Avian influenza, or bird flu, is a disease caused by influenza A viruses. These viruses are common in wild birds and can spread to domestic poultry. While avian influenza viruses circulate among bird populations, they can infect other animal species, though human infections have been infrequent.
The Current Situation in the United States
A strain of Highly Pathogenic Avian Influenza (HPAI) known as H5N1 is affecting bird and mammal populations in the United States. The virus is widespread in wild birds and has led to outbreaks in commercial poultry and backyard flocks, sometimes requiring culling to control its spread.
Beginning in late March 2024, H5N1 was detected in dairy cows. As of late 2024, hundreds of dairy herds across numerous states have confirmed cases, with California becoming a major center of the outbreak. This expansion into cattle is being closely watched by the Centers for Disease Control and Prevention (CDC) and the U.S. Department of Agriculture (USDA).
Federal agencies are offering financial aid and biosecurity guidance to poultry and dairy producers to manage the economic impact and control the virus’s spread. The USDA has invested funds into a strategy aimed at curbing the virus’s reach. The number of affected dairy herds is continuously updated to track the virus’s movement.
Transmission and Spread
The H5N1 virus circulates among birds through direct contact with an infected animal’s saliva, nasal secretions, and feces. Wild birds, particularly waterfowl, are natural carriers and can spread the virus over long distances during migration, introducing it to domestic poultry and other environments.
The virus can spread from birds to other animals, including mammals, through similar means. Direct contact with an infected bird or exposure to a contaminated environment, such as a farm, are the main routes of infection for mammals like dairy cattle. Early investigations into the cattle outbreaks pointed to the movement of infected cows between states and contaminated farm equipment as contributing factors. More recent findings suggest other potential pathways on farms, including mechanical transport by insects like house flies.
Once in a cattle herd, the virus can spread between cows. Research has explored mouth-to-teat transmission and transmission from calves through nursing. While the virus has shown it can adapt to new species, sustained transmission between humans has not been documented, as the virus has not acquired the ability to pass easily from person to person.
Human Health Implications
While the risk of H5N1 infection to the general public remains low, human cases have occurred, primarily among individuals with direct exposure to infected animals. The symptoms of H5N1 in humans can vary widely. Some individuals may experience mild illness, such as an eye infection (conjunctivitis) or upper respiratory symptoms similar to the common flu. In other cases, the infection can lead to severe pneumonia and other serious respiratory complications.
The individuals at the highest risk of exposure are those who have close and prolonged contact with infected birds or other animals. This group includes poultry and dairy farm workers, veterinarians, and those involved in culling operations to control outbreaks. Most of the documented human cases in the United States have been traced back to exposure on dairy farms or poultry facilities where an H5N1 outbreak was present.
As of mid-2025, there have been over 50 confirmed human cases in the U.S. since the outbreak expanded in 2024, with one reported death. Health agencies like the CDC are monitoring people who have been exposed to infected animals and tracking case numbers. The increase in human cases, though small relative to the number of exposed workers, provides the virus more opportunities to potentially adapt to human hosts, a development that scientists are monitoring closely.
Food Supply Safety
Federal agencies and food safety experts maintain that the commercial food supply in the United States is safe. For poultry and eggs, the risk is managed through proper handling and cooking. Cooking poultry to an internal temperature of 165°F effectively kills the H5N1 virus, as well as other pathogens.
A primary concern for consumers has been the safety of milk and dairy products, given the outbreaks in cattle. The U.S. Food and Drug Administration (FDA) has stated that the commercial milk supply is safe due to the routine process of pasteurization. This heating process is effective at inactivating the H5N1 virus, and tests on retail pasteurized dairy products have been negative for any viable virus.
For raw milk and products made from it, the FDA advises caution. The agency has recommended that milk from infected cows should not be used to produce raw milk products. Research is ongoing to determine if the aging process for raw milk cheese is sufficient to eliminate the virus, but initial findings suggest it may not be fully effective. Therefore, consuming pasteurized dairy products remains the safest option.