Avian influenza
Avian influenza refers to influenza (flu) Type A viruses that are adapted to avians (birds). These viruses occur naturally among wild aquatic birds (such as ducks and geese) worldwide and can infect domestic poultry (like chickens and turkeys) as well as other bird and animal species. Avian influenza viruses do not normally infect humans. However, sporadic human infections with avian influenza viruses have occurred, most commonly following direct or close contact with infected animals. This page offers resources and links to information about avian influenza for different audiences.
The CDC also provides additional information on avian influenza.
Avian influenza A virus infections in birds
These influenza A viruses occur naturally among wild aquatic birds (e.g., ducks and geese) worldwide. These birds can carry avian influenza A viruses in their intestines and respiratory tract, but usually do not get sick. However, avian influenza A viruses are very contagious among birds, and some of these viruses can sicken and even kill certain domesticated bird species including chickens, ducks, and turkeys.
Infected birds can shed avian influenza A viruses in their saliva, nasal secretions, and feces. Susceptible birds become infected when they have contact with the virus as it is shed by infected birds. They also can become infected through contact with surfaces that are contaminated with virus from infected birds.
Avian influenza A viruses are classified into two categories: low pathogenic avian influenza (LPAI) A viruses, and highly pathogenic avian influenza (HPAI) A viruses. These categories refer to the molecular characteristics of the virus and the virus’ ability to cause disease and mortality in chickens in a laboratory setting. It does not describe the virus’ ability to cause disease in humans.
The first outbreak of a HPAI in poultry in the US occurred in 1924 and since then, there have been many such outbreaks caused by different subtypes of avian influenza A viruses. An HPAI H5N1 subtype first arose in 1996 in southeast Asia and has since diversified into multiple lineages that have been detected around the world. Several of these lineages have resulted in human infections and some have also affected other kinds of mammals.
In 2020, a new HPAI H5N1 lineage emerged as a result of combination of two different influenza A avian viruses. This virus spread globally in wild birds, has caused poultry outbreaks worldwide, has resulted in some human infections and has been identified as a cause of illness in multiple other mammal species.
Avian influenza A virus infections in dairy cattle
On April 1, 2024. one human HPAI H5N1 infection was confirmed in a person with exposure to dairy cows in Texas that were presumed to be infected with the virus. This is thought to be the first instance of mammal to human spread of HPAI A(H5N1) virus. Since May of 2024, additional, human cases in people who had exposure to infected dairy cows have been identified. These cases have generally had mild illness and frequently involve conjunctivitis as a primary symptom. Most exposures are thought to occur as a result of infected milk coming into contact with people’s eyes. That latest human case counts are available from CDC.
Avian influenza A virus infections in humans
Although avian influenza A viruses do not typically infect humans, rare cases of human infection with these viruses have been reported. Most human infections with avian influenza A viruses have occurred following direct or close contact with infected poultry or their living environment. In 2024, as herds of dairy cattle became infected with avian influenza, there have also been human infections identified in some people who have worked with infected cows.
Illness in humans infected with avian influenza viruses can range from mild to severe.
The spread of avian influenza A viruses from one ill person to another has been reported very rarely, and has been limited, inefficient and not sustained. However, because of the possibility that avian influenza A viruses could change and gain the ability to spread easily between people, monitoring for human infection and person-to-person transmission is extremely important for public health.
The wide geographic spread of HPAI H5N1 viruses in wild birds, poultry, and some other mammals, including in cows, could create additional opportunities for people to be exposed to these viruses. Therefore, there could be an increase in sporadic human infections resulting from bird and animal exposures, even if the risk of these viruses spreading from birds to people has not increased. The current risk to the general public from bird flu viruses remains low. People who have job-related or recreational exposure to infected birds or animals, including cows, are at greater risk of becoming infected. There are monitoring, testing and treatment recommendations for people known to be exposed to HPAI H5N1 viruses to prevent severe illness and spread to other people.
How CDC is monitoring influenza data among people to better understand the current avian influenza A (H5N1) situation | Bird Flu | CDC This link provides information on what CDC is doing to monitor the people who are at increased risk of exposure to H5N1.If you suspect avian influenza in a patient due to symptoms and recent travel/exposure/risk, please contact the Division of Epidemiology immediately at 617-983-6800 for further direction on specimen collection and next steps. For more information on testing and surveillance, please visit the Infection Control, Testing, and Surveillance page.
The CDC provides additional information on Avian Influenza in Humans.
The CDC provides additional information on the current situation with Avian Influenza (H5)
Massachusetts Department of Agricultural Resources Diagnostic Testing Services
Additional resources
- CDC’s Traveler Health Notices
- U.S. Department of Agriculture (USDA) Avian Influenza
- USGS National Wildlife Health Center Avian Influenza
- World Health Organization Avian and other zoonotic influenza
- Division 6.2 Infectious Substance Shipping Guide: Classification, Packing, Marking and Labeling
- For more information about influenza visit the DPH Flu page, or call the Massachusetts Bureau of Infectious Disease and Laboratory Sciences at (617) 983-6800