionary pressures also favor lower virulence in natural bird populations.
Conclusion
In summary, domestic poultry like chickens and turkeys are the birds most vulnerable to highly pathogenic strains of avian influenza. Their confinement, lack of prior immunity, and respiratory anatomy allow rapid transmission and replication of bird flu viruses. Ducks, geese, and other domestic fowl are also susceptible. Migratory waterfowl are natural reservoirs for low pathogenic flu strains that can mutate into highly lethal variants.
A range of wild bird species, including waterfowl, seabirds, raptors, and scavengers have also tested positive or died from certain avian flu strains under natural conditions and experimental studies. However, high mortality rates are still most frequently observed in domestic chicken and turkey flocks. More research is needed to fully understand relative susceptibility across the over 10,000 species of wild birds.
Effective surveillance, biosecurity, and contingency planning are critical to reduce the spread of avian influenza between wild birds, poultry flocks, and humans. With emerging viruses, there are still uncertainties about which hosts and ecological drivers pose the greatest risks. But domestic birds clearly remain on the front lines of this avian health threat.