Tuberculosis in Cattle
Tuberculosis of cattle (bovine TB) was once common in US cattle herds; damaging the health of cattle and, serving as an important source of human disease in the US. That was long ago. In 1917 the US began a program to protect public health by eliminating bovine TB from cattle and pasteurizing milk. This program has made bovine TB an extremely rare infection of cattle, and humans in the US today. Unfortunately bovine TB is still a reality in some countries; even as close as Mexico. World-wide, bovine TB remains a hazard to cattle health, and the organism still contributes to the total number of TB cases in humans.
The agent of tuberculosis in cattle is Mycobacterium bovis, an organism that infects mainly cattle, but can be transmitted to humans by ingestion of unpasteurized milk products, through broken skin, by aerosol inhalation, and rarely by eating undercooked contaminated meat. In humans, disease caused by M. bovis appears similar to Mycobacterium tuberculosis (human tuberculosis) infection. All cattle are susceptible to the organism, but dairy herds may be particularly at risk because of confinement conditions which favor transmission. Cattle transmit the disease among themselves mainly through direct contact and via inhalation; pre-weaned calves may also contract the disease through contaminated milk, and there are wildlife vectors.
Human infection with M. bovis is very rare today in the United States (most human TB cases are caused by M. tuberculosis, a related organism) because of pasteurization and meat inspection practices, but that was not always the case. Because the organism spreads easily within herds, serious efforts to control infection are necessary to prevent excessive economic loss. Eradication efforts in the United States began in 1917 and have since drastically reduced the incidence of the disease, but wild animal reservoirs and human carriers make complete elimination difficult in countries all over the world. Recently several States have lost their bovine TB free status. Reemergence of cattle tuberculosis in the US can be attributed to a few things, including a few persistent low-level infections in beef and dairy herds, infected white-tailed deer populations in the north, and importation of cattle from Mexico. It is also important to remember that humans infected with bovine TB have served as the source of infection to cattle
Tuberculosis in a cattle herd can often go undetected since the early stages of infection do not produce clinical signs. In later stages, signs are lethargy, weakness, anorexia, pneumonia, fever, and chronic moist cough. Most cases of bovine TB in the US are detected by veterinary meat inspectors from apparently healthy cattle, and the animal is then traced back to the source herd. Live cattle are tested for exposure to the Mycobacterium bovis organism by an intradermal injection of the tuberculin purified protein derivative into the tail fold. The site is checked in 72 hours for an immune response (swelling), indicating previous exposure to the organism. A positive result may require more specific testing to rule out exposure to closely-related organisms (false positives). For a previously infected herd to be considered free of the disease, multiple negative herd tests will be required.
Recently, bovine TB was diagnosed in cattle from a Nebraska ranch. Information on that investigation was available on the Nebraska Department of Agriculture's Bovine Tuberculosis Updates page.
[June 20th, 2009]
Sherry Westphal, PPVM Veterinary Student
University of Nebraska-Lincoln