| Tuberculosis (abbreviated as TB for | | | | the prevalence of M. microti infections |
| Tubercle Bacillus) is a common and | | | | has been underestimated. Other |
| deadly infectious disease caused by the | | | | pathogenic mycobacteria are known, such |
| bacterium Mycobacterium tuberculosis, | | | | as Mycobacterium avium and M. kansasii; |
| which most commonly affects the lungs | | | | these are part of the group defined as |
| (pulmonary TB) but can also affect the | | | | Nontuberculous mycobacteria (NTM). |
| central nervous system, lymphatic | | | | Nontuberculous mycobacteria are |
| system, circulatory system, | | | | mycobacteria that are not part of the M. |
| genitourinary system, bones and joints. | | | | tuberculosis complex, but cause |
| Over one third of the world's population | | | | pulmonary diseases resembling |
| now has the TB bacterium in their bodies | | | | tuberculosis. |
| and new infections are occurring at a | | | | Transmission |
| rate of one per second. Not everyone who | | | | TB is spread by aerosol droplets |
| is infected develops the disease and | | | | expelled by people with the active |
| asymptomatic latent TB infection is most | | | | disease of the lungs when they cough, |
| common. However, one in ten latent | | | | sneeze, speak, or spit. These infectious |
| infections will progress to active TB | | | | droplets are 0.5 to 5 µm in diameter |
| disease which, if left untreated, will | | | | and about 40,000 can be produced by a |
| kill more than half of its victims. In | | | | single sneeze. People with prolonged, |
| 2004, 14.6 million people had active TB | | | | frequent, or intense contact are at |
| and there were 8.9 million new cases and | | | | highest risk of becoming infected, with |
| 1.7 million deaths, mostly in developing | | | | an estimated 22% infection rate. A |
| countries. A rising number of people in | | | | person with untreated, active |
| the developed world contract | | | | tuberculosis can infect 10-15 other |
| tuberculosis because their immune | | | | people per year. Others at risk include |
| systems are compromised by | | | | those from areas where TB is common, |
| immunosuppressive drugs, substance | | | | patients immunocompromised by conditions |
| abuse, or HIV/AIDS. | | | | such as HIV/AIDS, residents and |
| The rise in HIV infection levels and the | | | | employees of high-risk congregate |
| neglect of TB control programs have | | | | settings, health care workers who serve |
| caused a resurgence of tuberculosis, and | | | | high-risk clients, medically |
| drug-resistant strains of TB are also | | | | underserved, low-income populations, |
| emerging. The World Health Organization | | | | high-risk racial or ethnic minority |
| declared TB a global health emergency in | | | | populations, children exposed to adults |
| 1993, and the Stop TB Partnership | | | | in high-risk categories, and people who |
| proposed a Global Plan to Stop | | | | inject illicit drugs. |
| Tuberculosis which aims to save 14 | | | | Transmission can only occur from people |
| million lives between 2006 and 2015. | | | | with active—not latent— TB disease. |
| In the past, tuberculosis was called | | | | The probability of transmission from one |
| Consumption, because it seemed to | | | | person to another depends upon the |
| consume people from within with bloody | | | | quantity of the infectious droplets |
| cough, fever, pallor, and long | | | | expelled by the patient, the |
| relentless wasting. Other names included | | | | effectiveness of ventilation, the |
| Phthisis (Greek for consumption) and | | | | duration of exposure, and the virulence |
| phthisis pulmonalis; Scrofula, affecting | | | | of the Mycobacterium tuberculosis |
| the lymphatic system and resulting in | | | | strain. The chain of transmission can |
| swollen neck glands; Tabes mesenterica, | | | | therefore be broken by isolating |
| TB of the abdomen and Lupus vulgaris, TB | | | | patients with active disease and |
| of the skin; Wasting disease; White | | | | starting effective anti-tuberculous |
| plague, because sufferers appear | | | | therapy. |
| markedly pale; King's evil, because it | | | | About 90% of those infected with |
| was believed that a king's touch would | | | | Mycobacterium tuberculosis have |
| heal scrofula; and Pott's disease of the | | | | asymptomatic, latent TB infection |
| spine and joints. Miliary TB is an | | | | (sometimes called LTBI), with only a 10% |
| archaic term that is still occasionally | | | | lifetime chance that a latent infection |
| used, and is when the infection invades | | | | will progress to TB disease. However, if |
| the circulatory system resulting in | | | | untreated, the death rate for these |
| x-ray lesions with the appearance of | | | | active TB cases is more than 50%. |
| millet seeds. This form of TB is now | | | | TB infection begins when the |
| more commonly named disseminated TB. | | | | mycobacteria reach the pulmonary |
| Symptoms | | | | alveoli, where they invade and replicate |
| In the patients where TB becomes an | | | | within alveolar macrophages. The primary |
| active disease, 75% of these cases | | | | site of infection in the lungs is called |
| affect the lungs, where the disease is | | | | the Ghon focus. Bacteria are picked up |
| called pulmonary TB. Symptoms include a | | | | by dendritic cells, which do not allow |
| productive, prolonged cough of more than | | | | replication, although these cells can |
| three weeks duration, chest pain, and | | | | transport the bacilli to local |
| coughing up blood. Systemic symptoms | | | | (mediastinal) lymph nodes. Further |
| include fever, chills, night sweats, | | | | spread is through the bloodstream to the |
| appetite loss, weight loss, and easy | | | | more distant tissues and organs where |
| fatigability. | | | | secondary TB lesions can develop in lung |
| When the infection spreads out of the | | | | apexes, peripheral lymph nodes, kidneys, |
| lungs, extrapulmonary sites include the | | | | brain, and bone. |
| pleura, central nervous system in | | | | Tuberculosis is classified as one of the |
| (meningitis), lymphatic system in | | | | granulomatous inflammatory conditions. |
| (scrofula of the neck), genitourinary | | | | Macrophages, T lymphocytes, B |
| system in urogenital tuberculosis, and | | | | lymphocytes and fibroblasts are among |
| bones and joints in Pott's disease of | | | | the cells that aggregate to form a |
| the spine. An especially serious form is | | | | granuloma, with lymphocytes surrounding |
| disseminated, or miliary tuberculosis. | | | | the infected macrophages. The granuloma |
| Extrapulmonary forms are more common in | | | | functions not only to prevent |
| immunosuppressed persons and in young | | | | dissemination of the mycobacteria, but |
| children. Infectious pulmonary TB may | | | | also provides a local environment for |
| co-exist with extrapulmonary TB, which | | | | communication of cells of the immune |
| is not contagious. | | | | system. Within the granuloma, T |
| Bacterial species | | | | lymphocytes (CD4+) secrete cytokines |
| The cause of tuberculosis, Mycobacterium | | | | such as interferon gamma, which |
| tuberculosis (MTB), is a slow-growing | | | | activates macrophages to destroy the |
| aerobic bacterium that divides every 16 | | | | bacteria with which they are infected. T |
| to 20 hours; this is extremely slow | | | | lymphocytes (CD8+) can also directly |
| compared to other bacteria that have | | | | kill infected cells. |
| division times measured in minutes. In | | | | Importantly, bacteria are not always |
| contrast, one of the fastest growing | | | | eliminated within the granuloma, but can |
| bacteria is a strain of E. coli that can | | | | become dormant, resulting in a latent |
| divide roughly every 20 minutes. MTB is | | | | infection. Another feature of the |
| not classified as either Gram-positive | | | | granulomas of human tuberculosis is the |
| or Gram-negative because it does not | | | | development of cell death, also called |
| have the chemical characteristics of | | | | necrosis, in the center of tubercles. To |
| either. If a Gram stain is performed, | | | | the naked eye this has the texture of |
| MTB either stains very weakly | | | | soft white cheese and was termed caseous |
| Gram-positive, or not at all. MTB is a | | | | necrosis. |
| small rod-like bacillus which can | | | | If TB bacteria gain entry to the |
| withstand weak disinfectants and can | | | | bloodstream from an area of tissue |
| survive in a dry state for weeks. | | | | damage they spread through the body and |
| Normally, the bacteria can only grow | | | | set up many foci of infection, all |
| within a host organism, so in vitro | | | | appearing as tiny white tubercles in the |
| culture of M. tuberculosis took a long | | | | tissues. This severe form of TB disease |
| time to develop, but is now a routine | | | | is most common in infants and the |
| laboratory procedure. | | | | elderly and is called miliary |
| MTB is identified microscopically by its | | | | tuberculosis. Patients with this |
| staining characteristics: it retains | | | | disseminated TB have a fatality rate of |
| certain stains after being treated with | | | | approximately 20%, even with intensive |
| acidic solution, and is thus classified | | | | treatment. |
| as an "acid-fast bacillus" or AFB. In | | | | In many patients the infection waxes and |
| the most common staining technique, the | | | | wanes. Tissue destruction and necrosis |
| Ziehl-Neelsen stain, AFB are stained a | | | | are balanced by healing and fibrosis. |
| bright red which stands out clearly | | | | Affected tissue is replaced by scarring |
| against a blue background. Acid-fast | | | | and cavities filled with cheese-like |
| bacilli can also be visualized by | | | | white necrotic material. During active |
| fluorescent microscopy, and by an | | | | disease, some of these cavities are |
| auramine-rhodamine stain. | | | | joined to the air passages bronchi and |
| The M. tuberculosis complex includes 3 | | | | this material can be coughed up. It |
| other mycobacteria which can cause | | | | contains living bacteria and can |
| tuberculosis: M. bovis, M. africanum and | | | | therefore pass on infection. Treatment |
| M. microti. The first two are very rare | | | | with appropriate antibiotics kills |
| causes of disease in immunocompetent | | | | bacteria and allows healing to take |
| people, and M. microti is not usually | | | | place. Upon cure, affected areas are |
| pathogenic, although it is possible that | | | | eventually replaced by scar tissue |