Introduction/Causative Agent: Tuberculosis (TB) in cattle is a disease that is being eradicated in the United States. Due to state and federal cooperation, the number of cattle infected in the United States has dropped from 5 percent to 0.02 percent. Tuberculosis in cattle is usually caused by the bacterium Mycobacterium bovis. Mycobacterium bovis infections occur most commonly in cattle, but can be found in any other warm-blooded animal. No other tuberculosis bacterium has as great a host range as Mycobacterium bovis, and it is easily transmitted to humans.
Clinical Signs: Some animals may appear fine until slaughter, then the carcass is discovered to be so infected that it must be condemned. Other animals may show signs of weight loss, fever, enlarged lymph nodes, diarrhea, and breathing problems (dyspnea, tachypnea). The signs noticed will vary depending on what parts of the body are involved with the infection. Lesions or abscesses (necrotic granulomatous lesions) can form in almost any organ or body cavity of an infected animal. They may be found especially in the lungs, lymph nodes, and intestinal tract. They can also grow in the abdominal organs, reproductive organs, nervous system, and on bones. As the disease progresses, the lesions grow in size.
Disease Transmission: The most common route of transmission is by inhaling the bacteria that have been expelled by an infected animal. Other infections can occur when an animal ingests contaminated materials. Exhaled fluid droplets, feces, milk, semen, and vaginal discharge can contain the bacteria and cause infection. Young animals and even humans can contract the disease by drinking raw or unpasteurized milk from infected cows.
Animals are more at risk of contracting TB when they are housed in an enclosed area, such as a barn. Farms that have high numbers of animals in confined areas are also at an increased risk of disease spread. The Mycobacterium bovis organisms do not grow outside of a host and survive only a few weeks outside of the body.
Diagnosis: Several diagnostic tests, often used together, help to identify an infected animal. Some of the most common tests include the caudal fold tuberculin test, comparative cervical tuberculin test, necropsy, histopathology, acid fast staining, culture, and polymerase chain reaction.
When performing the caudal fold tuberculin test, 0.1 mL of tuberculin is placed in the caudal tail fold by a veterinarian. In about 72 hours the same veterinarian will come back and "read" the results of the initial injection. Animals that have been exposed to TB will respond to this test by producing a swelling reaction at the point of injection. This animal will be identified as a "reactor" and further testing is often done by a federal veterinarian.
Treatment: Identification of infected animals and slaughter of those that test positive is the only acceptable way to control this disease. Local veterinarians are routinely involved in testing of animals for TB. State and federal meat inspectors check the glands and organs of cattle in slaughter plants. If any infected carcasses are found, the source herd is identified and any infected animals are usually destroyed. In humans, the treatment is 95 percent effective and takes from 6 to 9 months.
Prevention: Maintaining a closed herd is one of the best ways to prevent this disease. If this is not possible, try to purchase animals from TB-free herds and obtain health histories of all animals purchased. Isolate all new animals until they can be confirmed TB-free. New animals should be tested upon arrival and again at 60 days. Maintaining good fencing will help prevent animals from mingling with neighboring animals.
If an animal tests positive for TB, it should be slaughtered immediately to prevent the potential spread of disease. The areas where the infected animal resided should also be cleaned and disinfected. Realize that prolonged exposure to heat, direct sunlight, or dry conditions will kill the bacteria. It can live a little longer under cold, dark, and moist conditions.