Tuberculosis (Mycobacterium tuberculosis)

Overview

Plain-Language Overview

Tuberculosis (TB) is an infectious disease caused by the bacterium Mycobacterium tuberculosis that primarily affects the lungs but can involve other parts of the body. It spreads through the air when a person with active TB coughs or sneezes, releasing tiny droplets containing the bacteria. The infection can cause symptoms like a persistent cough, fever, night sweats, and weight loss. TB can remain dormant in the body without symptoms or become active and cause serious illness. The disease mainly impacts the respiratory system and can lead to lung damage if untreated. Early detection and diagnosis are important to prevent spread and complications.

Clinical Definition

Tuberculosis (TB) is a chronic infectious disease caused by the aerobic, acid-fast bacillus Mycobacterium tuberculosis. It primarily affects the pulmonary system but can disseminate to extrapulmonary sites such as lymph nodes, bones, and the central nervous system. The pathogenesis involves inhalation of aerosolized bacilli, followed by macrophage phagocytosis and formation of granulomas with caseating necrosis. The disease manifests as either latent infection or active disease, with active TB characterized by productive cough, hemoptysis, fever, night sweats, and weight loss. TB is a major global health concern due to its potential for transmission, chronic morbidity, and development of drug-resistant strains. Diagnosis and management require integration of clinical, radiologic, microbiologic, and immunologic data.

Inciting Event

Inhalation of airborne droplets containing Mycobacterium tuberculosis from a person with active pulmonary tuberculosis.
Reactivation of latent tuberculosis due to immune suppression or systemic illness.
Exposure to multidrug-resistant M. tuberculosis strains in endemic or healthcare settings.

Latency Period

Weeks to months from initial infection to development of primary pulmonary granulomas.
Months to years of latent infection before possible reactivation and symptomatic disease.
Variable latency depending on host immune status and bacterial load.

Diagnostic Delay

Nonspecific symptoms such as chronic cough and weight loss mimic other respiratory diseases, delaying suspicion.
Low sensitivity of sputum smear microscopy in early or paucibacillary disease leads to missed diagnosis.
Limited access to advanced diagnostics like nucleic acid amplification tests in resource-poor settings.
Misattribution to common bacterial pneumonia or malignancy due to overlapping clinical and radiographic features.

Patient Population

Adults aged 20-50 years in endemic areas are most commonly affected by active tuberculosis.
Immunocompromised patients, including those with HIV/AIDS, have higher incidence and more severe disease.
Individuals in low- and middle-income countries bear the highest global burden of tuberculosis.
Healthcare workers and close contacts of active cases are at increased occupational risk.

Risk Factors

HIV infection causing impaired cell-mediated immunity is the strongest risk factor for active tuberculosis.
Close contact with individuals with active pulmonary tuberculosis increases transmission risk.
Immunosuppressive therapy such as corticosteroids or TNF-α inhibitors increases risk of reactivation.
Malnutrition and chronic illnesses like diabetes mellitus impair immune defenses against M. tuberculosis.
Living in crowded or poorly ventilated environments facilitates airborne spread of tuberculosis.

Clinical Presentation

Signs & Symptoms

Chronic cough lasting more than 3 weeks, often productive and sometimes hemoptysis.
Night sweats and fever are common systemic symptoms reflecting active infection.
Weight loss and fatigue indicate chronic inflammatory state and catabolism.
Chest pain may occur due to pleural involvement or cavitary lesions.
Lymphadenopathy and localized swelling in extrapulmonary tuberculosis sites.

History of Present Illness

Chronic productive cough lasting >3 weeks often with hemoptysis in active pulmonary tuberculosis.
Constitutional symptoms including fever, night sweats, weight loss, and fatigue develop insidiously.
Progressive dyspnea and chest pain may occur with extensive lung involvement or pleural tuberculosis.
Symptoms worsen gradually over weeks to months, reflecting slow mycobacterial growth and immune response.

Past Medical History

Previous latent tuberculosis infection or incomplete treatment increases risk of reactivation.
HIV infection or other immunodeficiency states predispose to active tuberculosis.
History of incarceration, homelessness, or residence in endemic areas increases exposure risk.
Prior use of immunosuppressive drugs such as corticosteroids or TNF-α inhibitors.

Family History

Close household contacts with active tuberculosis increase risk of transmission and infection.
No specific heritable genetic syndromes are strongly linked to tuberculosis susceptibility, but polymorphisms in immune genes may modulate risk.
Family history of immunodeficiency disorders may predispose to severe or disseminated tuberculosis.

Physical Exam Findings

Cervical lymphadenopathy (scrofula) is a common palpable finding in extrapulmonary tuberculosis.
Dullness to percussion and decreased breath sounds may be present over areas of pulmonary consolidation or cavitation.
Clubbing of fingers can occur in chronic pulmonary tuberculosis.
Cachexia and signs of chronic illness such as muscle wasting are often observed in advanced disease.
Pleural effusion signs such as decreased tactile fremitus and egophony may be present in tuberculous pleuritis.

Physical Exam Maneuvers

Tuberculin skin test (Mantoux test) is used to assess delayed-type hypersensitivity to Mycobacterium tuberculosis antigens.
Auscultation of lungs to detect crackles or bronchial breath sounds indicating active pulmonary involvement.
Chest percussion to identify areas of consolidation or pleural effusion.
Lymph node examination to detect enlargement or tenderness in common sites such as cervical and supraclavicular regions.
Assessment of neurological status in suspected tuberculous meningitis or spinal tuberculosis.

Common Comorbidities

HIV infection significantly increases risk of active tuberculosis and atypical presentations.
Diabetes mellitus impairs immune response and predisposes to tuberculosis reactivation.
Chronic kidney disease is associated with higher tuberculosis susceptibility due to immune dysfunction.
Malnutrition weakens host defenses and worsens tuberculosis outcomes.
Silicosis increases risk of pulmonary tuberculosis due to lung tissue damage.

Diagnostic Workup

Diagnostic Criteria

Diagnosis of tuberculosis relies on identification of acid-fast bacilli in sputum smear microscopy or culture of Mycobacterium tuberculosis from respiratory or extrapulmonary specimens. Chest radiography showing cavitary lesions or infiltrates in the upper lobes supports the diagnosis. The tuberculin skin test (TST) or interferon-gamma release assays (IGRAs) detect immune sensitization but cannot distinguish latent from active disease. Definitive diagnosis requires microbiologic confirmation by culture or nucleic acid amplification tests demonstrating the presence of M. tuberculosis complex. Histopathology showing caseating granulomas can aid diagnosis in extrapulmonary TB.

Pathophysiology

Key Mechanisms

Inhalation of aerosolized Mycobacterium tuberculosis leads to alveolar macrophage infection and granuloma formation.
Cell-mediated immunity with activated Th1 CD4+ T cells produces IFN-γ, which activates macrophages to contain infection.
Caseating granulomas form due to chronic immune response, causing tissue necrosis and lung cavitation.
Latent infection occurs when bacteria persist in a dormant state within granulomas, evading immune clearance.
Reactivation of latent bacilli occurs with immune suppression, leading to active disease and transmission.

Involvement	Details
Organs	Lungs are the main organs affected by tuberculosis, presenting with cavitary lesions and consolidation.
	Lymph nodes commonly enlarge due to granulomatous inflammation in extrapulmonary tuberculosis.
	Kidneys can be involved in disseminated tuberculosis, leading to renal granulomas and dysfunction.
Tissues	Lung parenchyma is the primary site of infection and granuloma formation in pulmonary tuberculosis.
Tissues	Lymphatic tissue is involved in immune response and may show caseating granulomas in disseminated disease.
Cells	Macrophages phagocytose Mycobacterium tuberculosis and form granulomas to contain infection.
	T helper 1 (Th1) cells produce interferon-gamma to activate macrophages against Mycobacterium tuberculosis.
	Multinucleated giant cells form part of granulomas characteristic of chronic tuberculosis infection.
Chemical Mediators	Interferon-gamma activates macrophages to enhance killing of Mycobacterium tuberculosis.
	Tumor necrosis factor-alpha (TNF-α) is critical for granuloma maintenance and containment of tuberculosis.
	Interleukin-12 (IL-12) promotes differentiation of naive T cells into Th1 cells essential for immune response.

Treatments

Pharmacological Treatments

Isoniazid
- Mechanism:
  - Inhibits mycolic acid synthesis, disrupting the Mycobacterium tuberculosis cell wall.
- Side effects:
  - Hepatotoxicity
  - Peripheral neuropathy
  - Rash
- Clinical role:
  - First-line
Rifampin
- Mechanism:
  - Inhibits DNA-dependent RNA polymerase, blocking RNA synthesis in Mycobacterium tuberculosis.
- Side effects:
  - Hepatotoxicity
  - Orange discoloration of body fluids
  - Drug interactions
- Clinical role:
  - First-line
Pyrazinamide
- Mechanism:
  - Disrupts mycobacterial membrane metabolism and transport functions under acidic conditions.
- Side effects:
  - Hepatotoxicity
  - Hyperuricemia
  - Arthralgia
- Clinical role:
  - First-line
Ethambutol
- Mechanism:
  - Inhibits arabinosyl transferase, impairing cell wall synthesis in Mycobacterium tuberculosis.
- Side effects:
  - Optic neuropathy
  - Red-green color blindness
- Clinical role:
  - First-line

Non-pharmacological Treatments

Isolation of infectious patients to prevent airborne transmission of tuberculosis.
Nutritional support to improve immune function during tuberculosis treatment.
Directly observed therapy (DOT) to ensure adherence to antitubercular medication regimens.

Prevention

Pharmacological Prevention

Isoniazid preventive therapy (IPT) for latent tuberculosis infection to reduce progression to active disease.
Rifampin prophylaxis as an alternative for isoniazid-intolerant patients.
Combination therapy with isoniazid and rifapentine weekly for 3 months as a shorter latent TB regimen.
Bacillus Calmette-Guérin (BCG) vaccine is not pharmacological but often given in endemic areas for prevention.
No other pharmacological agents are routinely used for tuberculosis prevention.

Non-pharmacological Prevention

Screening high-risk populations such as HIV-positive individuals and close contacts of active cases.
Isolation of infectious patients with airborne precautions to prevent transmission.
Use of N95 respirators by healthcare workers in contact with active tuberculosis patients.
Improving ventilation and ultraviolet germicidal irradiation in congregate settings to reduce airborne spread.
Public health measures including contact tracing and treatment adherence monitoring.

Outcome & Complications

Complications

Miliary tuberculosis results from hematogenous dissemination causing multi-organ failure.
Tuberculous meningitis leads to neurological deficits and high mortality if untreated.
Bronchiectasis and lung fibrosis develop from chronic pulmonary inflammation.
Pleural effusion and empyema can complicate pulmonary tuberculosis.
Airway obstruction from lymph node enlargement or endobronchial tuberculosis.

Short-term Sequelae	Long-term Sequelae
Acute respiratory distress from extensive pulmonary involvement or pleural effusion. Lymphadenitis causing painful swelling and possible abscess formation. Fever and systemic inflammatory response during active infection or immune reconstitution. Drug-induced hepatotoxicity from antituberculous therapy initiation. Transient worsening of symptoms due to immune reconstitution inflammatory syndrome (IRIS) in HIV co-infection.	Pulmonary fibrosis and restrictive lung disease from healed granulomatous lesions. Chronic bronchiectasis leading to recurrent infections and hemoptysis. Calcified granulomas visible on imaging as residual scars. Spinal deformities and neurological deficits from Pott disease (vertebral tuberculosis). Chronic pleural thickening causing restrictive lung physiology.

Short-term Sequelae

Long-term Sequelae

Acute respiratory distress from extensive pulmonary involvement or pleural effusion.
Lymphadenitis causing painful swelling and possible abscess formation.
Fever and systemic inflammatory response during active infection or immune reconstitution.
Drug-induced hepatotoxicity from antituberculous therapy initiation.
Transient worsening of symptoms due to immune reconstitution inflammatory syndrome (IRIS) in HIV co-infection.

Pulmonary fibrosis and restrictive lung disease from healed granulomatous lesions.
Chronic bronchiectasis leading to recurrent infections and hemoptysis.
Calcified granulomas visible on imaging as residual scars.
Spinal deformities and neurological deficits from Pott disease (vertebral tuberculosis).
Chronic pleural thickening causing restrictive lung physiology.

Differential Diagnoses

Tuberculosis (Mycobacterium tuberculosis) versus Histoplasmosis

Tuberculosis (Mycobacterium tuberculosis)	Histoplasmosis
Exposure to individuals with active pulmonary disease or residence in high TB prevalence regions	Exposure to bat or bird droppings in endemic areas such as the Ohio and Mississippi River valleys
Upper lobe cavitary lesions or apical infiltrates with possible miliary pattern	Diffuse reticulonodular infiltrates with mediastinal or hilar lymphadenopathy, often with calcifications
Positive acid-fast bacilli smear or culture for Mycobacterium tuberculosis	Positive urine or serum Histoplasma antigen test
Caseating granulomas with acid-fast bacilli on Ziehl-Neelsen stain	Granulomas with numerous intracellular yeast forms visible on fungal stains

Tuberculosis (Mycobacterium tuberculosis) versus Sarcoidosis

Tuberculosis (Mycobacterium tuberculosis)	Sarcoidosis
Asymmetric upper lobe infiltrates with cavitary lesions	Bilateral symmetric hilar and mediastinal lymphadenopathy without cavitation
Caseating granulomas with central necrosis	Noncaseating granulomas without necrosis
Positive acid-fast bacilli smear or culture	Elevated serum angiotensin-converting enzyme (ACE) levels and negative acid-fast bacilli stains
Progressive pulmonary symptoms with constitutional signs such as night sweats and weight loss	Chronic, often asymptomatic or with systemic symptoms like erythema nodosum

Tuberculosis (Mycobacterium tuberculosis) versus Lung cancer (especially squamous cell carcinoma)

Tuberculosis (Mycobacterium tuberculosis)	Lung cancer (especially squamous cell carcinoma)
Patchy or nodular infiltrates with cavitation primarily in upper lobes	Solitary pulmonary nodule or mass often with irregular borders and possible hilar enlargement
History of exposure to airborne Mycobacterium tuberculosis	History of heavy tobacco smoking
Acid-fast bacilli identified on sputum smear or culture	Malignant cells on cytology or biopsy
Chronic cough with fever, night sweats, and weight loss	Progressive respiratory symptoms with hemoptysis and weight loss, often without fever

Tuberculosis (Mycobacterium tuberculosis) versus Nontuberculous mycobacterial (NTM) infection

Tuberculosis (Mycobacterium tuberculosis)	Nontuberculous mycobacterial (NTM) infection
Growth of Mycobacterium tuberculosis complex in culture	Growth of slow-growing mycobacteria such as Mycobacterium avium complex in culture
Upper lobe cavitary lesions or miliary pattern	Bronchiectasis and nodular infiltrates predominantly in middle lobe and lingula
Positive NAAT for M. tuberculosis	Negative nucleic acid amplification test (NAAT) for M. tuberculosis with positive culture for NTM
Variable course with potential for active contagious pulmonary disease	Indolent course often in patients with underlying lung disease or immunosuppression

Tuberculosis (Mycobacterium tuberculosis) versus Pulmonary fungal infections (e.g., Coccidioidomycosis)

Tuberculosis (Mycobacterium tuberculosis)	Pulmonary fungal infections (e.g., Coccidioidomycosis)
Exposure to individuals with active tuberculosis or endemic TB regions	Residence or travel to southwestern United States or other endemic areas
Bilateral upper lobe infiltrates with cavitation or miliary pattern	Pulmonary nodules or cavities with surrounding consolidation, often unilateral
Positive acid-fast bacilli smear or culture	Positive serologic tests for Coccidioides antibodies
Caseating granulomas with acid-fast bacilli	Spherules containing endospores visible on histology