H5N1 Avian Influenza in Cattle: Symptoms, Milk Changes, and Biosecurity

H5N1 is a highly pathogenic avian influenza virus that was first confirmed in U.S. dairy cattle in March 2024. Even though it is best known as a bird flu virus, it can infect mammals, including dairy cows. In cattle, illness has been reported mainly in adult lactating dairy cows, with the udder and milk appearing to be major sites of virus activity.

In many affected herds, the most noticeable change is not severe breathing trouble. It is a sudden drop in milk production along with abnormal milk that may look thick, concentrated, or colostrum-like. Some cows also develop low-grade fever, reduced appetite, lethargy, mild respiratory signs, or manure changes. Compared with poultry, cattle often survive and recover, but the production impact can be substantial.

This is also a public health and food safety issue. Infected cows can shed high levels of virus in raw milk, which is why abnormal milk should be discarded and raw milk should not be consumed. Federal agencies continue to report that the commercial milk safety system, including pasteurization, is effective at inactivating H5N1.

If you manage dairy cattle and notice unusual milk changes or a herd-level milk drop, involve your vet promptly. Early testing, isolation, and biosecurity can help protect the rest of the herd and the people working with them.

H5N1 in cattle is caused by infection with highly pathogenic avian influenza A(H5N1) virus. Investigators believe the virus originally spilled over from wild birds into dairy cattle, and then spread between farms through a mix of animal movement and contaminated people, equipment, vehicles, clothing, or other shared items. USDA has emphasized that transmission can occur on things that move from farm to farm, including links between dairies and poultry facilities.

Within affected cows, the virus appears to have a strong attraction to the mammary gland, which helps explain why milk changes are so prominent and why milk can contain high levels of virus. That is different from the classic picture many people expect from influenza. In cattle, the udder may be more involved than the lungs.

Risk is not limited to the cow alone. Raw milk, splashes in the milking parlor, contaminated bedding, manure, and close contact with sick animals can all increase exposure risk for workers and other animals on the farm. Cats and some other mammals have also become infected after exposure to contaminated raw milk or infected animals.

This is why herd management matters as much as individual cow care. If your farm has unexplained milk changes, dead wild birds nearby, or recent cattle movement, your vet may consider H5N1 as one possible cause and help you decide what testing and reporting steps fit the situation.

Diagnosis starts with history and pattern recognition. Your vet will look at whether the affected animals are lactating dairy cows, how quickly milk production changed, what the milk looks like, whether appetite and rumination dropped, and whether there are mild fever, nasal discharge, or manure changes. Because these signs can overlap with mastitis, digestive disease, or other infectious problems, lab testing is important.

The main confirmatory test is PCR testing, usually on milk from affected cows and sometimes on nasal swabs or other approved samples. In many states, testing is coordinated through your state animal health official, USDA pathways, and National Animal Health Laboratory Network laboratories. Non-negative samples are typically forwarded for federal confirmation.

Your vet may also recommend looking at the herd picture, not only one cow. Bulk tank or herd-level surveillance programs are now part of the national response, and interstate movement of lactating dairy cattle has involved testing requirements and monitoring programs. These programs help detect infected herds earlier, even when some cases are mild or missed.

If H5N1 is suspected, do not wait to improve biosecurity while results are pending. Separating suspect cows, handling milk carefully, and protecting workers can start right away while your vet guides testing, reporting, and next steps.

Prevention centers on biosecurity. USDA advises farms to reduce spread on people, equipment, vehicles, and other items that move between farms. That means limiting unnecessary visitors, cleaning and disinfecting shared tools, changing boots and clothing between groups, and avoiding cross-traffic between dairy and poultry areas. If you bring in new cattle or move lactating dairy cows across state lines, follow current testing and movement requirements with your vet and state officials.

On the farm, watch closely for production changes, especially a sudden milk drop or thickened milk. Separate suspect cows promptly, milk them last if your vet recommends that workflow, and keep abnormal milk out of the bulk tank. Feed and water sources should be protected from wild birds as much as possible, and dead birds or unusual wildlife activity around the dairy should be reported through the channels your vet recommends.

Worker safety is part of prevention too. People handling sick cows, raw milk, or contaminated materials should use appropriate PPE and avoid touching their eyes, nose, or mouth with contaminated gloves or clothing. Splashes of raw milk are a known concern. Workers should monitor themselves for symptoms such as eye redness, fever, cough, or sore throat after exposure and follow public health guidance.

For food safety, pasteurized milk remains the safer choice. Federal agencies continue to state that pasteurization inactivates H5N1, while raw milk and some non-heat-treated raw milk products carry more concern. If you suspect H5N1 on your farm, your vet can help you build a prevention plan that matches your facilities, labor, and herd movement patterns.