MYCOBACTERIUM INFECTIONS (TUBERCULOSIS)
Tuberculosis (TB) is the most common bacterial cause of death in the world. Worldwide, it is the leading cause of death in HIV/AIDS patients. In the United States cases of TB have decreased to an all-time low, but cases are reported every year.
Other names for TB include consumption (the disease seemed to consume people from within), red death (for the bloody vomiting often seen), wasting disease, white plague (infected people become very pale), prosector’s wart (skin TB, transferred by contact with contaminated carcasses), and Koch’s disease (named for Robert Koch, who discovered the TB organism). Edgar Allen Poe wrote a short story, “The Masque of the Red Death,” which deals with TB.
Bacteria belonging to the genus Mycobacterium are responsible for TB. The three most important species are M. bovis (reservoirs are cattle, dogs, and pigs), M. avium (reservoirs are birds, pigs, and sheep), and M. tuberculosis (human reservoir). Nonzoonotic Mycobacterium leprae causes human leprosy.
Mycobacterium organisms are slow-growing, gram-positive, acid-fast, aerobic bacilli.
Humans are the ultimate reservoir for M. tuberculosis, but other animals, such as nonhuman primates, cattle, dogs, pigs, and psittacine birds, can become infected by a process known as reverse zoonosis. Cattle will test positive for TB when infected with M. tuberculosis, but generally will not become clinically ill. Pigs can become infected by eating table scraps from the table of an infected person. Granulomatous lesions develop in the gastrointestinal tract and associated lymph nodes. Dogs can develop granulomas in any part of the body, but if the pharynx is infected, the organism can be transferred back to people. Birds develop skin granulomas when infected with M. tuberculosis. Asian elephants in the United States have tested positive for M. tuberculosis and are presumably infected by their handlers. Elephants and handlers have to be tested annually.
M. avium is becoming a concern because of its increased appearance in immunocompromised people, especially HIV/AIDS patients. Birds, pigs and sheep are naturally susceptible to M. avium infection. Cattle, dogs, and cats appear to be resistant.
M. bovis is found primarily in cattle and buffalo, but will infect dogs, pigs, and humans.
The Mycobacterium species that cause TB are spread by aerosol droplets, contact with infected animals, contact with contaminated surfaces, and ingestion. M. bovis can be transmitted to people through unpasteurized milk and products made with unpasteurized milk. Aerosol droplets can be dispersed through talking, singing, laughing, coughing, or sneezing.