secondary tuberculosis

Secondary tuberculosis represents reactivation of a previously contained (latent) Mycobacterium tuberculosis infection, or less commonly reinfection with a new strain. It occurs when host cell-mediated immunity wanes, allowing the bacteria—previously walled off in Ghon complexes or granulomas from primary infection—to proliferate again. Risk factors include HIV/AIDS, immunosuppressive therapy, diabetes mellitus, malnutrition, chronic kidney disease, and advanced age.

Pathophysiologically, secondary TB preferentially affects the apical and posterior segments of the upper lobes because these areas have the highest oxygen tension, favoring growth of this obligate aerobe. This contrasts with primary TB, which typically involves the mid to lower lung zones (Ghon focus) with associated hilar lymphadenopathy (Ghon complex = Ghon focus + lymphadenopathy).

Histologically, secondary TB is characterized by caseating granulomas with central caseous necrosis surrounded by epithelioid histiocytes, Langhans giant cells, and a rim of lymphocytes. Cavitation is common due to extensive tissue destruction and liquefaction of caseous material, which can rupture into a bronchus, leading to hemoptysis and highly infectious aerosolized sputum.

Clinical features include chronic cough (often productive, sometimes with hemoptysis), low-grade fever, night sweats, weight loss, fatigue, and anorexia—classic "constitutional" symptoms. Chest X-ray typically shows fibrocavitary apical lesions, sometimes with calcification.

Complications include miliary TB (hematogenous dissemination), TB pneumonia, pleural effusion, Pott disease (vertebral TB), renal TB, and Addison disease (adrenal involvement). Diagnosis involves sputum acid-fast bacilli (AFB) smear and culture (gold standard), nucleic acid amplification testing (NAAT), and chest imaging. Treatment involves multidrug therapy (RIPE: rifampin, isoniazid, pyrazinamide, ethambutol) for at least 6 months.

This is a high-yield topic for boards, often tested by contrasting primary vs. secondary TB radiographic and pathologic findings, understanding reactivation risk factors, and recognizing clinical presentation.