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Tuberculosis affects both humans and animals. Cattle tuberculosis has affected animal and human health since ancient times. Not very long ago cattle and swine tuberculosis was one of the commonest diseases affecting livestock killing millions.
It is important for cattle rearers to get the “bovine tuberculosis free” accreditation because cattle serve as a reservoir for bovine tuberculosis in humans.
Cattle tuberculosis is caused by Mycobacterium bovis. The main reservoir of M. bovis infections in cattle is infected cattle. These infected cattle may also serve as the source of infection for man. Humans can contract bovine tuberculosis by eating the bacteria in raw milk and milk products.
Cattle tuberculosis also has the capability of spreading among other herbivores like deer, sheep, goats, horses, pigs, camels, llamas, tapirs, elk, elephants, rhinoceroses, opossums, ground squirrels, otters, seals, hares etc. They can also infect dogs, cats, ferrets, foxes, badgers, rats, primates, moles, raccoons, coyotes, lions, tigers, leopards, and lynx.
Tuberculosis in cattle is a chronic disease that usually takes many months to a year before the development of clinically evident infections. There is a high prevalence of latent and hidden cases as well. Most infected cattle only become clinically infected at advanced age.
Adult cattle are usually infected by inhaling invisible droplets containing the bacteria into their lungs while calves are more often infected by drinking contaminated milk. It is the mode of infection that determines the clinical signs of the disease.
In adult cows the lesions are seen in the lungs leading to pulmonary tuberculosis. The bacteria enter the lungs and begin to multiply and generally spread to the lymph nodes near the lungs. If the cattle has a good immune system the infection may remain limited to the lungs and could continue to be undetected for the productive life of the cow.
Once the infection overcomes the immune system other organs are affected. This includes kidneys, liver, spleen and the lymph nodes associated with these organs. Calves which ingest the bacteria in milk commonly develop lesions in the lymph nodes at the junction of the neck and head near their throats. They have swollen lymph nodes along the intestinal tract as well.
When cattle is infected with other bacteria similar to tuberculosis bacteria like Mycobacterium avium, they become sensitive to tuberculin testing and give false positive reactions when tests are being conducted to find cows with M. bovis infections. These other bacteria are more important as causes of infection in swine, sheep, goats and humans.