Colorado has highly pathogenic avian influenza – also known as HPAI or bird flu – on a dairy farm, the ninth state with confirmed cases. The U.S. Department of Agriculture’s National Veterinary Services Laboratories confirmed the virus on April 25, 2024, in a herd in northeast Colorado.

This farm is one of 35 dairy farms across the U.S. with verified cases of bird flu in cattle as of May 7, 2024, according to the USDA.

Bird flu is not new to Colorado. The state experienced an outbreak in poultry that began in 2022. Since then, the USDA’s Animal and Plant Health Inspection Service has reported that 6.3 million birds in nine commercial flocks and 25 backyard flocks have been affected by the virus. The most recent detection was in February 2024.

But this is the first time the disease has made cattle in Colorado sick.

I’m a veterinarian and epidemiologist at Colorado State University who focuses on infectious diseases in dairy cows. I spent many years on a USDA incident management team working on multiple cattle and poultry disease outbreaks, and I’m leading the efforts at Colorado State University to study this novel outbreak.

The first cases of bird flu in cattle

Bird flu was first detected in dairy cattle in Texas and Kansas in March 2024.

Colorado State University faculty responded to the outbreak by forming a multistate group with state departments of agriculture, the USDA and other universities to gain a better perspective of how this virus is transmitted between farms and among cows. The team is coordinating the sampling and testing of sick and healthy cows on affected farms to understand which animals are shedding the virus, meaning they are more likely to spread the disease, and for how long.

We are also working to identify mitigation steps to help control this disease. Our network of animal health specialists is working with dairy producers and informing them of new data on a weekly basis.

Detecting bird flu in cattle

In February 2024, veterinarians and researchers began testing the blood, urine, feces, milk and nasal swab samples of sick cows. The virus was detected most frequently in raw milk, suggesting the disease may have been spread to other cows during the milking process.

More recent laboratory tests have also detected the virus in cows’ nasal secretions for a short time before the virus presents in their raw milk.

In late April, the U.S. Food and Drug Administration and USDA began testing commercial milk samples. To date, authorities have not detected any live virus in these samples.

That’s expected because the pasteurization process, which heats milk to at least 161 degrees Fahrenheit (72 degrees Celsius) for at least 15 seconds, kills the virus. Pasteurization times and temperatures used in the U.S. are designed to kill bacterial pathogens, but they are working against this virus.

Raw milk, as its name implies, is not pasteurized. The CDC has linked drinking raw milk to many foodborne illnesses, including E. coli and salmonella. The presence of the virus that causes bird flu, H5N1, is an additional reason for concern.

Dairy producers are required to divert abnormal milk and milk from sick cows from the food supply to protect consumers.

In addition to milk, the USDA Food Safety and Inspection Service has tested samples of commercial ground beef from states with sick cows. No virus had been found in beef as of May 1, 2024.

Slowing the spread of the disease

At this early stage of the outbreak in dairy cows, researchers don’t know exactly how bird flu spreads in cattle, so recommendations to contain it may shift as more is learned.

I have seen many infected cows and they look dull and depressed, similar to how humans feel during a viral infection. Many infected cows have signs similar to humans with the flu, such as a fever.

Many dairy producers separate sick cows in hospital pens, away from healthy ones, so sick cows can be easily monitored and treated.

Since the virus has been found in nasal secretions in early infection, it’s possible the water tanks for the herds’ drinking water could be a source of infection. Farmers should continue to clean these tanks at least weekly – and even more often in hospital pens – as a best practice.

Infected cows can recover

The good news is that most cows get better. Like a human with the flu, they respond to anti-inflammatory drugs and oral fluids.

A small percentage of cows do get secondary bacterial infections and die or are humanely euthanized. Some cows recover from the infection but stop producing milk and are removed from the herd and usually slaughtered for beef.

Because the virus is detected most often in milk from sick cows, our team recommends dairy producers continue following best milking practices on the dairy farm, including disinfecting the cow’s teats before and after milking, even healthy cows.

Just one case of human conjunctivitis due to bird flu was reported in a Texas dairy farmworker in late March. The worker was likely exposed through direct contact with milk from an infected cow or from rubbing their eyes with their hands or gloves that had come in contact with contaminated milk. The CDC recommends farmworkers wear personal protective equipment, including eye protection, when in direct or close physical contact with raw milk.

How dairy producers can protect herds

Viruses can end up on farms through the movement of cattle, people, vehicles, equipment and wild birds.

The U.S. dairy industry has a Secure Milk Supply Plan that addresses foreign and emerging dairy cattle diseases like bird flu. The plan calls for increased biosecurity practices on farms during disease outbreaks.

Biosecurity practices include limiting cattle movement on and off farms, allowing only required personnel access to cattle, preventing vehicles and equipment from other farms from entering cattle areas, and cleaning and disinfecting vehicles entering and leaving dairy farms. Following these practices should greatly reduce the opportunity for the virus to enter new herds.

Birds also carry the virus. They are more difficult to control because of their easy access to feed and water on dairy farms. State and federal fish and game departments and wildlife agencies work with farmers to reduce the risk of diseases spread by wild birds. These include programs to limit the number of birds attracted to dairy farms while respecting rules protecting these species.

Producers who observe cows with clinical signs of bird flu should let their veterinarians know so that proper testing can be done to confirm the presence of bird flu. If a test result is positive, the lab that conducts the test must report it to the USDA. As USDA and affected states continue to track the disease, an accurate estimate of affected farms will allow investigators to determine how the virus is spreading from farm to farm – and if we are making progress in containing it.

Jason Lombard receives funding from USDA. He is affiliated with the National Mastitis Council and the American Association of Bovine Practitioners. He is a former USDA employee.