What is Leprosy (Hansen Disease)?
Leprosy, also known as Hansen disease, is a chronic bacterial infection caused by Mycobacterium leprae (and M. lepromatosis) that primarily affects the skin, peripheral nerves, and mucosa of the upper respiratory tract. 1
Causative Organism
The disease is caused by acid-fast bacilli of the Mycobacterium leprae complex, which includes both M. leprae and the more recently identified M. lepromatosis 2, 3. These organisms have a particular tropism for cooler body tissues, explaining their predilection for skin and peripheral nerves.
Clinical Spectrum
The disease manifests along a clinical spectrum that reflects variations in the host's cell-mediated immune response to the organism 1, 2. The CDC guidelines 1 define four major clinical forms:
Tuberculoid Form
- One or a few well-demarcated, hypopigmented, and anesthetic skin lesions
- Frequently shows active, spreading edges with a clearing center
- Peripheral nerve swelling or thickening may occur
- Represents a strong cell-mediated immune response with few bacilli
Lepromatous Form
- Multiple erythematous papules and nodules or infiltration of the face, hands, and feet
- Lesions distributed bilaterally and symmetrically
- Progressive thickening of the skin
- Represents weak cell-mediated immunity with high bacterial loads
Borderline (Dimorphous) Form
- Skin lesions showing characteristics of both tuberculoid and lepromatous forms
- Intermediate immune response
Indeterminate Form
- Early lesions, usually hypopigmented macules
- Lacks developed tuberculoid or lepromatous features
- May progress to other forms
Key Diagnostic Features
The hallmark of leprosy is the combination of skin lesions with peripheral nerve involvement, particularly the loss of sensation in affected areas 1, 2. This neurocutaneous syndrome distinguishes leprosy from many other dermatologic conditions 4.
Laboratory Confirmation
According to CDC guidelines 1, confirmed diagnosis requires:
- Demonstration of acid-fast bacilli in skin or dermal nerve obtained from full-thickness skin biopsy of a lepromatous lesion
Modern diagnostics also include 2:
- Skin smears
- Histopathology showing characteristic granulomas
- Polymerase chain reaction (PCR) for bacterial DNA
- Nerve conduction studies to detect subclinical neuropathy
Clinical Significance
Peripheral nerve damage is irreversible once it occurs, making early recognition and treatment critical 2. The disease can cause:
- Permanent sensory loss leading to unnoticed injuries
- Motor weakness and muscle atrophy
- Deformities of fingers, toes, and limbs
- Aesthetic and functional impairment 4
Leprosy Reactions
Type 1 and type 2 reactions can cause rapid nerve deterioration and require immediate anti-inflammatory treatment 2. Type 2 reactions (erythema nodosum leprosum) may present with painful lesions, sometimes with atypical features like bullae and necrosis 5.
Transmission and Epidemiology
Recent evidence supports zoonotic transmission from nine-banded armadillos and Eurasian red squirrels 3. The disease remains endemic in certain regions and is increasingly seen as an imported disease in developed countries due to migration patterns 5, 6.