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TB Disease | Active & latent TB, how you get TB

Active TB disease & latent TB infection

TB is a bacterial disease usually caused by an organism called Mycobacterium tuberculosis. The TB bacteria usually affect the lungs, but can affect other parts of the body. If you are infected with TB bacteria you won’t necessarily become sick, because you can have either latent TB or active TB disease.1Basic TB Facts”, CDC www.cdc.gov/TB/topic/basics/default.htm

Latent TB

Latent or inactive TB infection (LTBI) occurs when a person carries the TB bacteria within their body, but the bacteria are present in very small numbers and are kept under control by the body’s immune system and do not cause disease. People with LTBI do not feel sick and do not have any symptoms of TB. It is usually only known that someone has latent TB because they have had a TB test, such as the TB skin test. People with latent TB are not infectious and cannot pass the bacteria on to other people.2Basic TB Facts”, CDC www.cdc.gov/TB/topic/basics/default.htm

Most people with latent TB never develop active TB disease and never become sick. Overall about 5 to 10% of people with latent TB who do not receive treatment for it, will develop active TB disease at some time in their lives.3“Basic TB Facts”, CDC www.cdc.gov/TB/topic/basics/risk.htm

There are though some people who are known to have a higher risk of progressing from latent TB to active TB disease.4“Guidelines for Preventing the Transmission of Mycobacterium tuberculosis in Health-Care Settings 2005″, CDC, 5 www.cdc.gov/tb/publications/guidelines/infectioncontrol.htm

These include:

  • Infants and children aged less than 4 years
  • People infected within the previous two years
  • People infected with HIV
  • People who have certain clinical conditions, or conditions which compromise their immune system, such as people with diabetes, and people with chronic renal failure.

TB disease

Active TB or TB disease occurs when the TB bacteria have started to multiply and they become numerous enough to overcome the body’s immune system or defences. Active TB disease usually causes a person to feel ill, and in certain circumstances they are able to pass the TB bacteria on to other people.5“The Difference Between Latent TB Infection and Active TB Disease”, CDC,www.cdc.gov/TB/publications/factsheets/general/LTBIandActiveTB.htm

Some people develop active TB disease soon after they have become infected, before their immune system can fight the TB bacteria. Other people don’t get sick and have active TB disease when they first become infected, they have latent TB, but they get sick and have active TB disease years later when their immune system becomes weak for another reason, such as infection with HIV, or some other health problem.

How do you get TB?

TB is spread through the air from one person to another. A person can get TB by inhaling TB bacteria that have been released into the air by a person with active TB. When a person with active TB disease of the lungs or throat coughs, sneezes, or talks, droplets containing the TB bacteria are released into the air.

People with active TB disease are not equally infectious. Generally it is only people with TB of the throat or lungs who are infectious.6“Guidelines for the Investigation of Contacts of Persons with Infectious Tuberculosis”, CDC,www.cdcnpin.org/scripts/tb/cdc.asp Also, the most important factor for people who can be infectious is whether someone is on effective TB treatment, as effective treatment dramatically reduces the number of infectious droplets released by a person with active TB disease. The strength of a person’s cough can also affect the number of droplets released. Children with TB are generally not infectious.7Starke, J “Tuberculosis in Children”, Semin Respir Crit Care Med, 2004;25(3)www.medscape.com/viewarticle/484123

TB bacteria can sometimes stay alive in the air for a few hours, especially in small places with no fresh air. Fresh air and sunlight make it harder for TB bacteria to stay alive. The fresh air scatters the bacteria and the sunlight kills them.8“Get the Facts About TB Disease”, CDC www.cdc.gov/tb/publications/pamphlets/default.htm

It is not known why some people who are exposed to TB bacteria become infected when other people don’t. The probability of becoming infected if exposed to TB bacteria, depends primarily on the concentration of infectious droplets in the air, and the duration of exposure to a person with infectious TB disease. The closer the proximity and the longer the duration of exposure, the higher the risk is of being infected.9“Guidelines for Preventing the Transmission of Mycobacterium tuberculosis in Health-Care Settings 2005″, CDC, 5 www.cdc.gov/tb/publications/guidelines/infectioncontrol.htm

TB is not transmitted:

  • through food and water,
  • or by kissing,
  • or by skin contact such as shaking hands,
  • or by touching a toilet seat,
  • or by sharing a toothbrush.

What are the symptoms of TB disease?

The symptoms of TB disease depend on which area of the body has been infected. If someone has pulmonary TB disease, which is TB in the lungs, then they may have a bad cough that lasts longer than two weeks, they may have pain in their chest and they may cough up blood or phlegm from deep inside their lungs. Other symptoms of TB disease include weakness or fatigue, weight loss, lack of appetite, chills, fever and night sweats.10“Get the Facts About TB Disease”, CDC www.cdc.gov/tb/publications/pamphlets/default.htm

It is though very difficult to diagnose TB disease via symptoms alone, because the symptoms are not exclusive to TB.

Diagnosing TB disease and latent TB infection

Electron micrograph of Mycobacterium tuberculosis bacteria, the bacteria that cause TB disease. © CDC/Dr Ray Butler

Electron micrograph of Mycobacterium tuberculosis bacteria, the bacteria that cause TB disease. © CDC/Dr Ray Butler

Whether the person has symptoms or not, TB is often difficult to diagnose. A diagnosis of active TB disease is usually only certain when there is definite evidence of TB bacteria. Some of the diagnostic tools look directly for the bacteria, although others such as the chest X-ray look for the effect of the bacteria on the person suspected of having TB disease.TB tests include the TB skin test, the IGRA tests, sputum microscopy, culture as well as the new Xpert MTB/RIF test.

Major problems with TB tests are the lack of accuracy as well as the time they take, and with newer tests the cost.

Treatment of TB disease

Active TB disease can usually be cured. The treatment consists of a combination of TB drugs that must usually be taken over at least six months. But the TB treatment will only be successful if the drugs are taken exactly as required for the entire length of time.

Drug resistant TB

Drug resistance is one of the major problems affecting the worldwide control of TB. A person can have drug resistant TB either because they have become infected with a drug resistant strain, or because resistance developed during the time that they were taking drug treatment for TB.

There is more about drug resistant TB generally, as well as more specifically XDR TB and totally drug resistant TB.

TB disease worldwide

TB is estimated to have killed 1.5 million people in 2013. There were an estimated 1.14 million deaths from TB disease in HIV negative people, and then a further 360,000 deaths from HIV and TB co-infection. In addition an estimated 9 million people developed active TB disease.11“Global Tuberculosis Report 2014″, WHO, Geneva, 2014 www.who.int/tb/publications/global_report/ A third of the world’s population, nearly two billion people, are estimated to have latent TB infection. There are more TB statistics.

Major efforts are being made in global TB control but with varying degrees of success.

TB and HIV

In 2013 an estimated 1.1 million (13%) of the 9 million people who worldwide developed active TB disease were HIV positive.12“Global Tuberculosis Report 2014″, WHO, Geneva, 2014 www.who.int/tb/publications/global_report/

People living with HIV are up to 37 times more likely to develop TB disease than people who are HIV negative.13“The Global Plan TO STOP TB 2011 – 2015″, WHO, Geneva, 2010, vi www.stoptb.org/global/plan/ But although HIV increases the likelihood of progression from latent TB infection to active TB disease, it is not known whether HIV infection increases the risk of become infected if someone is exposed to TB bacteria.14“Guidelines for Preventing the Transmission of Mycobacterium tuberculosis in Health-Care Settings 2005″, CDC, 6 www.cdc.gov/tb/publications/guidelines/infectioncontrol.htm HIV and TB co-infection means that a person is infected with HIV and also has TB bacteria in their body.

Globally in 2013, there were an estimated 1.5 million AIDS related deaths, and of these people an estimated 360,000 (24%) also had TB.15“UNAIDS Fact Sheet 2014″, UNAIDS, 2014 www.unaids.org/en/resources/campaigns/2014/2014gapreport/factsheet/

How can TB be prevented?

There are a number of steps that can be taken to prevent the spread of TB, and these include:

  • The use of the BCG vaccine;
  • drug treatment to prevent people with latent TB from developing TB disease;
  • infection control measures to prevent health care workers and other people in contact with people with active TB disease, from becoming infected;
  • and drug treatment for people with active TB disease will also prevent transmission of TB bacteria to other people, as drug treatment makes a person less infectious.


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