What is tuberculosis?

Coughing, sneezing, singing, even speaking can all make the air we share hazardous at times.
Tuberculosis, or “consumption” if you’ve just emerged from a novel by Charles Dickens, is an infectious disease caused by a bacterium, Mycobacterium tuberculosis. It mainly affects the lungs (although can also infect other parts of your body), and is usually spread to other people via the coughs and sneezes of those infected. In addition to being a potentially serious, dramatic affliction suitable for any self-respecting 19th-century character, this disease continues to be a major concern worldwide to this day. Tuberculosis is often referred to as TB.

Your healthy lungs

Your lungs, if everything is working as it should be, resemble large pink sponges that expand when you inhale a mouthful of air. Their main job is to transport oxygen from the air into your bloodstream and release carbon dioxide from your body into the air. To do this, small sacs located deep inside your lungs, called alveoli, fill up like balloons, which enables this gas exchange to occur. The alveoli are very thin, just one cell layer thick, which allows very efficient transport of oxygen into your blood capillaries, and carbon dioxide out.
image of healthy alveoli performing gas exchange of oxygen and carbon dioxide to blood cells

What happens if you get infected with M. tuberculosis?

In most cases of tuberculosis, the bacteria spreads slowly and widely throughout the lungs. There are two distinct types, latent and active:
  • Latent tuberculosis - this means you are infected with tuberculosis bacteria, but you do not feel sick or have any signs or symptoms of tuberculosis disease. In this case, your immune system limits the infection by enclosing the tuberculosis bacteria within a tough calcified shell, known as a granuloma. The granulomas protect your lungs from any damage the bacteria might do. As long as the bacteria are contained, you will not have any symptoms, and are not contagious, which means you can’t infect anyone else. The presence of tuberculosis granulomas can be seen on a chest X-ray.
  • Active tuberculosis - whether or not tuberculosis remains latent, or progresses to active tuberculosis depends on the health of your immune system. When you are healthy and everything is functioning properly, your body is able to keep the bacteria contained and under control. Sometimes, your immune system may become weaker and is no longer able to control the growth of the tuberculosis bacteria; for example, if you have a disease that attacks your immune system, like HIV/AIDS. When this happens, the calcified shell of the granuloma can deteriorate and your tuberculosis infection may transition from latent to active. This can occur anytime from weeks, to years after you were first infected. Once the tuberculosis granulomas open, the bacteria are able to emerge, inhabit your lungs, and damage the surrounding tissue. The damage causes the spongy, balloon-like tissue of the alveoli to harden and become fibrous, making them useless for gas exchange.
Image of healthy alveoli blocking gas exchange to and from blood cells
Eventually, the lung tissue cells themselves begin to deteriorate and die. When the bacteria reach a part of your lungs which connects to an airway, they are expelled when you cough or sneeze. This releases them in an aerosol of fine droplets that may be inhaled by another person (droplet transmission). Like the flu, tuberculosis is an airborne disease, meaning that the infectious droplets can travel long distances and remain suspended in the air for a long time. Consequently, it is possible for the bacteria to circulate throughout entire buildings, and you can catch tuberculosis by entering a room that an infected person left hours ago. Masks that filter the air (e.g., N95 masks) are often used by healthcare professionals who are caring for patients with tuberculosis, and these patients are often isolated in negative pressure rooms that prevent the contaminated air from escaping from the room.
Unfortunately, because your body naturally wants to eject any unwanted particles in your lungs, you are likely to be coughing a lot if you have active tuberculosis. As the bacteria continue to attack your lung tissues, the damage and inflammation becomes more extensive, and you may even begin to cough up blood.

Symptoms

Many of the worst symptoms of active tuberculosis arise as a direct result of the extensive tissue damage that the bacteria do to your lungs. Typical signs and symptoms of active tuberculosis include a bad dry cough lasting for more than three weeks that may cause you to cough up bloody sputum. You are also likely to experience night sweats, fever and weight loss (the reason why historically tuberculosis was called “consumption”) as your body tries to fight off the infection.
For about one in five people, the infection is so severe that cavities begin to form within the lung tissue. If these areas start to bleed, tuberculosis bacteria are able to enter the bloodstream. If this happens, they can travel to other parts of your body, causing additional symptoms. A tuberculosis infection outside of the lungs is called an extrapulmonary tuberculosis infection and most commonly occurs in your lymphatic system, your genitourinary system, and/or in your bones and joints. However, in some cases extrapulmonary tuberculosis is disseminated, which means the infection has spread widely throughout your whole body.
Image illustrating the effects of extrapulmonary tuberculosis

What puts you at risk of catching tuberculosis?

Tuberculosis is contagious, meaning that you can catch it from other people, especially if you live with someone who is infected. However, there are many other situations that can also increase your chances of getting tuberculosis. These include times of life when your immune system is weaker; for example when you are very young or very old, or when you have certain diseases that reduce immune function, such as HIV/AIDS, diabetes or cancer, or are receiving immunosuppressive medications for these or other diseases. The risk of catching tuberculosis is also linked to crowded, poorly ventilated living conditions. This is a common situation for people living in poverty, for whom malnutrition, lack access to medical care, and substance abuse may all contribute to an increased risk of becoming infected. It is worth noting that poverty is not the only driver of crowding and inadequate living conditions. In many countries, prisons also make transmission of tuberculosis very efficient, for these reasons. Finally, drug resistant tuberculosis has been reported in 100 countries worldwide, including sub-Saharan Africa, India, China and Pakistan.1^1 Around 10% of people are infected with this form, which is very hard to treat.1^1 Your chances of catching tuberculosis is higher if you live in or travel to these regions, where it is becoming a major public health problem.

How likely are you to get tuberculosis?

Tuberculosis is the world’s second most deadly infectious disease after HIV/AIDS. In 2013, around 9 million people got active tuberculosis and 1.5 million people died of the disease, mainly in low- and middle-income countries.2^2 HIV (human immunodeficiency virus) attacks and weakens your immune system. This reduces your ability to fight off other infections including tuberculosis, making it around 30 times more likely you will become infected with both HIV and tuberculosis.3^3 This type of coinfection is often very serious, and tuberculosis is a leading cause of death of people with HIV infection.
Map illustrating TB prevalence around the world

How to avoid getting tuberculosis

You need to avoid being in close proximity to people who have active tuberculosis if you want to avoid getting it yourself. This means avoiding spending time in an enclosed space with anyone who has active tuberculosis. Sometimes this is impossible, for example if you work in a healthcare setting where there are patients with active tuberculosis, or if you are living with someone with active tuberculosis. In these circumstances, it is a good idea to wear a protective mask, and to encourage and support the infected person or persons to take medication for their tuberculosis infection. It is very important to take the medication exactly as directed, and to finish the entire course of the medication to prevent the bacteria developing drug resistant strains. With proper treatment, active tuberculosis quickly becomes noninfectious.
There is a tuberculosis vaccine, called the BCG (Bacillus Calmette-Guerin) vaccine that is widely given to newborn children and infants in many countries worldwide because it can prevent severe tuberculosis. Unfortunately, it is not very effective in adults. One reason for this may be that adults are likely to have been exposed to other strains of mycobacterium in the environment that give partial immunity against M. tuberculosis, and that this prevents the development of the full immune response to the vaccine that is necessary to protect against future infection. It is also worth noting that a vaccine is not a treatment for a disease, it prevents a disease from occurring, so must be given before you have become infected.

How is tuberculosis diagnosed and treated?

The most common way to diagnose whether or not you have tuberculosis is a tuberculosis skin test, also known as a tuberculin test. Your healthcare provider will inject a small amount of a substance called tuberculin purified protein derivative, or PPD (prepared from dead tuberculosis bacteria), usually just under the skin of your inside forearm. If you develop a hard red bump during the following 48 to 72 hours, and depending on its size, you have tested positive for tuberculosis. Unfortunately, this test is not 100% reliable and sometimes gives false positives, or false negatives, as well as generally giving a positive result if you have been vaccinated against tuberculosis. Because of this, if you test negative, but are in, or have been in, a situation that puts you at high risk for tuberculosis, your healthcare provider may order a blood test to confirm whether or not your skin test was a false negative.
Illustration of a skin test for TB
If your skin test is positive, your healthcare provider will likely want you to have a chest X-ray or a CT scan. He will be looking for tuberculosis granuloma, or other changes in your lungs that often occur when you have active tuberculosis, to confirm the diagnosis. You may also be asked to provide a sample of sputum, which will be tested for tuberculosis bacteria. If bacteria are detected, they can then be tested to see whether or not you are infected with a drug resistant strain. This information helps your health care provider decide which medicine will work best for you.
Tuberculosis is a treatable and curable disease. There are at least 10 anti-tuberculosis drugs available to treat tuberculosis infection. However, unlike many other bacterial infections, which only require a short course of medicine, tuberculosis drugs generally need to be taken for at least six to nine months. This is because the bacteria can remain latent, or dormant in your lungs for long periods of time without making you sick. It is very important to get treated properly to eliminate both latent and active tuberculosis bacteria. Your healthcare provider will choose the antibiotics you need based on whether or not your infection is latent or active, where the infection is in your body (lungs or elsewhere), your general state of health, and whether or not you have a drug resistant strain.
Treatment of latent tuberculosis - if you have tested positive for latent tuberculosis, your healthcare provider may recommend that you take either isoniazid or rifampin to prevent a first, or recurrent episode of active tuberculosis. This approach is typically used to help people who are at particularly high risk of developing active tuberculosis including infants and young children, people with a recent infection (within the last 2 years), people infected with tuberculosis and HIV, and people who may have a weak immune system brought about by another disease, such as diabetes, or chronic kidney failure.
Treatment of active, drug-sensitive tuberculosis - most cases of active, drug-sensitive tuberculosis are treated with a standard six-month course of four anti-tuberculosis drugs. The most common ones are isoniazid, rifampin, ethambutol and pyrazinamide. These drugs are very effective; however, the infection will only be cured if the drugs are taken properly, and for the prescribed length of time. If you stop taking the drugs early, or skip doses, the tuberculosis bacteria may mutate and become resistant to the drugs. Drug resistant tuberculosis is much more difficult to treat, and is more likely to be spread from one person to the next.
Treatment of drug resistant tuberculosis - if you are infected with a drug-resistant strain of tuberculosis, you will likely need a different combination of antibiotics, and you may have to take them for 2 years or more. All of these medicines are very toxic to your liver and may cause serious side effects.

Consider the following:

  • Drug resistant tuberculosis is very widespread, and strains of tuberculosis that are resistant to a single anti-tuberculosis drug have been found in all countries surveyed worldwide.2 Unfortunately, multidrug resistant strains known as multidrug resistant tuberculosis (MDR-tuberculosis), that don’t respond to isoniazid and rifampin, the two most important 1st-line anti-tuberculosis drugs, have also appeared. MDR-tuberculosis is curable using combinations of other 2nd-line anti-tuberculosis drugs; although these are expensive and not always available, especially in countries with limited resources. More recently, even more extreme drug resistant strains have appeared, called extensively drug-resistant tuberculosis (XDR-tuberculosis). These strains can be treated with aggressive drug regimens, but many more people die of this form of tuberculosis than MDR-tuberculosis or drug-sensitive tuberculosis.
  • It is very important for people with HIV infection to be tested for tuberculosis. Why do you think that might be? People with HIV and latent tuberculosis are at very high risk of progression to active tuberculosis because their immune system is already weakened by the HIV infection. Without treatment, active tuberculosis can quickly become a serious illness leading to death. If detected early, people with HIV/tuberculosis co-infection can start treatment for latent tuberculosis and prevent its progression to more serious disease.

Attribution

This article is licensed under a CC-BY-NC-SA 4.0 license. https://creativecommons.org/licenses/by-nc-sa/4.0/

References

  1. World Health Organization. (2015). What is multi-drug resistant tuberculosis and how do we control it? Online Q&A (reviewed March 2015). Retrieved from http://www.who.int/features/qa/79/en/
  2. World Health Organization. (2015). Tuberculosis. Fact sheet No. 104. Retrieved from http://www.who.int/mediacentre/factsheets/fs104/en/
  3. World Health Organization. (2015). HIV/AIDS; Tuberculosis and HIV. Retrieved from http://www.who.int/hiv/topics/tb/about_tb/en/
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