Avian influenza (AI), commonly known as bird flu, is a disease caused by Type A influenza viruses that primarily circulate among bird populations. This viral infection has a natural reservoir in wild aquatic birds, where it has existed for centuries, but it can also infect domestic poultry and other species. The severity of the disease varies dramatically, ranging from a mild, almost unnoticeable infection to a rapidly fatal systemic illness. Understanding how the virus spreads and affects different avian groups is key to grasping its impact on commercial poultry operations and global wildlife ecosystems.
The Spectrum of Illness in Individual Birds
Avian influenza is categorized into two main forms based on the severity of the disease it causes in poultry: Low Pathogenicity Avian Influenza (LPAI) and Highly Pathogenicity Avian Influenza (HPAI). LPAI strains generally result in mild or subclinical infections, often presenting only as minor respiratory symptoms like ruffled feathers, mild coughing, or a slight drop in egg production in domestic birds. In wild birds, LPAI frequently circulates without causing any visible signs of illness, allowing the host to function as an asymptomatic carrier.
HPAI indicates a severe, systemic infection that affects multiple internal organs, not just the respiratory tract. This systemic spread is linked to a specific change in the virus’s genetic code that allows it to use the enzyme furin to enter cells throughout the host’s body. HPAI infection in susceptible individuals, particularly chickens and turkeys, can result in mortality rates approaching 100%, often within 48 hours of exposure.
Symptoms of HPAI are severe and often rapid, including sudden death without prior clinical signs. Affected birds may exhibit a lack of coordination, stumbling, or twisting of the head and neck, indicating neurological involvement. Other physical signs include swelling of the eyelids, comb, and wattles, as well as hemorrhages on the legs and internal organs.
Mechanisms of Viral Transmission and Spread
The primary mechanism for the spread of avian influenza among birds is the fecal-oral route, which is particularly relevant for the virus’s natural hosts, such as waterfowl. Infected birds shed the virus in high concentrations through their saliva, nasal secretions, and feces. When these droppings contaminate shared water sources, the virus can be easily ingested by other birds.
Direct physical contact between sick and healthy birds is another major transmission pathway, especially within dense flocks. This includes pecking, preening, and close proximity, allowing for the exchange of respiratory droplets and viral secretions. Aerosol transmission, where the virus travels through the air in fine droplets or dust particles, also plays a role, particularly in crowded environments like poultry houses.
The virus can also spread indirectly through fomites, which are inanimate objects contaminated with viral material. This is a significant factor in the rapid spread within domestic poultry farms, as contaminated equipment, feed, water, and the clothing of farm workers can carry the virus between facilities. Wild migratory birds serve as long-distance carriers, transporting the virus along major flyways and introducing it to new geographic regions.
Differential Susceptibility Across Bird Species
The impact of avian influenza is highly dependent on the host species; some groups act as reservoirs while others face near-certain death. Wild aquatic birds, including ducks, geese, and gulls, are considered the natural reservoir hosts. These species often carry LPAI without showing illness and have evolved a degree of tolerance that allows them to shed the virus into the environment for extended periods.
In contrast, gallinaceous domestic poultry like chickens and turkeys are acutely susceptible to HPAI strains. In these species, the virus can rapidly overwhelm the immune system, leading to the catastrophic mortality rates that define HPAI outbreaks. Their heightened susceptibility allows the virus to replicate unchecked.
Among wild birds, susceptibility to HPAI varies widely, but some species are particularly vulnerable to emerging strains. Raptors and scavengers, such as bald eagles and vultures, often contract the disease after consuming infected carcasses. Specific wild species, including wood ducks and laughing gulls, have also been shown to be highly susceptible to HPAI, experiencing significant illness and death.
Consequences for Avian Populations and Ecosystems
The emergence and global spread of HPAI strains have caused large-scale consequences for both wild and domestic bird populations. Since 2021, the current highly pathogenic H5N1 strain has resulted in the deaths of hundreds of millions of birds worldwide, triggering a panzootic event. This mass mortality can lead to severe population bottlenecks, particularly in wild species that congregate in dense colonies to nest.
Colonial nesting seabirds, such as northern gannets and common murres, are especially vulnerable, as the close quarters facilitate rapid viral transmission and catastrophic die-offs. The loss of adult breeding stock during these events severely disrupts the population structure, leading to reproductive failure and reduced recruitment of new individuals for several subsequent years.
The virus poses a particular threat to species already facing conservation challenges, with HPAI detected in numerous at-risk birds, including the endangered California condor. For small or isolated populations, a single outbreak of HPAI can result in localized extinction. Beyond direct mortality, the mass die-offs disrupt broader ecosystem functions by removing birds that serve as predators, scavengers, or seed dispersers. The movement of the virus into over 70 mammalian species, often through scavenging infected carcasses, demonstrates the ripple effect across the entire food web.