Tuberculosis (TB) is a potentially serious infectious disease that mainly affects the lungs. It is caused by a bacterium called Mycobacterium tuberculosis. Tuberculosis primarily affects the lungs, but it can also damage other parts of the body, such as the brain, kidneys or spine. Tuberculosis spreads through the air from one person to another. The bacteria are put into the air by a person with TB disease of the lungs or throat when they cough, sneeze, speak or sing. People nearby may breathe in these bacteria and become infected. Tuberculosis bacteria usually attack the lungs, but they can spread to any part of the body such as the kidney, spine, and brain. If not treated properly, TB disease can be fatal.
There are two stages of tuberculosis: latent TB infection and active TB disease. Latent TB infection means the person has been exposed to tuberculosis bacteria and their body has fought off the germs. They are not sick themselves, and they cannot spread the germs to others. This type of TB infection is sometimes called “latent” or “inactive” tuberculosis. The person with latent TB infection has no symptoms and cannot spread the disease. They may develop active TB disease in the future and spread the germs if they do not receive treatment for latent TB infection.
Active TB disease means that the person has developed symptoms of tuberculosis infection and can spread the bacteria to others. People with active TB disease in their lungs or throat can cough and spread the bacteria through the air. They may spread the TB bacteria to others before they are diagnosed and begin treatment. Some symptoms of active TB disease that affect the lungs include coughing, chest pains, coughing up blood or sputum, weakness or fatigue, weight loss, lack of appetite, chills, fever and sweating at night. Some symptoms related to TB infection in other organs could include headaches, stiff neck, pain or swelling in the abdomen, back or joints. These symptoms will depend on what other part of the body is affected.
There are several risk factors associated with developing active TB disease versus remaining latently infected. Those at higher risk of developing active disease if infected with the TB bacteria include people with HIV infection or other conditions that weaken the immune system, infants and young children, people who abuse alcohol or intravenous drugs, and those who have been recently infected with TB bacteria. Some health conditions like diabetes, cancer, or kidney failure also increase the risk of progressing from latent TB infection to active TB disease by lowering immunity. The use of immune suppressing medications like corticosteroids, TNF-alpha blockers, and other arthritis or cancer drugs can also increase the risk. Poverty, homelessness, and malnutrition all increase the odds of developing active TB once someone is exposed to the bacteria.
Tuberculosis is highly contagious, but infection only happens after prolonged exposure to someone with the active, respiratory form of the disease. This means spending many hours a week in an enclosed space with someone who is coughing due to active, untreated TB of the lungs or throat. Casual contact is unlikely to result in transmission. When an infected person coughs, sneezes or speaks, they expel infectious aerosol droplets 0.5 to 5 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. It is estimated that a single bacterium has the capability of causing a new infection.
Some other factors influence how contagious a person with active TB disease might be. The degree of contagiousness depends on factors such as the number of bacilli expelled, the effectiveness of the patient’s cough, the environment in which exposure occurs and the extent of lung cavitation. People with advanced or extensive TB disease tend to be more contagious. Patients are most infectious when untreated or in the early stages of treatment before they are no longer coughing. Factors associated with worse ventilation such as crowding, indoor exposure, and lack of sunlight in buildings may increase transmission risk. In developed countries with effective TB control programs, the risk of infection after casual contact is below 1%.
Diagnosis involves a medical examination of the patient’s medical history and known risk factors combined with several tests to look for tuberculosis infection or disease. These tests may include:
Tuberculin skin test (TST): This test involves injecting a small amount of fluid (called tuberculin) into the skin in the forearm. A person who has been exposed to the tuberculosis bacteria will develop a firm, raised red area at the injection site within 2 days. This skin test is not always definitive and may produce false positive or negative results.
Interferon-gamma release assays (IGRAs): These are blood tests that can detect a TB infection more specifically than a tuberculin skin test. They are not affected by previous BCG vaccinations or most nontuberculous environmental mycobacteria. The main IGRA tests approved for use in the United States are the Quantiferon-TB Gold test and T-Spot TB test.
Chest x-ray: An x-ray of the chest can show abnormalities caused by active TB disease, such as enlarged lymph nodes, scar tissue, or cavities in the upper lungs. X-rays cannot confirm the diagnosis and other tests are usually needed as well.
Sputum examination test: This involves coughing up mucus samples from deep inside the lungs and checking them under a microscope for tuberculosis bacteria. Detecting acid-fast bacilli in the sputum suggests active pulmonary TB disease. This test may need several samples collected on different days to improve accuracy.
Biopsy: If samples from sputum or other bodily fluids are inconclusive, the doctor may remove a small piece of tissue from the lungs or other areas for laboratory testing. Samples can then be examined to check for tuberculosis bacteria and determine which drugs are effective against the bacteria.
Treatment of latent TB infection involves taking one oral antibiotic daily for 3 to 9 months. Treatment prevents or heals the dormant TB infection so it cannot become active later. For active TB disease, treatment requires taking multiple antibiotics daily. A combination of several drugs is used to reduce the risk that any surviving TB bacteria will become resistant to the effects of one or more medications. The typical treatment plan for active disease is a combination of four drugs for 2 months followed by two drugs for 4 more months, for a total of 6 months. It is very important to complete the full course of treatment to ensure the bacteria are completely eliminated.
Preventing tuberculosis involves limiting exposure to the bacteria. This encompasses curing infected persons to lessen transmission, adequate sanitation and hygiene measures, contact tracing, screening high-risk groups and prompt treatment of latent and active cases. Bacille Calmette–Guérin (BCG) is a vaccine that provides some degree of protection for children against severe forms of TB such as TB meningitis or miliary TB. BCG vaccination does not always protect people from getting infected with latent TB or developing active disease later in life when immunity wanes. Overall, it is estimated that the BCG vaccine protects children from disseminated forms of tuberculosis by approximately 50-80%.
Additional factors that help control the spread include testing individuals exposed to infected people for latent TB infection and using preventive antibiotic treatment for those infected to reduce the risk of progression to active disease. Communities also often screen groups at high risk like those experiencing homelessness or those recently arriving from endemic areas. Public health departments monitor tuberculosis rates and work to contact trace those exposed. Coordinating treatment between departments ensures completion of therapy and prevents the spread of drug resistant strains. With cooperation between healthcare providers and patients, communities can maintain control levels and avoid new outbreaks.
Tuberculosis is an infectious disease causing significant global health problems. With appropriate diagnosis, antibiotic treatment can cure active infections and preventative therapy can halt progression from latent infection. While still a threat today, improved sanitation, contact tracing and widespread access to care have greatly reduced rates in many nations compared to the pre-antibiotic era. Ongoing prevention efforts remain important, especially in high-risk groups, to continue driving down transmission. Coordinated global strategies and public health collaboration aim to make tuberculosis a rare disease worldwide in the coming decades. Adherence to treatment regimens remains key to cure active cases and halt the development of drug resistant strains.
Andrew Stroud