Tuberculosis

Symptoms

Although your body may harbor the bacteria that cause tuberculosis, your immune system often can prevent you from becoming sick. For this reason, doctors make a distinction between:

• Latent TB. In this condition, you have a TB infection, but the bacteria remain in your body in an inactive state and cause no symptoms. Latent TB, also called inactive TB or TB infection, isn't contagious.
• Active TB. This condition makes you sick and can spread to others.

Why do I have to take treatment when I have no symptoms?

If you have latent TB infection, although you do not have symptoms now, there is an increased risk that you will develop active TB disease in the future. Treatment of latent TB infection before it develops into active TB disease, decreases this risk and involves taking fewer medicines than if you have active TB.

Why do I need to take the TB medicine for so long?

TB germs are killed very slowly. It takes at least 3 months for the medicine to kill all the germs. The length of the treatment depends on what medicines you are given. You must continue to take your medicine until all the TB germs are dead, even though you have no symptoms of TB disease.

Signs and symptoms of active TB include:

• Unexplained weight loss
• Fatigue
• Fever
• Night sweats
• Chills
• Loss of appetite

Tuberculosis usually attacks your lungs. Signs and symptoms of TB of the lungs include:

• Coughing that lasts three or more weeks
• Coughing up blood
• Chest pain, or pain with breathing or coughing

Tuberculosis can also affect other parts of your body, including your kidneys, spine or brain. When TB occurs outside your lungs, symptoms vary according to the organs involved. For example, tuberculosis of the spine may give you back pain, and tuberculosis in your kidneys might cause blood in your urine.

Causes

Tuberculosis is caused by an organism called Mycobacterium tuberculosis. The bacteria spread from person to person through microscopic droplets released into the air. This can happen when someone with the untreated, active form of tuberculosis coughs, speaks, sneezes, spits, laughs or sings. Rarely, a pregnant woman with active TB may pass the bacteria to her unborn child.

Although tuberculosis is contagious, it's not especially easy to catch. You're much more likely to get tuberculosis from a family member or close co-worker than from a stranger. Most people with active TB who've had appropriate drug treatment for at least two weeks are no longer contagious.

TB infection vs. active TB

If you breathe TB bacteria into your lungs, one of four things might happen:

• You don't become infected with TB. Your immune system immediately destroys the germs and clears them from your body.
• You develop latent TB infection. The germs settle in your lungs and begin to multiply. Within several weeks, however, your immune system successfully "walls off" the bacteria in your lungs, much like a scab forming over a wound. The bacteria may remain within these walls for years — alive, but in a dormant state. In this case, you're considered to have TB infection and you'll test positive on a TB skin test. But you won't have symptoms and won't transmit the disease to others.
• You develop active TB. If your immune defenses fail, TB bacteria begin to exploit your immune system cells for their own survival. The bacteria move into the airways in your lungs, causing large air spaces (cavities) to form. Filled with oxygen — which the bacteria need to survive — the air spaces make an ideal breeding ground for the bacteria. The bacteria may then spread from the cavities to the rest of your lungs as well as to other parts of your body. If you have active TB, you're likely to feel sick. Even if you don't feel sick, you can still infect others. Without treatment, many people with active TB die. Those who survive may develop long-term symptoms, such as chest pain and a cough with bloody sputum, or they may recover and go into remission.
• You develop active TB years after the initial infection. After you've had latent TB for years, the walled-off bacteria may suddenly begin multiplying again, causing active TB, also known as reactivation TB. It's not always clear what triggers this reactivation, but it most commonly happens after your immune system becomes weakened. Your resistance may be lower because of aging, drug or alcohol abuse, malnutrition, chemotherapy, prolonged use of prescription medications such as corticosteroids or TNF inhibitors, and diseases such as HIV/AIDS.

Only about one in 10 people who have TB infection goes on to develop active TB. The risk is greatest in the first two years after infection and is much higher if you have HIV infection.

HIV and TB

Since the 1980s, the number of cases of tuberculosis has increased dramatically because of the spread of HIV, the virus that causes AIDS. Tuberculosis and HIV have a deadly relationship — each drives the progress of the other.

Infection with HIV suppresses the immune system, making it difficult for the body to control TB bacteria. As a result, people with HIV are many times more likely to get TB and to progress from latent to active disease than are people who aren't HIV-positive.

TB is one of the leading causes of death among people with AIDS, especially outside the United States. One of the first indications of HIV infection may be the sudden onset of TB, often in a site outside the lungs.

Drug-resistant TB

Another reason TB remains a major killer is the increase in drug-resistant strains of the bacterium. Ever since the first antibiotics were used to fight TB 60 years ago, the germ has developed the ability to survive attack, and that ability gets passed on to its descendants. Drug-resistant strains of TB emerge when an antibiotic fails to kill all of the bacteria it targets. The surviving bacteria become resistant to that particular drug and frequently other antibiotics as well. Today, for each major TB medication, there's a TB strain that resists its treatment.

The major cause of TB drug resistance is inadequate treatment, either because the wrong drugs are prescribed or because people don't take their entire course of medication.

There are two types of drug-resistant TB:

• Multidrug-resistant TB (MDR TB). This form of TB can't be killed by the two most powerful antibiotics for TB, isoniazid and rifampin. Although MDR TB can be successfully treated, it's much harder to combat than is regular TB and requires long-term therapy — up to two years — with drugs that are very expensive and can cause serious side effects. People with untreated MDR TB can transmit this serious type of TB to others.
• Extensively drug-resistant TB (XDR TB). XDR TB is a less common form of MDR TB in which the bacteria resist isoniazid and rifampin as well as most of the alternative or second line drugs used to treat TB. XDR TB has shown up across the world, including 49 cases in the United States between 1993 and 2006. Treatment for XDR TB is challenging and lengthy and leads to serious side effects and a higher rate of failure. Recently, the first cases of completely drug-resistant TB were reported — the bacteria could not be killed by any available TB drug. 

Risk factors

Anyone can get tuberculosis, but certain factors increase your risk of the disease. These factors include:

• Lowered immunity. A healthy immune system can often successfully fight TB bacteria, but your body can't mount an effective defense if your resistance is low. A number of factors can weaken your immune system. Having a disease that suppresses immunity, such as HIV/AIDS, diabetes, end-stage kidney disease, certain cancers or the lung disease silicosis, can reduce your body's ability to protect itself. Your risk is also higher if you take corticosteroids, certain arthritis medications, chemotherapy drugs or other drugs that suppress the immune system.
• Close contact with someone with infectious TB. In general, you must spend an extended period of time with someone with untreated, active TB to become infected yourself. You're more likely to catch the disease from a family member, roommate, friend or close co-worker.
• Country of origin. People from regions with high rates of TB — especially sub-Saharan Africa, India, China, the islands of Southeast Asia and Micronesia, and parts of the former Soviet Union — are more likely to develop TB. In the United States, more than half the people with TB were born in a different country. Among these, the most common countries of origin were Mexico, the Philippines, India and Vietnam.
• Age. Older adults are at greater risk of TB because normal aging or illness may weaken their immune systems. They're also more likely to live in nursing homes, where outbreaks of TB can occur.
• Substance abuse. Long-term drug or alcohol use weakens your immune system and makes you more vulnerable to TB.
• Malnutrition. A poor diet or one too low in calories puts you at greater risk of TB.
• Lack of medical care. If you are on a low or fixed income, live in a remote area, have recently immigrated to the United States or are homeless, you may lack access to the medical care needed to diagnose and treat TB.
• Living or working in a residential care facility. People who live or work in prisons, immigration centers or nursing homes are all at risk of TB. That's because the risk of the disease is higher anywhere there is overcrowding and poor ventilation.
• Living in a refugee camp or shelter. Weakened by poor nutrition and ill health and living in crowded, unsanitary conditions, refugees are at especially high risk of TB infection.
• Health care work. Regular contact with people who are ill increases your chances of exposure to TB bacteria. Wearing a mask and frequent hand washing greatly reduce your risk.
• International travel. As people migrate and travel widely, they may expose others or be exposed to TB bacteria.

Complications

Without treatment, tuberculosis can be fatal. Drug-resistant strains of the disease are more difficult to treat.

Untreated active disease typically affects your lungs, but it can spread to other parts of the body through your bloodstream. Complications vary according to the location of TB bacteria:

• Lung damage can occur if TB in your lungs (pulmonary TB) isn't diagnosed and treated early.
• Severe pain, abscesses and joint destruction may result from TB that infects your bones.
• Meningitis can occur if TB infects your brain and central nervous system.
• Miliary TB is TB that has spread throughout your entire body, a serious complication.

Diagnosis

If your doctor suspects TB, you will need a complete medical evaluation and tests for TB infection.

Skin test

The most commonly used diagnostic tool for TB is a simple skin test. Although there are two methods, the Mantoux test is preferred because it's more accurate.

For the Mantoux test, a small amount of a substance called PPD tuberculin is injected just below the skin of your inside forearm. You should feel only a slight needle prick. Within 48 to 72 hours, a health care professional will check your arm for swelling at the injection site, indicating a reaction to the injected material. A hard, raised red bump (induration) means you're likely to have TB infection. The size of the bump determines whether the test results are significant, based on your risk factors for TB.

The Mantoux test isn't perfect. A false-positive test suggests that you have TB when you really don't. This is most likely to occur if you're infected with a different type of mycobacterium other than the one that causes tuberculosis, or if you've recently been vaccinated with the bacillus Calmette-Guerin (BCG) vaccine. This TB vaccine is seldom used in the United States, but widely used in countries with high TB infection rates.

On the other hand, some people who are infected with TB — including children, older people and people with AIDS — may have a delayed or no response to the Mantoux test.

Blood tests

Blood tests may be used to confirm or rule out latent or active TB. These tests use sophisticated technology to measure the immune system's reaction to Mycobacterium tuberculosis. These tests are quicker and more accurate than is the traditional skin test. They may be useful if you're at high risk of TB infection but have a negative response to the Mantoux test, or if you received the BCG vaccine.

Further testing

If the results of a TB test are positive (referred to as "significant"), you may have further tests to help determine whether you have active TB disease and whether it is a drug-resistant strain.

These tests may include:

• Chest X-ray or CT scan. If you've had a positive skin test, your doctor is likely to order a chest X-ray. In some cases, this may show white spots in your lungs where your immune system has walled off TB bacteria. In others, it may reveal a nodule or cavities in your lungs caused by active TB. A computerized tomography (CT) scan, which uses cross-sectional X-ray images, may show more subtle signs of disease.
• Culture tests. If your chest X-ray shows signs of TB, your doctor may take a sample of your stomach secretions or sputum — the mucus that comes up when you cough. The samples are tested for TB bacteria, and your doctor can have the results of special smears in a matter of hours. Samples may also be sent to a laboratory where they're examined under a microscope as well as placed on a special medium that encourages the growth of bacteria (culture). The bacteria that appear are then tested to see if they respond to the medications commonly used to treat TB. Your doctor uses the results of the culture tests to prescribe the most effective medications for you. Because TB bacteria grow very slowly, traditional culture tests can take four to eight weeks.
• Other tests. Testing called nuclear acid amplification (NAA) can detect genes associated with drug resistance in Mycobacterium tuberculosis. This test is generally available only in developed countries. A test used primarily in developing countries is called the microscopic-observation drug-susceptibility (MODS) assay. It can detect the presence of TB bacteria in sputum in as little as seven days. Additionally, the test can identify drug-resistant strains of the TB bacteria.

References

http://www.hsa.ie/eng/Topics/Biological_Agents/Tuberculosis/

http://www.webmd.com/lung/understanding-tuberculosis-basics

https://www.hse.ie/eng/health/immunisation/hcpinfo/guidelines/chapter22.pdf

http://www.tmb.ie/vaccinations/tuberculosis

http://www.who.int/topics/tuberculosis/en/