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Mycobacterium tuberculosis |
This is the classic agent of human tuberculosis. Human beings
are the reservoir for this species of Mycobacterium, although
animals can be infected as a "reverse zoonosis."
(See below)
M. tuberculosis disease in people:
Primary infection with M. tuberculosis occurs by inhalation
of the organism in droplets that are aerosolized by an infected
person. The organism replicates initially in cells of the terminal
airways, after which it is taken up by and replicates in alveolar
macrophages. Macrophages distribute the organism to other areas
of the lungs and the regional lymph nodes. Once a cell-mediated
hypersensitivity immune response develops, replication of the
organism decreases and the bacteria become restricted to developing
granulomas.
- This initial phase of the infection may be subclinical.
Reactivation of infection occurs if the cellular immune response
is weakened. The organism begins to replicate again, and foci
of infection undergo caseation and cavitation, with release of
the organism into the bronchial tree and shedding in the sputum.
- Reactivation occurs in about 5% of infected people overall,
but in 50-100% of AIDS patients that are not maintained on anti-TB
drugs.
- Reactivation may lead to progressive pulmonary TB (cough,
hemoptysis and chronic debilitation/weight loss) and/or extrapulmonary
TB.
- TB of the larynx is of great concern because these patients
are highly infectious for other people simply through talking.
Diagnosis of M. tuberculosis in humans:
- intradermal injection of tuberculin (TB skin test) (DTH test)
- may be negative in cases of advanced disease or in AIDS patients
- chest radiograph for calcified or cavitating lesions typical
of TB
- acid fast stain of the sputum
- The presence of organisms in the sputum is indicative of
active infection.
Can M. tuberculosis infect animals?:
Humans are the ultimate reservoir for M. tuberculosis,
but animals can occasionally be infected by exposure to a person
who is shedding the organism, hence a "reverse zoonosis."
- Infection of cattle with M. tuberculosis can cause
false (+) skin testing for M. bovis, but cattle generally
do not become clinically sick when infected with M. tuberculosis.
- Pigs can be infected by eating food scraps from the household
of an infected person. Mycobacterium tuberculosis infection
in pigs leads to granulomas in the GI tract and associated lymph
nodes.
- Dogs develop granulomas in various parts of the body and,
if the organism is replicating in the pharynx, can transmit the
organism on to other people!
- Non-human primates develop classic TB as in humans.
- Birds can develop cutaneous granulomas due to M. tuberculosis
infection.
- Three outbreaks of TB have occurred in recent years among
mongooses and suricates/meerkats in Botswana. The animals probably
become infected when searching through trash piles from human
dwellings. Both indigenous people and tourists (a worrisome side-effect
of eco-tourism) may be sources of infection for these animals.
- Approximately 3.3% of the captive Asian elephants in the
U.S. have been infected, presumably from infected handlers in
their countries of origin. Since 1998, USDA has required that
all elephants be tested yearly, and recommends that their handlers
also be tested each year.
