Bird Flu in Milk: Is Your Dairy Supply Safe to Drink?

The detection of H5N1 bird flu in United States dairy cattle has raised public concern about the safety of the milk supply. This discovery, confirmed in March 2024, was the first time the virus was identified in dairy cows. Subsequent testing found genetic traces of the virus in some commercial milk samples, prompting questions about potential risks to consumers.

Avian Influenza Transmission to Dairy Cows

Highly Pathogenic Avian Influenza (HPAI) H5N1 is a virus primarily affecting wild birds and poultry. Its jump to dairy cattle represents a significant cross-species transmission, likely occurring through contact with infected wild birds. Once in a herd, the virus can spread from cow to cow through contact with contaminated milking equipment.

The outbreak has affected herds across numerous states, and the identified strain is the same one circulating in wild birds. Infected cows show symptoms like a drop in milk production, fever, and changes in milk consistency. While the virus is severe in birds, the illness in cattle has been mild, with most animals recovering.

Virus Presence and Viability in Milk

Investigations into H5N1 in dairy cows focused on its presence in milk. Milk taken directly from infected cows revealed high concentrations of viral RNA, the virus’s genetic material. It is important to distinguish these genetic fragments from a live, infectious virus, as standard tests can detect RNA even after a virus has been killed.

Studies confirmed that raw, unpasteurized milk from infected cows can contain live H5N1 virus, posing a transmission risk. Because of this, regulations mandate that milk from sick animals must be diverted and destroyed, preventing it from entering the commercial food supply.

Pasteurization’s Role in Ensuring Milk Safety

The commercial milk supply undergoes pasteurization, a process that kills harmful microorganisms by heating milk to a specific temperature. The most common method is High-Temperature Short-Time (HTST) pasteurization, which heats milk to at least 161°F (72°C) for 15 seconds.

Federal agencies like the FDA and USDA conducted studies confirming this process effectively inactivates the H5N1 virus. While genetic fragments have been detected in retail milk, subsequent tests have found no live, infectious virus in these pasteurized products. These findings support the assessment that the commercial milk supply is safe.

Elevated Risks of Raw Milk Consumption

Raw milk is not pasteurized to kill harmful pathogens and carries risks from germs like Salmonella and E. coli. The emergence of H5N1 in dairy cattle introduces a new concern for raw milk consumers.

Since live H5N1 virus has been found in the unpasteurized milk of infected cows, consumption creates a potential transmission route. Incidents, such as farm cats dying after consuming raw milk from infected cows, highlight this danger. Public health agencies strongly advise against consuming raw milk or products made from it.

Public Health Guidance and Human Risk Assessment

Health authorities maintain the overall risk to the general public is low. This assessment is based on pasteurization’s effectiveness and the fact that most people lack close contact with livestock. The primary risk is for individuals with prolonged, unprotected exposure to infected animals, like farmworkers and veterinarians.

A few human cases have been linked to the outbreak, almost exclusively involving dairy farm workers with direct contact to infected cows. Reported symptoms have been mild, such as conjunctivitis, and there is no evidence of human-to-human transmission.

The CDC recommends farmworkers use personal protective equipment (PPE) like gloves and eye protection. For the general population, the advice is to consume only pasteurized milk and dairy products. Ongoing surveillance in cattle and humans continues to inform the public health response.