Bovine Tuberculosis in Cows: Symptoms, Testing, and Herd Impact

Bovine tuberculosis, often called bovine TB or bTB, is a chronic infectious disease caused by Mycobacterium bovis. It mainly affects cattle, but it can also infect other mammals, including deer, goats, dogs, wildlife, and people. In the United States, it is considered rare, but it remains important because it can spread within herds, affect interstate movement, and create public health concerns.

The disease often develops slowly. Some cows look normal for a long time, while infection is quietly contained inside granulomas, sometimes called tubercles, in the lungs, lymph nodes, or other organs. As disease progresses, affected cattle may lose condition, cough, or show signs linked to where those lesions are located.

For many producers, the biggest impact is not dramatic illness in one cow. It is the herd-level effect. A positive or suspect result can trigger official testing, movement restrictions, traceback work, slaughter surveillance, and decisions about test-and-remove versus depopulation. That is why early communication with your vet and animal health officials matters so much.

Bovine tuberculosis is caused by the bacterium Mycobacterium bovis. The most common route of spread is inhalation of infected respiratory droplets from an infected animal. Cattle can also become infected by ingesting contaminated milk, feed, or water. In some situations, wildlife reservoirs can contribute to ongoing exposure pressure for livestock.

Once the bacteria enter the body, they are taken up by immune cells and may become walled off inside granulomas. These lesions can stay stable for long periods or slowly spread through lymphatic and blood vessels to the lungs, lymph nodes, udder, intestines, liver, spleen, reproductive tract, or other tissues. That slow, hidden course is one reason herd surveillance is so important.

Risk factors often include introducing cattle of unknown status, fence-line or shared-resource contact with infected animals, wildlife exposure in affected regions, and use of unpasteurized milk in calf feeding programs. Your vet can help assess which risks matter most on your farm and how to build a practical prevention plan around them.

Diagnosis usually starts with official live-animal testing rather than symptoms alone. In U.S. cattle, the routine screening test is the caudal fold tuberculin test, which uses USDA-licensed bovine purified protein derivative injected into the caudal fold and read about 72 hours later. Any swelling or palpable change can make the animal a suspect, and that result must be handled through official channels.

If a cow is classified as suspect on the screening test, follow-up testing may include the comparative cervical test or an interferon-gamma blood test, depending on the case and regulatory plan. These tests help sort out true infection from cross-reactions caused by other mycobacteria. False negatives can also happen, especially in early infection, advanced disease, older cattle, or recently calved cows.

Definitive confirmation usually relies on finding the organism in tissues through culture, PCR, and pathology, often after slaughter or necropsy. Culture can take weeks, so herd decisions may begin before every final lab result is back. If bovine TB is confirmed or strongly suspected, your vet will work with State and Federal animal health officials on identification, movement control, traceback, and herd-level testing.

Prevention starts with herd biosecurity and careful sourcing. Work with your vet to buy cattle from herds with strong health records, maintain accurate identification, and review any testing or movement requirements before animals arrive. Quarantine new additions when practical, and avoid sharing airspace, water, feed, or nose-to-nose contact with cattle of unknown status.

Milk safety matters too. Because M. bovis can spread through contaminated milk, calves should not be fed risky raw milk from suspect animals, and milk for people should be pasteurized. Good sanitation around milking equipment, calf feeding tools, and manure handling also helps reduce exposure opportunities.

In regions where wildlife exposure is a concern, prevention may also include feed storage protection, fencing strategies, limiting wildlife access to water and mineral sources, and prompt removal of attractants. If your herd is in an area with known bovine TB activity, ask your vet about a herd-specific surveillance and response plan so you are not making urgent decisions under pressure.