Difference Between TB Infection and TB Disease

Tuberculosis (TB) is a complex and multifaceted disease caused by the bacterium Mycobacterium tuberculosis.While TB infection represents a dormant state where the bacteria are contained by the immune system, TB disease occurs when the bacteria become active and cause symptoms. Latent TB can reactivate and become active TB, especially if the immune system weakens due to factors such as HIV, diabetes, or immunosuppressive treatments.

Comparative table: TB infection (latent TB) and TB disease (active TB)

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TB Infection (Latent TB Infection)

In TB infection, M. tuberculosis bacteria enter the body and establish a primary infection, usually in the lungs. The body's immune system mounts a defense, encapsulating the bacteria within granulomas, which are small, localized clusters of immune cells. These granulomas keep the bacteria in a dormant state, preventing them from causing active disease. Individuals with latent TB infection do not exhibit symptoms and are not contagious. However, the bacteria remain alive within the granulomas and can reactivate if the immune system becomes compromised.

Symptoms and Diagnosis

• Individuals with latent TB infection are asymptomatic. They do not feel sick and have no signs or symptoms of TB. 
• The infection is typically detected through a positive tuberculin skin test (TST) or an interferon-gamma release assay (IGRA), which indicate an immune response to the bacteria. Despite the absence of symptoms, The TST involves injecting a small amount of tuberculin purified protein derivative (PPD) into the skin and measuring the reaction after 48-72 hours. A positive test indicates TB infection but cannot distinguish between latent and active TB
•  IGRAs, such as the QuantiFERON-TB Gold test, measure the release of interferon-gamma in response to TB antigens and offer a more specific and convenient alternative to the TST.

Treatment Procedures

The goal of treating latent TB infection is to prevent the progression to active TB disease. Treatment typically involves a course of antibiotics, such as isoniazid or rifapentine, taken for several months. This treatment reduces the risk of developing TB disease but requires adherence to ensure effectiveness and minimize the risk of drug resistance.

TB Disease (Active TB)

TB disease occurs when the immune system is unable to contain the bacteria, leading to their proliferation and spread within the body. This can happen soon after the initial infection or years later, particularly if the immune system becomes weakened. The bacteria can cause active disease in various organs, but the lungs are the most common site. When the bacteria become active, they cause tissue damage and elicit a more pronounced immune response, leading to the characteristic symptoms of TB disease.

Symptoms

The most common form, typically presents these symptoms

• Persistent cough lasting three weeks or more
•  chest pain, and hemoptysis (coughing up blood).
•  Other general symptoms include fever, night sweats, weight loss, and fatigue. 
• Extrapulmonary TB, which affects organs other than the lungs, can cause symptoms specific to the affected organ, such as lymphadenopathy in TB lymphadenitis or neurological deficits in TB meningitis.

Diagnosis

Diagnosing active TB disease requires a combination of clinical evaluation, imaging, and microbiological tests.

• Chest radiography (X-ray) : commonly used to detect abnormalities in the lungs consistent with TB.
• Microbiological confirmation involves sputum smear microscopy, culture, and nucleic acid amplification tests (NAATs) to identify M. tuberculosis.
•  Sputum culture is the gold standard for diagnosis but can take several weeks.
•  Rapid molecular tests like Xpert MTB/RIF can detect TB and resistance to rifampicin within hours, facilitating timely treatment.

Treatment

Treating active TB disease is more complex and involves a combination of antibiotics taken for at least six months. The standard regimen includes isoniazid, rifampicin, pyrazinamide, and ethambutol for the initial two months, followed by isoniazid and rifampicin for the remaining four months. Adherence to the treatment regimen is crucial to ensure a cure and prevent the development of multidrug-resistant TB (MDR-TB). In cases of drug-resistant TB, treatment involves second-line drugs, which are less effective, more toxic, and require longer treatment durations.

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Key differences between TB infection and TB disease

TB Infection (Latent TB):

• Dormant Bacteria: Bacteria are inactive.
• Asymptomatic: No symptoms.
• Non-Contagious: Cannot spread TB to others.
• Diagnosis: Positive tuberculin skin test (TST) or interferon-gamma release assay (IGRA).
• Treatment: Preventive treatment to stop progression to active disease.

TB Disease (Active TB):

• Active Bacteria: Bacteria are multiplying and causing illness.
• Symptomatic: Symptoms include persistent cough, fever, night sweats, and weight loss.
• Contagious: Can spread TB to others.
• Diagnosis: Chest X-ray, sputum tests, and other microbiological tests.
• Treatment: Requires a longer course of antibiotics to cure the disease and prevent transmission.