Tuberculosis is a widespread, and in many cases fatal, infectious disease caused by various strains of mycobacteria, usually Mycobacterium tuberculosis.
Tuberculosis typically attacks the lungs, but can also affect other parts of the body. It is spread through the air when people who have an active TB infection cough, sneeze, or otherwise transmit respiratory fluids through the air. Most infections do not have symptoms, known as latent tuberculosis.
About one in ten latent infections eventually progresses to active disease which, if left untreated, kills more than 50% of those so infected.
The classic symptoms of active TB infection are a chronic cough with blood-tinged sputum, fever, night sweats, and weight loss (the latter giving rise to the formerly common term consumption). Infection of other organs causes a wide range of symptoms.
Diagnosis of active TB relies on radiology (commonly chest X-rays), as well as microscopic examination and microbiological culture of body fluids.
Diagnosis of latent TB relies on the tuberculin skin test (TST) and/or blood tests. Treatment is difficult and requires administration of multiple antibiotics over a long period of time. Social contacts are also screened and treated if necessary.
Antibiotic resistance is a growing problem in multiple drug-resistant tuberculosis (MDR-TB) infections. Prevention relies on screening programs and vaccination with the bacillus Calmette-Guérin vaccine.
One-third of the world's population is thought to have been infected with M. tuberculosis, with new infections occurring in about 1% of the population each year. In 2007, an estimated 13.7 million chronic cases were active globally, while in 2010, an estimated 8.8 million new cases and 1.5 million associated deaths occurred, mostly in developing countries.
The absolute number of tuberculosis cases has been decreasing since 2006, and new cases have decreased since 2002.
The rate of tuberculosis in different areas varies across the globe; about 80% of the population in many Asian and African countries tests positive in tuberculin tests, while only 5–10% of the United States population tests positive.
More people in the developing world contract tuberculosis because of a poor immune system, largely due to high rates of HIV infection and the corresponding development of AIDS.
Tuberculosis may infect any part of the body, but most commonly occurs in the lungs (known as pulmonary tuberculosis).
Extrapulmonary TB occurs when tuberculosis develops outside of the
lungs, although extrapulmonary TB may coexist with pulmonary TB, as
well.
General signs and symptoms include fever, chills, night sweats, loss of appetite, weight loss, and fatigue. Significant nail clubbing may also occur.
Scanning electron micrograph of M. tuberculosis
The main cause of TB is Mycobacterium tuberculosis, a small, aerobic, nonmotile bacillus. The high lipid content of this pathogen accounts for many of its unique clinical characteristics.
It divides every 16 to 20 hours, which is an extremely slow rate compared with other bacteria, which usually divide in less than an hour. Mycobacteria have an outer membrane lipid bilayer.
If a Gram stain is performed, MTB either stains very weakly "Gram-positive" or does not retain dye as a result of the high lipid and mycolic acid content of its cell wall.
MTB can withstand weak disinfectants and survive in a dry state for weeks. In nature, the bacterium can grow only within the cells of a host organism, but M. tuberculosis can be cultured in the laboratory
Using histological stains on expectorated samples from phlegm (also called "sputum"), scientists can identify MTB under a regular (light) microscope. Since MTB retains certain stains even after being treated with acidic solution, it is classified as an acid-fast bacillus (AFB).
The most common acid-fast staining techniques are the Ziehl–Neelsen stain, which dyes AFBs a bright red that stands out clearly against a blue background, and the auramine-rhodamine stain followed by fluorescence microscopy.
The M. tuberculosis complex (MTBC) includes four other TB-causing mycobacteria: M. bovis, M. africanum, M. canetti, and M. microti. M. africanum is not widespread, but it is a significant cause of tuberculosis in parts of Africa.
M. bovis was once a common cause of tuberculosis, but the introduction of pasteurized milk has largely eliminated this as a public health problem in developed countries.
M. canetti is rare and seems to be limited to the Horn of Africa, although a few cases have been seen in African emigrants. M. microti is also rare and is mostly seen in immunodeficient people, although the prevalence of this pathogen has possibly been significantly underestimated.
Other known pathogenic mycobacteria include M. leprae, M. avium, and M. kansasii. The latter two species are classified as "nontuberculous mycobacteria" (NTM). NTM cause neither TB nor leprosy, but they do cause pulmonary diseases that resemble TB.
This is a particular problem in sub-Saharan Africa, where rates of HIV are high. Of people without HIV who are infected with tuberculosis, about 5–10% develop active disease during their lifetimes; in contrast, 30% of those coinfected with HIV develop the active disease.
Tuberculosis is closely linked to both overcrowding and malnutrition, making it one of the principal diseases of poverty.
Those at high risk thus include: people who inject illicit drugs, inhabitants and employees of locales where vulnerable people gather (e.g. prisons and homeless shelters), medically underprivileged and resource-poor communities, high-risk ethnic minorities, children in close contact with high-risk category patients, and health-care providers serving these patients.
Chronic lung disease is another significant risk factor. Silicosis increases the risk about 30-fold.Those who smoke cigarettes have nearly twice the risk of TB compared to nonsmokers.
Other disease states can also increase the risk of developing tuberculosis. These include alcoholism and diabetes mellitus (three-fold increase).
Certain medications, such as corticosteroids and infliximab (an anti-αTNF monoclonal antibody) are becoming increasingly important risk factors, especially in the developed world.
Also a genetic susceptibility element exists, for which the overall importance remains undefined.When people with active pulmonary TB cough, sneeze, speak, sing, or spit, they expel infectious aerosol droplets 0.5 to 5.0 µm in diameter.
A single sneeze can release up to 40,000 droplets. Each one of these droplets may transmit the disease, since the infectious dose of tuberculosis is very small (the inhalation of fewer than 10 bacteria may cause an infection).
People with prolonged, frequent, or close contact with people with TB are at particularly high risk of becoming infected, with an estimated 22% infection rate. A person with active but untreated tuberculosis may infect 10–15 (or more) other people per year.
Transmission should only occur from people with active TB - those with latent infection are not thought to be contagious.
The probability of transmission from one person to another depends upon several factors, including the number of infectious droplets expelled by the carrier, the effectiveness of ventilation, the duration of exposure, the virulence of the M. tuberculosis strain, the level of immunity in the uninfected person, and others.
The cascade of person-to-person spread can be circumvented by effectively segregating those with active ("overt") TB and putting them on anti-TB drug regimens. After about two weeks of effective treatment, subjects with nonresistant active infections generally do not remain contagious to others.
If someone does become infected, it typically takes three to four weeks before the newly infected person becomes infectious enough to transmit the disease to others.
Tuberculosis typically attacks the lungs, but can also affect other parts of the body. It is spread through the air when people who have an active TB infection cough, sneeze, or otherwise transmit respiratory fluids through the air. Most infections do not have symptoms, known as latent tuberculosis.
About one in ten latent infections eventually progresses to active disease which, if left untreated, kills more than 50% of those so infected.
The classic symptoms of active TB infection are a chronic cough with blood-tinged sputum, fever, night sweats, and weight loss (the latter giving rise to the formerly common term consumption). Infection of other organs causes a wide range of symptoms.
Diagnosis of active TB relies on radiology (commonly chest X-rays), as well as microscopic examination and microbiological culture of body fluids.
Diagnosis of latent TB relies on the tuberculin skin test (TST) and/or blood tests. Treatment is difficult and requires administration of multiple antibiotics over a long period of time. Social contacts are also screened and treated if necessary.
Antibiotic resistance is a growing problem in multiple drug-resistant tuberculosis (MDR-TB) infections. Prevention relies on screening programs and vaccination with the bacillus Calmette-Guérin vaccine.
One-third of the world's population is thought to have been infected with M. tuberculosis, with new infections occurring in about 1% of the population each year. In 2007, an estimated 13.7 million chronic cases were active globally, while in 2010, an estimated 8.8 million new cases and 1.5 million associated deaths occurred, mostly in developing countries.
The absolute number of tuberculosis cases has been decreasing since 2006, and new cases have decreased since 2002.
The rate of tuberculosis in different areas varies across the globe; about 80% of the population in many Asian and African countries tests positive in tuberculin tests, while only 5–10% of the United States population tests positive.
More people in the developing world contract tuberculosis because of a poor immune system, largely due to high rates of HIV infection and the corresponding development of AIDS.
Signs and symptoms
General signs and symptoms include fever, chills, night sweats, loss of appetite, weight loss, and fatigue. Significant nail clubbing may also occur.
Causes
Mycobacteria
It divides every 16 to 20 hours, which is an extremely slow rate compared with other bacteria, which usually divide in less than an hour. Mycobacteria have an outer membrane lipid bilayer.
If a Gram stain is performed, MTB either stains very weakly "Gram-positive" or does not retain dye as a result of the high lipid and mycolic acid content of its cell wall.
MTB can withstand weak disinfectants and survive in a dry state for weeks. In nature, the bacterium can grow only within the cells of a host organism, but M. tuberculosis can be cultured in the laboratory
Using histological stains on expectorated samples from phlegm (also called "sputum"), scientists can identify MTB under a regular (light) microscope. Since MTB retains certain stains even after being treated with acidic solution, it is classified as an acid-fast bacillus (AFB).
The most common acid-fast staining techniques are the Ziehl–Neelsen stain, which dyes AFBs a bright red that stands out clearly against a blue background, and the auramine-rhodamine stain followed by fluorescence microscopy.
The M. tuberculosis complex (MTBC) includes four other TB-causing mycobacteria: M. bovis, M. africanum, M. canetti, and M. microti. M. africanum is not widespread, but it is a significant cause of tuberculosis in parts of Africa.
M. bovis was once a common cause of tuberculosis, but the introduction of pasteurized milk has largely eliminated this as a public health problem in developed countries.
M. canetti is rare and seems to be limited to the Horn of Africa, although a few cases have been seen in African emigrants. M. microti is also rare and is mostly seen in immunodeficient people, although the prevalence of this pathogen has possibly been significantly underestimated.
Other known pathogenic mycobacteria include M. leprae, M. avium, and M. kansasii. The latter two species are classified as "nontuberculous mycobacteria" (NTM). NTM cause neither TB nor leprosy, but they do cause pulmonary diseases that resemble TB.
Risk factors
A number of factors make people more susceptible to TB infections. The most important risk factor globally is HIV; 13% of all people with TB are infected by the virus.This is a particular problem in sub-Saharan Africa, where rates of HIV are high. Of people without HIV who are infected with tuberculosis, about 5–10% develop active disease during their lifetimes; in contrast, 30% of those coinfected with HIV develop the active disease.
Tuberculosis is closely linked to both overcrowding and malnutrition, making it one of the principal diseases of poverty.
Those at high risk thus include: people who inject illicit drugs, inhabitants and employees of locales where vulnerable people gather (e.g. prisons and homeless shelters), medically underprivileged and resource-poor communities, high-risk ethnic minorities, children in close contact with high-risk category patients, and health-care providers serving these patients.
Chronic lung disease is another significant risk factor. Silicosis increases the risk about 30-fold.Those who smoke cigarettes have nearly twice the risk of TB compared to nonsmokers.
Other disease states can also increase the risk of developing tuberculosis. These include alcoholism and diabetes mellitus (three-fold increase).
Certain medications, such as corticosteroids and infliximab (an anti-αTNF monoclonal antibody) are becoming increasingly important risk factors, especially in the developed world.
Also a genetic susceptibility element exists, for which the overall importance remains undefined.When people with active pulmonary TB cough, sneeze, speak, sing, or spit, they expel infectious aerosol droplets 0.5 to 5.0 µm in diameter.
A single sneeze can release up to 40,000 droplets. Each one of these droplets may transmit the disease, since the infectious dose of tuberculosis is very small (the inhalation of fewer than 10 bacteria may cause an infection).
People with prolonged, frequent, or close contact with people with TB are at particularly high risk of becoming infected, with an estimated 22% infection rate. A person with active but untreated tuberculosis may infect 10–15 (or more) other people per year.
Transmission should only occur from people with active TB - those with latent infection are not thought to be contagious.
The probability of transmission from one person to another depends upon several factors, including the number of infectious droplets expelled by the carrier, the effectiveness of ventilation, the duration of exposure, the virulence of the M. tuberculosis strain, the level of immunity in the uninfected person, and others.
The cascade of person-to-person spread can be circumvented by effectively segregating those with active ("overt") TB and putting them on anti-TB drug regimens. After about two weeks of effective treatment, subjects with nonresistant active infections generally do not remain contagious to others.
If someone does become infected, it typically takes three to four weeks before the newly infected person becomes infectious enough to transmit the disease to others.
