Three Types of Tuberculosis Infection
Primary, Secondary Reactivated and Disseminated TB
An abbreviation for Tubercle bacillus, TB is the result of infection with a slow growing, wax-coated acid-fast bacterium that can remain viable in dried aerosol droplets for 8 months. Although few of those infected with the bacteria will actually contract the tuberculosis, it takes very few M. tuberculosis bacteria to start the disease process.
Transmission of Tuberculosis
The majority of TB infections are restricted to the lungs, after inhalation of respiratory droplets expelled when an infected individual coughs, sneezes or speaks. Still, it is possible for the bacteria to disseminate and cause infection in other areas of the body.
Tuberculosis Infection
Infection with Tuburcle bacillus (most often M. tuberculosis) is characterized by the formation of tubercles, hard nodules in the lungs that are the result of interaction between the bacteria and the host’s immune system. When M. tuberculosis enters the lungs, phagocytic cells of the host’s immune system, called macrophages, engulf the pathogen, but are unable to digest the bacteria due to its waxy mycolic acid cell wall.
The Mycobacteria then begin to multiply within the macrophages, eventually killing these cells that are supposed to protecting the host. The cycle continues as the bacteria released from the dead macrophages are then engulfed by other macrophages. While this immune system drama is being played out internally, the host normally shows few external signs of infection, other than a possible fever.
The infected macrophages result in an inflammatory response (heat, swelling, dilated capillaries) which attracts more macrophages until the site of infection is completely surrounded by many of these compressed phagocytic cells. Inflammation triggers other cells within the host to essentially quarantine the area by depositing collagen fibers around the packed macrophages, forming an enclosed infection within the lung called a tubercle. The cells at the center of the tubercle may eventually die, producing either an area of necrosis or an actual cavity.
Types of Tuberculosis
Tuberculosis is divided into three clinically important categories:
- Primary TB
- Secondary Reactivated TB
- Disseminated TB
Primary Tuberculosis
Primary tuberculosis refers to the infection process which eventually eliminates the pathogen or results in a salemate between the Mycobacteria and the immune system. With most TB infections, the immune system is able to contain, although not eliminate, the Mycobacteria within the tubercle, preventing the spread of bacteria and progression of the disease. M. tuberculosis can remain in this impasse of dormant infection for many years.
Secondary or Reactivated Tuberculosis
The infection can become reactivated if the Mycobacteria are able to rupture the tubercle and spread through the lungs. This reactivation typically happens to those with a weakened or supressed immune system.
Disseminated Tuberculosis
The spread of the disease within the body may result if infected macrophages moving through the blood and lymph transport the bacteria to other sites. Once infected, symptoms of disseminated TB relate to the locations infected. The antiquated term ‘consumption’ arose from the myriad of symptoms associated with disseminated tuberculosis, when those infected seemed to slowly waste away.
More Information on Infectious Disease
To learn more about this and other infectious diseases, see the Mayo Clinic Infectious Disease Center or the National Institutes of Health website. The Suite101 article What Causes a False TB Skin Test provides additional information on tuberculosis testing.
Additional Sources
CDC Website (2008) Division of Tuberculosis Elimination.
Bauman, R. (2005) Microbiology. Pearson Benjamin Cummings.
