Tuberculosis, an infectious disease caused by bacteria of the genus -Mycobacterium, is rare in the United States but is considered a re-emerging infectious disease. Most infections in horses are caused by Mycobacterium bovis and are likely acquired by close contact with infected cattle. The disease affects practically all species of vertebrates, and, before control measures were adopted, was an important disease of humans and domestic animals. Signs and lesions are generally similar in the various species.
Three main types of tubercle bacilli are: human (Mycobacterium tuberculosis), bovine (M. bovis), and avian (M. avium). The 2 mammalian types are more closely related to each other than to the avian type. All types may produce infection in host species other than their own. Ingestion (particularly of contaminated feed) occurs, especially with M. avium. The breathing in of infected droplets expelled from the lungs of an infected person or animal may also result in infection.
The signs reflect the extent and location of lesions. Generalized signs include progressive emaciation, lethargy, weakness, loss of appetite, and fever. Horses are relatively resistant to M. tuberculosis but are susceptible to M. bovis. Horses infected with M. bovis often have signs related to the respiratory form of the disease, including coughing and difficulty breathing. Infection may be found in the liver, abdominal lymph nodes, lungs, and other sites.
M. bovis may be transmitted to humans and other animals and represents a public health risk. If a horse is suspected of having advanced tuberculous lesions, it must be reported to the appropriate public health authorities. Treatment of a horse with tuberculosis should be discussed with your veterinarian.