Avian influenza, commonly known as “bird flu,” is a highly contagious viral disease that can infect birds and, in rare cases, humans. There are many different strains of avian influenza viruses that circulate constantly in wild aquatic birds worldwide. While most strains do not infect or cause illness in humans, some have developed the ability to infect humans and cause severe illness. The introduction of avian flu viruses from wild birds into domestic poultry populations can quickly lead to large outbreaks that devastate poultry industries. Monitoring wild bird populations for avian flu viruses allows scientists to identify potentially dangerous strains before they spread to domestic birds and potentially humans.
How do wild birds spread avian flu?
Wild aquatic birds, like ducks and geese, are the natural reservoir for avian influenza viruses. These birds often carry influenza viruses in their intestines and upper respiratory tract, but usually do not get sick from them. However, they readily shed these viruses in their saliva, mucous, and feces as they migrate across continents. If a new strain of avian flu emerges that is highly pathogenic, meaning it causes severe illness or death in domestic birds, the viruses can then spread rapidly between bird populations.
Not all wild bird species play an equal role in spreading avian flu. Many studies have shown that dabbling ducks, such as mallards, are key players in the global spread of avian flu viruses. These ducks often migrate long distances, sharing wetlands and aquatic habitats with other wild and domestic bird populations along their flyways. This provides opportunities for novel avian flu strains to mingle and exchange genetic material with local bird flu strains. The newly mixed viruses can then continue to spread further. However, avian flu outbreaks are rarely reported in wild birds, indicating they have developed immune tolerance to these viruses.
How did avian flu spread to domestic poultry in the US?
In the 1990s and early 2000s, there were several introductions of Eurasian-origin H5 and H7 avian influenza viruses from wild aquatic birds into domestic poultry in the United States. Notable examples include:
- Pennsylvania, 1983: H5N2
- Pennsylvania, 1986: H5N2
- New York, 1994: H7N2
- California, 2000: H6N2
- California, 2002: H5N2
- Texas, 2004: H5N2
In these cases, the viruses were likely shed by wild waterfowl and transmitted into nearby domestic poultry farms. Once introduced into poultry, these viruses can rapidly mutate into highly pathogenic strains that cause outbreaks in commercial operations. Strong surveillance, quarantine, and culling measures are required to contain outbreaks and prevent further spread.
Pennsylvania 1983-1986
In 1983, an H5N2 strain caused an outbreak on a Pennsylvania farm with severe illness and mortality in chickens and turkeys. After quarantine and culling eradicated this outbreak, a related strain emerged again in 1986, indicating that the virus continued to circulate in local wild birds.
New York 1994
In 1994, an H7N2 strain outbreak occurred in commercial egg-laying flocks in New York and rapidly spread to 10 farms across the state. This outbreak was linked to live bird markets in New York City that were receiving birds from the infected farms. It was the first time an avian flu virus was transmitted from birds to humans in the US when a poultry worker became infected but had mild illness.
California 2000-2002
From 2000-2002, H6N2 and H5N2 strains caused outbreaks in California meat turkeys. Genetic analysis showed the viruses were nearly identical to viruses circulating in regional migratory ducks, indicating wild birds introduced the strains into the turkey flocks. Over 3.5 million birds died or were euthanized, resulting in approximately $161 million in economic losses.
Texas 2004
In 2004, H5N2 caused outbreaks in commercial broiler operations in Texas. Epidemiological investigation identified contact with free-range feral waterfowl as the likely source. Over 6,600 birds died or were culled at a cost of $7 million.
How did recent Asian-origin H5 avian flu reach North America?
In 2014-2015, Eurasian-origin H5 avian influenza viruses were introduced from wild birds into domestic poultry in Canada and the Pacific, Central, and Mississippi flyways of the United States. The affected flyways overlap with migratory paths of Northern pintails and other wild waterfowl arriving from Asia. These H5 strains are genetically related to other Asian-origin viruses like H5N1 and H5N8 that have spread through Europe, Asia, and the Middle East in the 2000s. The timing coincided with seasonal migratory bird travel from Eurasian wintering areas to North American spring breeding grounds.
Key outbreak events:
- December 2014: H5N2 detected in British Columbia, Canada geese
- December 2014: H5N8 detected in Washington, captive gyrfalcon fed wild duck meat
- January 2015: H5N1 detected in Washington, captive gyrfalcon
- January 2015: H5N8 detected in California, backyard chickens
- March 2015: H5N2 detected in Minnesota, turkey farm
- April 2015: H5N2 detected on commercial turkey farms in South Dakota, Missouri, Kansas, Arkansas, Iowa, Wisconsin, and Oregon
These outbreaks marked the first time these Eurasian H5 viruses were detected in North America. Over 50 million commercial US turkeys and chickens died or were culled, as the viruses rapidly spread between farms to become the most expensive animal health disaster in US history.
How do avian flu viruses spread between poultry facilities?
Once an avian flu virus infects a poultry farm, viruses can spread between facilities in several ways:
Between farms
- Shared farm equipment and vehicles
- Visits by workers between infected and clean farms
- Wind dispersal of viral particles between nearby operations
- Rodents or wild birds traveling between farms
Across states and regions
- Movement of infected live birds and poultry products
- Transport of contaminated feed, egg cartons, crates, and other supplies
Implementing secure biosecurity measures is critical to limiting the spread of viruses between facilities, including restricted access, dedicated protective clothing, and disinfection protocols. Suspending the interstate transport of any live birds from areas with active outbreaks can help limit large-scale spread.
Year | State(s) | Strain | Affected birds | Economic loss |
---|---|---|---|---|
1983 | Pennsylvania | H5N2 | 17 million | $60 million |
1994 | New York | H7N2 | 1,200 | $13 million |
2000-02 | California | H6N2, H5N2 | 15 million | $161 million |
2004 | Texas | H5N2 | 6,600 | $7 million |
2014-15 | WA, OR, ID, CA, UT, MT, MN, IA, MO, AR, WI, IN, OH | H5N8, H5N2 | 50.5 million | $3.3 billion |
Conclusion
In summary, wild aquatic birds are the ultimate source of influenza A viruses circulating globally in avian populations. Through migration and sharing of habitats, they can introduce avian flu viruses from one region into poultry flocks of a distant region. Several independent introductions of Eurasian avian flu viruses have occurred from wild birds into North American domestic poultry in recent decades. If a strain emerges with increased virulence or transmissibility, it can lead to widespread outbreaks in commercial operations. Rigorous monitoring of wild bird populations for novel avian flu strains, along with robust biosecurity measures on farms, is critical for detecting threats early and limiting economic impacts.