Latent tuberculosis (TB) refers to a condition where the Mycobacterium tuberculosis bacteria are present in the body but remain inactive. Individuals with latent TB infection do not experience symptoms and cannot spread the bacteria to others. Despite being inactive, these bacteria can become active later, leading to TB disease. Globally, approximately one-third of the population is infected with latent TB, making its treatment a significant public health concern.
Distinguishing Latent from Active Tuberculosis
The distinction between latent TB infection and active TB disease lies in the bacteria’s state, the presence of symptoms, and contagiousness. In latent TB, the bacteria are contained by the immune system and do not cause illness; individuals have no symptoms and cannot transmit the infection. The bacteria remain dormant within the body.
Conversely, active TB disease occurs when the bacteria overcome the immune system, multiply, and cause noticeable symptoms. These symptoms can include a persistent cough, chest pain, weight loss, fever, and night sweats. Individuals with active TB can spread the bacteria to others through the air. Treating latent TB is important because approximately 5% to 10% of infected individuals will develop active, contagious TB disease months or even years after initial infection.
Diagnosing Latent Tuberculosis
Identifying latent TB infection involves detecting the body’s immune response to Mycobacterium tuberculosis bacteria. The Tuberculin Skin Test (TST) is a traditional method, involving an injection of a purified protein derivative (PPD) under the skin of the forearm. A healthcare professional measures the reaction (induration) after 48 to 72 hours; a positive result indicates exposure to TB bacteria. However, the TST can yield false positives in individuals who have received the Bacillus Calmette-Guérin (BCG) vaccine or have been exposed to certain non-tuberculous mycobacteria.
Interferon-Gamma Release Assays (IGRAs) offer an alternative diagnosis. These blood tests measure the immune response by detecting interferon-gamma released by white blood cells after exposure to specific TB antigens. IGRAs are not affected by prior BCG vaccination and can be more reliable in immunocompromised individuals compared to TST. A positive result from either test indicates TB infection, prompting further medical evaluation, including a chest X-ray, to rule out active TB disease before considering latent TB treatment.
Treatment Regimens for Latent Tuberculosis
Treatment for latent tuberculosis aims to eliminate the inactive bacteria, preventing progression to active disease. Healthcare providers consider several regimens, involving isoniazid, rifampin, or a combination of isoniazid and rifapentine. The Centers for Disease Control and Prevention (CDC) and the National Tuberculosis Coalition of America prioritize shorter, rifamycin-based regimens due to their effectiveness, safety, and higher completion rates.
One common short-course option is daily rifampin for four months. This regimen has shown similar efficacy to longer isoniazid monotherapy in preventing active TB and often has better treatment completion rates. Another preferred short regimen is a 3-month course of once-weekly isoniazid plus rifapentine, which can be administered through directly observed therapy (DOT) to ensure adherence. This regimen has demonstrated favorable safety profiles and completion rates.
Isoniazid monotherapy for six or nine months remains an alternative, particularly if shorter regimens are not suitable due to factors like drug interactions with rifamycins. While effective in preventing TB disease, isoniazid monotherapy regimens have a higher risk of toxicity and lower completion rates compared to the shorter, rifamycin-based options. The choice of regimen depends on individual patient factors, including age, coexisting medical conditions like HIV infection, potential drug interactions, and the susceptibility of the presumed source case’s bacteria to specific drugs.
Important Considerations During Treatment
Undergoing treatment for latent tuberculosis involves practical considerations, especially regarding potential side effects and the importance of medication adherence. All patients receiving latent TB treatment should be monitored monthly for adherence, signs and symptoms of active TB disease, and adverse reactions. Medications used to treat latent TB, particularly isoniazid, can cause liver toxicity, leading to symptoms like nausea, vomiting, abdominal pain, or jaundice (yellowing of the skin or eyes). Patients should be advised to report any such symptoms to their healthcare provider immediately, as severe liver injury may necessitate stopping treatment.
Other possible side effects can include flu-like symptoms, rashes, itching, or tingling and numbness in the hands and feet, particularly with isoniazid. Rifampin can cause orange discoloration of urine, tears, saliva, and other bodily fluids, which is a harmless but noticeable effect. Completing the full course of medication is important for the treatment to be effective in preventing progression to active TB disease.