Bovine tuberculosis (bTB) is a serious chronic disease that primarily affects cattle. Other mammals, like deer and elk, can also be affected by bTB.
Leslie Verteramo

bTB is caused by a type of bacteria called Mycobacterium bovis (M. bovis), affecting mainly the lungs and the lymph nodes of the animal. This infection causes granulomas or tubercles, hence the name tuberculosis, in the infected tissue.

Transmission of bTB occurs via respiratory droplets, saliva, milk and bodily discharges from an infectious animal. In a cold, dark and humid environment, M. bovis can survive close to a year; however, under a hot and dry environment with direct sunlight, M. bovis can only survive a few weeks. Sharing feed and watering sites with infected animals, including wildlife, allows the bacteria to spread in the herd.

Human tuberculosis (TB), on the other hand, is caused mainly by the bacteria Mycobacterium tuberculosis, different but related to M. bovis. TB symptoms include fever, cough, chest pain, weakness and weight loss. TB can be effectively treated with antibiotics but can be fatal if left untreated. Although the Center of Disease Control (CDC) reported that about 4% of the U.S. population have latent TB, the number of active TB cases is about 9,000. Most active cases originate from latent TB people. People with latent TB are asymptomatic and cannot spread the disease but can potentially develop active TB in the future, especially if their immune system is weak or they are in a high-risk group (e.g., diabetic). TB is not a public health problem in the U.S.

Humans can develop tuberculosis from Mycobacterium bovis as well, but this is rare in the U.S. According to the CDC, less than 2% of all human TB cases in the U.S. originated from M. bovis. Most cases of human TB from M. bovis are from people who consumed unpasteurized milk or dairy products from M. bovis-infected cows. The risk of infection from respiratory droplets from infected animals to humans is possible but is not the main way of transmission.

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Milk produced from bTB- infected cows contain M. bovis. Pasteurization, which is required for all commercial milk, eliminates M. bovis and other harmful bacteria in milk, making it safe for humans. Consumers are not at risk of infection by consuming pasteurized dairy products from infected cows.

Relative to the general population, farmworkers who work in farms with bTB-infected cattle are at a higher risk of infection, as well as people who are in close contact with infected animals like deer and elk. These people may not always develop TB symptoms. Because of the low TB prevalence and risk of infection in the U.S., vaccination against TB (BCG vaccine) is not recommended to the general public. This is not the case in developing countries, where most of the population is administered the BCG vaccine due to a higher prevalence.

Infected cattle at early stages of bTB are hard to detect visually. The best way to test cattle for bTB is through tuberculin tests (caudal fold tuberculin test or comparative cervical tuberculin test). Once an animal has been infected with M. bovis, it may take more than a year before the animal becomes infectious and begins to show symptoms. Before the infectious stage and after about a month of becoming infected with M. bovis, the animal is in the reactive stage of infection. At the reactive stage, an animal can react (test positive) to the tuberculin tests but may not infect other animals.

Cows at the reactive stage may not show symptoms of infection, but studies found that their milk production can decrease. A study in Ireland showed that bTB-reactive cows produced about 10% less milk than non-reactive, healthy cows. There are no similar studies in U.S. herds due to the low bTB prevalence.

At the infectious stage, the infected animal can show the following symptoms: weight loss, chronic coughing and lethargy. When any of these symptoms are present, the farmer should separate the infected animal from the rest of the herd to avoid spreading the disease and must contact their accredited veterinarian immediately. bTB can be confirmed after a postmortem test.

Fortunately, cases of confirmed bTB cases in the U.S. are very rare. As a reference, in the past few years the number of bTB-positive animals detected at slaughter in the U.S. were about 10 per year. The only area in the U.S. that is not accredited free of bTB is a region in northeastern Michigan where bTB is endemic. In this affected region, wild deer are reservoir of the bacteria. There are strict controls for cattle movement in this area and, because deer do not migrate long distances, the risk of bTB spreading to other regions is very low. Nevertheless, dairy managers should always follow biosafety protocols to minimize the risk of introducing an infection (bTB or any other disease) into their herd and safeguard the health of their workers.

bTB is commonly introduced into a herd through exposure to infected cattle or wildlife. Some recommendations from the Animal and Plant Health Inspection Service (APHIS) of the USDA to minimize bTB infection in a farm include the following:

Recommendations to minimize bTB infection and spread in a herd

  • Test all your livestock.

  • Raise your own replacement stock (when possible).

  • Avoid contact with neighboring and other herds.

  • Provide adequate ventilation in your barn.

  • Do not allow people near your herd if they have been in contact with other herds.

  • Disinfect vehicles and equipment if they were in other farms.

  • Avoid sharing equipment with other farms.

  • Do not spread manure from other farms.

Recommendations to minimize bTB infection when purchasing cattle

  • Purchase animals from accredited bTB-free herds.

  • Test animals before purchasing.

  • Isolate the animal for 60 days and re-test it before joining the herd.

Recommendations to minimize bTB infection in humans

  • Do not consume unpasteurized milk and dairy products.

  • High-risk individuals (e.g., diabetic, weak immune system) should avoid close contact with potential bTB-infected cattle.

  • Farm workers should seek medical attention as soon as any TB symptoms develop.

  • People who plan to visit dairy operations in other countries (especially developing countries) should wear shoe covers while in the farm and disinfect their shoes after the visit, be mindful of any TB symptoms and follow the official protocols upon returning.

Always consult your local accredited veterinary and follow APHIS reports of bTB prevalence in your area, including wildlife. For more information on bTB, visit the National Tuberculosis Eradication Program, of the APHIS - USDA.

References omitted but are available upon request. Click here to email an editor.

Leslie J. Verteramo is a research associate at Cornell University. Email Leslie J. Verteramo.