Pathophysiology of Tuberculosis Disease

Tuberculosis (TB), formerly called consumption, infects one-third of the world's population, killing three million people a year. It has been a global health problem since ancient times; 3,000-year-old Egyptian mummies show signs of damage from TB.

Cause

• TB is caused by members of the genus Mycobacterium, especially Mycobacterium tuberculosis. Mycobacteria have waxy cell walls that are difficult to Gram stain, and these protect the bacteria and help them live outside the host. Mycobacteria are highly resistant to drying and can remain viable for six to eight months in dried sputum.

Infection

• When the patient inhales a droplet of tuberculosis or particle of dried sputum, the organisms are phagocytized (or "eaten") by the white blood cells of the immune system, but this does not kill them. Inside the white blood cells, the living microbes slowly multiple until the white blood cells burst.

In the Lungs

• After bursting from the white blood cells, if the tuberculosis is in the lungs, an acute inflammatory response follows, causing pneumonia-like symptoms. The bacteria form lesions in the lungs, which solidify into nodules called tubercles. If the tubercles are near blood vessels, they can perforate the vessels and cause hemorrhage, resulting in blood-tinged spit.

Other Locations

• Disseminated tuberculosis, frequently seen in AIDS patients, can be anywhere in the body, and the infected cells become like casts. The tubercles multiply in a cell, destroying the organelles inside, and a clump of microbes in the shape of the former cell remains. Tuberculosis can also infect the bones or the intestine through drinking unpasteurized milk, though intestinal tuberculosis can more likely be caused by swallowing infected sputum.

Treatment

• TB is treated with isoniazid and rifapin for at least a year. There is a vaccine called BCG that is used in the developing world, but it has not been approved for use in the United States.