What is Tuberculosis?
Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis. Once a person has been infected and if they are not treated, the bacterium can persist for a long time.

Nearly a third of the world's population (2 billion people) are infected with M. tuberculosis. Eight million people develop TB disease every year and an estimated 2-3 million people (1 every 15 seconds) die from TB each year. The significant toll of TB on human life prompted the World Health Organization to declare TB a "global emergency" in 1993.

When a person first gets infected, (by breathing in the bacteria) the TB multiply in the small air sacs of the lungs. A small number enter the bloodstream and spread throughout the body. Depending on the infected person's immune system, the infection can remain latent (dormant), or it can develop into active TB disease quite quickly. A small proportion of people who get infected are able to kill off the infection naturally. About 5-10% of people who have latent TB infection will develop the disease at some time in their lives, but the risk is greatest in the first two year's after infection. If the body is unable to kill off the bacteria the patient can be successfully treated with long courses of multiple antibiotics.

Active TB disease usually occurs in the lungs (pulmonary TB), but it can also occur in other parts of the body. This is known as extrapulmonary (outside the lungs) TB when the bacteria are carried to other parts of the body where they multiply and cause disease. Around 40% of all TB is extrapulmonary. 

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How is TB spread?
Human TB is passed from person to person through the air. People with active pulmonary (lung) TB spray the bacteria into the air when they cough, sneeze, talk, or otherwise expel air, dispersing droplets containing M. tuberculosis. People nearby can breathe the infectious droplets into their lungs and become infected. Infection usually requires prolonged (8 hours or more) contact with a person who is expelling TB bacteria into the air.

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What is latent TB infection?
When most people become infected with M. tuberculosis their own immune system limits the multiplication of the bacteria preventing the illness from developing. A small proportion of people who get infected are able to kill off the infection naturally. However, in many people the bacteria are not killed but are kept under control by the body's immune system. These people do not have any symptoms and cannot spread the disease. The infection is said to be latent (dormant). People can have latent TB for many years and often for a large part of their life without any sign of the disease. However, latently infected people can develop active TB at any time especially if their immune systems are weakened allowing the bacteria to multiply. If diagnosed, latent TB can be treated with antibiotics to prevent progression to active TB. The T-SPOT.TB^® test is designed to detect latent TB infection.

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What is active TB disease?
Active TB disease occurs when a person's immune system cannot prevent the M. tuberculosis bacteria from multiplying in the body. Conversion to TB disease is most common within the first 2 years of infection, but it can be delayed for many years. If infected people are, or become immunosuppressed (their immune system is weakened as a result of certain illness or medical treatment) they are less able to control the infection so conversion/reactivation to active disease is more likely. The following conditions may trigger reactivation to active disease:

• HIV infection
• certain medical treatments including:
  • cancer therapy
  • rheumatoid arthritis therapy (using anti-TNF α agents)
  • drugs associated with organ transplants (which suppress the immune system)
  • high-dose corticosteroid therapy
• cancer
• leukemia or Hodgkin's disease
• severe renal (kidney) disease
• drug or alcohol abuse
• diabetes mellitus (where the body is unable to properly use sugar from the diet)
• silicosis (lung disease caused by inhaling certain types of dust)
• severe weight loss or malnourishment

About 60% of TB disease is pulmonary (in the lungs) which is highly infectious since coughing, sneezing and even talking will spray M. tuberculosis bacteria into the air. If it is left untreated, it can cause serious illness and possibly death. TB can also occur in other organs especially in the brain, spine, lymph glands, and kidneys. Extrapulmonary TB can be very dangerous, particularly tubercular meningitis in children. TB is usually curable with effective antibiotic treatment. Usually a combination of four drugs is recommended for initial treatment. The choice of drugs used and the length of treatments will vary.

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What are the symptoms of active TB disease?
The symptoms will depend on where the TB bacteria are active and growing. The following symptoms are usually seen in active pulmonary (lung) TB disease:

• bad cough lasting longer than 2 weeks
• pain in the chest
• coughing up blood or sputum
• weight loss
• poor appetite
• weakness or fatigue 
• chills
• fever
• night sweats 

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Is there a test for latent TB infection?
Until recently the only way of testing for latent TB was through the use of the Tuberculin Skin Test (TST). This test is over one-hundred years old and involves injecting a small amount of killed TB bacteria into the skin and measuring any bump that forms 2-3 days later. The test can often be inaccurate, particularly in those people who have been vaccinated against TB (with the BCG vaccine) or people who have a suppressed immune system. These inaccuracies can occur in up to half of the skin tests done. The usual skin test used is called the Mantoux test.

More recently the T-SPOT.TB test has been introduced which uses a special technique of enumerating the type of cells (T cells) which fight infection in the body. The test uses a small sample of blood to test for TB infection. The T-SPOT.TB test is highly sensitive and specific and overcomes the many problems associated with the tuberculin skin test.

- Result is available next day (as opposed to 2-3 days later with TST)
- Requires only 1 visit to a doctor or nurse (as opposed to 2 with TST)

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What is the Tuberculin Skin Test (TST)?
The tuberculin skin test was developed over one-hundred years ago and is a test for TB infection.  It is also known as the Mantoux test. The TST uses a small needle to inject Purified Protein Derivative (PPD), also sometimes called 'tuberculin', into the arm just under the skin. Tuberculin is a mixed protein preparation, which is produced from killed TB bacteria. If a lump (known as an induration) appears at the site of the injection 48-72 hours after the injection, it is measured, and if it is large enough the test is considered to be positive. To increase the accuracy of the test, the size of the lump that is considered positive, is different for different groups of people, depending on whether they have other illnesses or are on certain medical treatments.

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What is BCG (Bacillus Calmette-Guerin) vaccination?
The BCG vaccine contains a living, weakened strain of Mycobacterium bovis - the TB which affects cattle. Normally it does not cause illness in humans, but it is similar enough to Mycobacterium tuberculosis to offer some protection against TB infection in children (but has very limited protective value in adults). BCG vaccinated people may produce a positive Tuberculin Skin Test (TST), even if they are not infected with TB. This is a common cause of false-positive TST results.

The search for more effective vaccines against TB remains a key public health priority; however, despite much promising work, it will still be many years before a new improved vaccine becomes available.

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Are there any other kinds of Mycobacteria that cause disease?
Other bacteria that are closely related to Mycobacterium tuberculosis can cause infections. One such organism, Mycobacterium avium, can cause a serious illness in people with weakened immune systems. However, people with a healthy immune system will usually be able to control the M. avium infection.

M. avium infections are relatively common and as the organism is so similar to M. tuberculosis their presence can give a false positive result in the tuberculin skin test. There is no cross-reactivity between the T-SPOT.TB test and M. avium.  Besides M. avium, there are a number of less common mycobacterial species; however, these rarely cause clinical disease.

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How important is TB treatment?
Treatment is very important. Treatment for TB involves multiple antibiotics and is usually for six to nine months. It is extremely important that people with TB do not stop their treatment early, even if they are feeling better. TB that is not treated appropriately can come back and may become resistant to drugs, making it more difficult to treat.

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Why is the treatment period so long?
Bacteria need to be actively dividing to be killed by antibiotics. The replication cycle of Mycobacterium tuberculosis is very slow and so a lengthy treatment is required to ensure that all of the bacteria are killed. If the treatment is inconsistent or too short, some bacteria may survive, potentially allowing TB disease to reactivate. There is also the potential for the bacteria to develop resistance to the drugs, a particularly dangerous form of TB.

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What should I do if I think I might have TB?
Contact your doctor. 

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