Tuberculosis (TB) remains a significant global health concern, caused by the bacterium Mycobacterium tuberculosis. This bacterium can exist within the body in two primary forms: active TB disease and latent TB infection. While active TB presents with symptoms and is contagious, latent TB represents a state where the bacteria are present but remain inactive and do not cause illness. This article focuses specifically on understanding and managing latent tuberculosis.
Understanding Latent Tuberculosis
Latent tuberculosis infection (LTBI) occurs when a person is infected with Mycobacterium tuberculosis but does not develop active TB disease. The bacteria reside in the body without causing symptoms. Individuals with latent TB cannot spread the bacteria to others because they are dormant and not actively multiplying or being expelled.
The body’s immune system plays a significant role in controlling the infection, effectively “walling off” the bacteria. This containment prevents the bacteria from multiplying and causing active disease. Although present, the bacteria are kept in a dormant state.
Identifying Latent TB
Detecting latent tuberculosis involves diagnostic tests to identify Mycobacterium tuberculosis infection without active disease. The Tuberculin Skin Test (TST), also known as the Mantoux test, is one common method. This test involves injecting a small amount of tuberculin purified protein derivative (PPD) under the skin of the forearm. A healthcare professional then measures the reaction after 48 to 72 hours.
Interferon-Gamma Release Assays (IGRAs) represent another diagnostic approach, offering a blood test alternative. Examples include the QuantiFERON-TB Gold Plus test and T-SPOT.TB. These tests measure the immune system’s reaction to Mycobacterium tuberculosis antigens by detecting interferon-gamma. A positive result from either the TST or an IGRA indicates infection with TB bacteria. Testing is recommended for individuals with close contact to active TB, those from high-prevalence countries, and those with weakened immune systems.
Treating Latent TB
Treating latent tuberculosis is a proactive measure aimed at preventing the dormant bacteria from reactivating and causing active TB disease. This preventive treatment is important because individuals with untreated latent TB have a 5-10% lifetime risk of developing active TB, with the highest risk occurring in the first two years after infection. The choice of treatment regimen depends on several factors, including the patient’s age, potential drug interactions, and the likelihood of medication adherence.
Common treatment options include regimens involving isoniazid (INH) and rifampin (RIF). Shorter courses, such as a 3-month regimen of isoniazid and rifapentine, or a 4-month daily regimen of rifampin, may be prescribed. Longer courses, like 6 to 9 months of daily isoniazid, are also effective. Completing the entire prescribed course of medication is important to reduce the risk of progression to active TB disease.
Factors Influencing Progression
While the immune system keeps Mycobacterium tuberculosis in a latent state, certain factors can weaken these defenses, increasing the risk of progression. A compromised immune system is a concern, often seen in individuals with conditions like HIV infection. The human immunodeficiency virus directly attacks and weakens the immune system, making it less capable of containing the TB bacteria.
Other medical conditions also elevate this risk, including diabetes, chronic kidney disease, and silicosis. Medications that suppress the immune system, such as corticosteroids or TNF-alpha inhibitors used for autoimmune diseases or organ transplantation, can also lead to active disease. A recent TB infection, within the last two years, carries a higher risk of progression.