Tuberculosis (TB) is an illness caused by the bacterium Mycobacterium tuberculosis, primarily affecting the lungs but able to impact other body parts. While many people associate TB with active illness, latent TB infection is a distinct condition. In this state, the bacteria are present but inactive, causing no symptoms or signs of disease. Latent TB represents a hidden reservoir of the bacteria.
Understanding Latent TB Infection
Latent tuberculosis infection occurs when Mycobacterium tuberculosis bacteria are inside the body in a dormant state. Individuals with latent TB experience no symptoms and cannot spread the bacteria. This differs from active TB disease, where bacteria multiply and cause illness.
Despite their inactivity, the bacteria in latent TB are alive and can become active later, potentially leading to active TB disease. Approximately one-quarter of the global population is estimated to have latent TB infection. The immune system typically walls off TB organisms, keeping them inactive; this state can persist for a lifetime.
How Latent TB Spreads
The bacteria that cause tuberculosis, Mycobacterium tuberculosis, spread through the air. Transmission occurs when a person with active TB disease of the lungs or throat coughs, sneezes, speaks, or sings, releasing tiny droplets containing bacteria into the air. Others can then inhale these airborne droplets, leading to potential infection.
Only people with active TB disease can transmit the bacteria. A person with latent TB is not contagious and cannot spread the bacteria. Infection generally requires prolonged exposure to someone with active TB.
Who Should Be Tested for Latent TB?
Testing for latent TB infection is recommended for specific populations with an increased risk of exposure or progression to active disease. These include:
- Those with close contact to someone diagnosed with active TB.
- Those living in or traveling to high TB rate areas.
- Healthcare workers due to occupational exposure.
- Those with weakened immune systems, such as HIV or organ transplant recipients.
- Those with certain medical conditions increasing risk.
Diagnosing Latent TB
Diagnosing latent TB involves specific tests to detect bacteria in the body. The Tuberculin Skin Test (TST), also known as the Mantoux tuberculin skin test, is one common method. A small amount of purified protein derivative (PPD) solution is injected under the forearm skin. A positive reaction, indicated by a raised, firm red bump, suggests infection with TB bacteria. However, the TST can sometimes yield false positive or negative results.
Another diagnostic tool is the Interferon-Gamma Release Assay (IGRA), such as the QuantiFERON-TB Gold test. IGRAs are blood tests that measure the immune system’s response to TB bacteria. These tests are generally more specific than the TST and are less affected by prior BCG vaccination. A positive TST or IGRA indicates Mycobacterium tuberculosis infection, but further evaluation, including a chest X-ray and symptom assessment, is necessary to rule out active TB disease.
Treating Latent TB and Preventing Progression
Treating latent TB infection prevents active TB disease, which can be severe and transmissible. Without treatment, approximately 5% to 15% of people with latent TB may develop active disease, often within the first few years after infection. Treatment regimens typically involve specific antibacterial medications, such as isoniazid or rifampin.
The duration of treatment can vary, with some regimens lasting for several months, for example, 9 months of isoniazid. Completing the prescribed course of medication is important to prevent progression to active disease. Regular follow-up with a healthcare provider helps ensure adherence and monitors for side effects. This preventive treatment can significantly reduce the risk of future active TB.