Tuberculosis

Introduction

Tuberculosis (TB) is an infectious disease caused by the bacterium Mycobacterium tuberculosis. It primarily affects the lungs but can also affect other parts of the body, such as the kidneys, spine, and brain. TB is a major global health concern, with millions of new cases reported each year. In this guide, we will cover the important aspects of TB that are frequently tested in the United States Medical Licensing Examination (USMLE).

Epidemiology

TB is a worldwide epidemic, with the highest burden seen in developing countries. According to the World Health Organization (WHO), approximately 10 million people developed TB in 2019, and 1.4 million died from the disease. It is important to note that TB is more prevalent in immunocompromised individuals, such as those with human immunodeficiency virus (HIV) infection.

Transmission

TB is primarily transmitted through the inhalation of respiratory droplets containing Mycobacterium tuberculosis. When an infected individual coughs, sneezes, or talks, they release these droplets into the air, which can be inhaled by others nearby. Close and prolonged contact with an infected person increases the risk of transmission.

Pathogenesis

After inhalation, the bacteria reach the alveoli in the lungs and are phagocytosed by alveolar macrophages. In most cases, the immune system is able to control the infection by forming granulomas, which isolate the bacteria. However, in some individuals, the bacteria can overcome the immune response and cause active disease.

Clinical Presentation

TB can present with a wide range of symptoms, which can sometimes be nonspecific. The most common symptoms include:

• Persistent cough (usually lasting more than 2 weeks)
• Hemoptysis (coughing up blood)
• Fatigue and weakness
• Weight loss
• Fever and night sweats
• Chest pain

In extrapulmonary TB, symptoms will depend on the affected organ system. For example, TB meningitis may present with headache, altered mental status, and neck stiffness.

Diagnosis

The diagnosis of TB requires a combination of clinical evaluation, radiographic findings, and laboratory tests. The following are commonly used diagnostic tools:

• Tuberculin skin test (TST): A purified protein derivative (PPD) is injected intradermally, and a positive reaction indicates exposure to TB. However, it cannot differentiate between latent and active infection.
• Interferon-gamma release assays (IGRAs): Blood tests that detect the release of interferon-gamma by T cells in response to TB antigens. Similar to TST, it cannot differentiate between latent and active infection.
• Chest X-ray: Can show characteristic findings such as upper lobe infiltrates, cavitations, and pleural effusions.
• Sputum smear microscopy: Microscopic examination of stained sputum samples for acid-fast bacilli (AFB).
• Nucleic acid amplification tests: Polymerase chain reaction (PCR) tests that detect Mycobacterium tuberculosis DNA/RNA in sputum samples.
• Culture and drug susceptibility testing: The gold standard for diagnosing TB and determining the appropriate drug regimen.

Treatment

TB treatment consists of multiple drugs to prevent the development of drug resistance. The standard initial therapy for drug-susceptible TB includes a combination of four drugs: isoniazid, rifampin, pyrazinamide, and ethambutol. This regimen is usually continued for 2 months, followed by isoniazid and rifampin for an additional 4 months.

Prevention

Preventive strategies play a crucial role in controlling the spread of TB. The Bacillus Calmette-Guérin (BCG) vaccine provides variable protection against severe forms of TB, particularly in children. It is important to identify and treat individuals with latent TB infection to prevent disease progression and transmission. This is achieved through isoniazid preventive therapy (IPT) or rifampin-based regimens.

Conclusion

Tuberculosis is a significant global health issue that requires prompt diagnosis, appropriate treatment, and preventive measures. Understanding the epidemiology, pathogenesis, clinical presentation, and diagnostic tools of TB is essential for medical professionals preparing for the USMLE. By familiarizing yourself with this information, you will be better equipped to diagnose and manage TB cases effectively.