Avian influenza, commonly known as bird flu, is a disease caused by type A influenza viruses that primarily infect birds. These viruses naturally circulate among wild bird populations, but can also cause illness in domestic poultry and spread to mammals, including humans. The potential for these viruses to cross species barriers and cause severe disease makes avian influenza a topic of ongoing public health interest.
Understanding Avian Influenza
Avian influenza viruses are part of the influenza A family, categorized by specific surface proteins: hemagglutinin (H) and neuraminidase (N). There are 16 H subtypes and 9 N subtypes, leading to combinations like H5N1 and H7N9. These viruses are naturally found in wild aquatic birds, where they typically cause mild or no illness. However, certain highly pathogenic avian influenza (HPAI) strains, like H5N1 and H7N9, can lead to severe disease and high mortality in domestic poultry and, rarely, in humans. HPAI classification is based on severity in domestic chickens, not its impact on other species.
How Avian Influenza Spreads
Avian influenza viruses are shed in the saliva, nasal secretions, and feces of infected birds. Birds can become infected through direct contact with infected birds or indirectly via contaminated environments like droppings, bedding, or equipment. This spread is particularly rapid in poultry flocks.
Transmission to humans typically occurs through close contact with infected birds or heavily contaminated environments. This includes touching infected birds (alive or dead), their droppings, contaminated surfaces, or preparing infected poultry. Live bird markets can also be a source of human infection. While human-to-human transmission is rare, limited instances have been observed, usually within small family clusters without sustained spread.
Health Impacts
In birds, avian influenza symptoms vary widely depending on the virus strain and bird species. Low pathogenic strains may cause mild signs like ruffled feathers, reduced egg production, or no symptoms at all. Highly pathogenic strains, however, can result in severe breathing difficulties, a significant drop in egg production, swelling of the head, comb, and wattles, and sudden death.
For humans, symptoms can range from mild, such as eye redness (conjunctivitis) or mild flu-like illness including fever, cough, and sore throat, to severe respiratory disease. More serious complications include pneumonia, acute respiratory distress syndrome, and multi-organ failure. The incubation period for human infection is typically 1 to 5 days, but can extend up to 10 days. Certain strains, like H5N1, have been associated with a high mortality rate in humans, with approximately 468 deaths reported from 972 confirmed cases between 2003 and February 2025.
Public Health Measures and Global Preparedness
Public health efforts to manage avian influenza involve strategies to prevent its spread and mitigate its impact. Surveillance systems monitor outbreaks in bird populations, including wild birds and poultry, to detect the virus early and track its evolution. This includes testing wild birds that may carry the virus without showing symptoms.
Biosecurity measures are important in poultry farming to prevent the introduction and spread of the virus. These include limiting contact between domestic poultry and wild birds, maintaining strict hygiene for farm equipment and personnel, and controlling access to farms. If an outbreak occurs in poultry, culling affected flocks is a standard measure to contain the virus.
Vaccination plays a role in both animal and human health. Vaccines are available for poultry to protect against specific strains. For humans, several licensed H5N1 vaccines exist, primarily for individuals at high risk of exposure, such as poultry workers. These vaccines can induce antibodies that may offer some protection against circulating strains. Researchers are also developing new vaccine technologies, such as mRNA-based vaccines, for more rapid development and deployment in response to emerging strains.
The ongoing spread of H5N1 in various species, including recent infections in dairy cows across multiple U.S. states, shows the virus’s adaptability and the need for vigilance. While human-to-human transmission remains rare, the potential for the virus to adapt and spread more easily among people emphasizes the importance of monitoring and global preparedness to address future pandemic threats. This collaborative effort involves public health agencies, veterinary organizations, and researchers worldwide.