Leprosy (Hansen's Disease) (Mycobacterium leprae)

Overview

Plain-Language Overview

Leprosy (Hansen's Disease) is a chronic infection that mainly affects the skin and nerves. It is caused by the bacterium Mycobacterium leprae, which grows slowly and can lead to skin lesions and nerve damage. This nerve damage can cause a loss of feeling, which increases the risk of injuries and infections. The disease primarily affects the peripheral nervous system, leading to muscle weakness and deformities if untreated. Early symptoms include light or reddish skin patches with reduced sensation. Over time, untreated leprosy can cause significant disability due to nerve and tissue damage.

Clinical Definition

Leprosy (Hansen's Disease) is a chronic granulomatous infection caused by the obligate intracellular bacterium Mycobacterium leprae. It primarily targets the skin and peripheral nerves, leading to a spectrum of clinical manifestations based on the host immune response. The disease is classified into tuberculoid and lepromatous forms, representing strong cell-mediated immunity with localized disease versus weak immunity with widespread infection, respectively. The hallmark pathology includes nerve inflammation and demyelination, resulting in sensory loss and muscle weakness. The disease is significant due to its potential for causing permanent neuropathy, deformities, and disability. Transmission occurs mainly via respiratory droplets, and the incubation period is typically several years.

Inciting Event

Inhalation or skin contact with infectious droplets or secretions from untreated patients
Exposure to Mycobacterium leprae through prolonged close contact with infected individuals
Transmission via nasal secretions is the primary route of infection
Zoonotic transmission from infected armadillos in endemic regions
Entry of bacilli into peripheral nerves and skin macrophages initiates infection

Latency Period

Long incubation period of 2 to 10 years before clinical symptoms develop
Variable latency depending on host immunity, with tuberculoid forms presenting earlier than lepromatous forms
Slow bacterial replication rate contributes to prolonged asymptomatic phase
Delayed nerve involvement may precede skin manifestations by months to years
Subclinical infection can persist for years before diagnosis

Diagnostic Delay

Indolent onset with subtle skin lesions and mild sensory loss leads to underrecognition
Misdiagnosis as common dermatologic or neuropathic conditions such as eczema or diabetic neuropathy
Lack of awareness in non-endemic areas delays consideration of leprosy
Paucibacillary forms have low bacterial load making slit-skin smear less sensitive
Stigma and social isolation may delay patient presentation

Patient Population

Individuals in endemic tropical and subtropical regions are most commonly affected
All age groups can be affected but diagnosis is more common in young adults
Both sexes are affected with no strong sex predilection
Close contacts of infected individuals such as household members have higher incidence
People with prolonged exposure to armadillos in the southern United States represent a zoonotic risk group

Risk Factors

Close and prolonged contact with untreated patients increases transmission risk
Living in endemic areas such as India, Brazil, and Southeast Asia is a major epidemiologic risk factor
Immunosuppression or impaired cell-mediated immunity predisposes to lepromatous disease
Genetic susceptibility involving HLA-DR and other immune-related genes influences disease susceptibility and type
Poor socioeconomic conditions and overcrowding facilitate spread of M. leprae

Clinical Presentation

Signs & Symptoms

Painless hypopigmented or erythematous skin patches with sensory loss
Peripheral neuropathy causing numbness, paresthesias, and muscle weakness
Nerve thickening and tenderness often palpable along major peripheral nerves
Nasal congestion and epistaxis in some cases due to mucosal involvement
Eye involvement causing lagophthalmos and corneal anesthesia leading to vision loss

History of Present Illness

Progressive hypopigmented or erythematous skin patches with sensory loss are typical initial symptoms
Numbness and paresthesias in peripheral nerves develop gradually
Thickened peripheral nerves with associated pain or tenderness may be reported
Muscle weakness and deformities such as claw hand or foot drop occur in advanced disease
Episodes of acute neuritis or erythema nodosum leprosum reactions cause sudden worsening

Past Medical History

Previous untreated or inadequately treated leprosy increases risk of relapse or reactions
History of immunosuppressive conditions or therapies may worsen disease severity
Prior exposure to endemic areas or contact with known cases is relevant
No specific vaccinations or prior infections directly affect risk
Chronic neuropathies or skin conditions may confound presentation

Family History

Family members with leprosy increase risk due to shared environment and genetics
Certain HLA haplotypes associated with susceptibility cluster in families
No classic Mendelian inheritance pattern but familial aggregation is common
Genetic polymorphisms in immune response genes influence disease phenotype
No known hereditary syndromes directly cause leprosy but familial exposure is key

Physical Exam Findings

Hypopigmented or erythematous skin patches with loss of sensation due to peripheral nerve involvement
Thickened peripheral nerves, especially ulnar, peroneal, and greater auricular nerves
Muscle weakness and atrophy in affected areas from nerve damage
Nerve tenderness on palpation indicating active inflammation
Claw hand deformity from ulnar nerve damage in advanced cases

Diagnostic Workup

Diagnostic Criteria

Diagnosis is established by identifying at least one of the following: hypopigmented or erythematous skin lesions with sensory loss, thickened peripheral nerves, or detection of acid-fast bacilli on skin smear or biopsy. Histopathology shows granulomatous inflammation with foamy macrophages in lepromatous forms or well-formed granulomas in tuberculoid forms. The Fite-Faraco stain is used to visualize Mycobacterium leprae in tissue samples. Clinical classification and nerve involvement guide diagnosis and management.

Pathophysiology

Key Mechanisms

Chronic granulomatous inflammation caused by Mycobacterium leprae infection primarily affecting skin and peripheral nerves
Cell-mediated immunity determines disease spectrum, with strong Th1 response leading to tuberculoid leprosy and weak Th1/strong Th2 response causing lepromatous leprosy
Nerve damage results from direct bacterial invasion and immune-mediated inflammation causing demyelination and axonal loss
Type 1 (reversal) and Type 2 (erythema nodosum leprosum) reactions represent immune-mediated exacerbations contributing to acute nerve injury
Intracellular survival of M. leprae within macrophages and Schwann cells facilitates chronic infection and tissue destruction

Involvement	Details
Organs	Skin is the main organ affected, presenting with hypopigmented or erythematous anesthetic plaques.
Organs	Peripheral nerves undergo demyelination and fibrosis leading to sensory and motor deficits characteristic of leprosy.
Tissues	Peripheral nerve tissue is the primary site of damage causing sensory loss and deformities in leprosy.
Tissues	Skin tissue shows granulomatous inflammation and infiltration by infected macrophages in leprosy lesions.
Cells	Macrophages phagocytose Mycobacterium leprae and are the primary host cells harboring the bacteria.
	Schwann cells are infected by Mycobacterium leprae, leading to nerve damage and sensory loss.
	T helper 1 (Th1) cells mediate the cell-mediated immune response in tuberculoid leprosy, controlling bacterial growth.
	T helper 2 (Th2) cells predominate in lepromatous leprosy, leading to ineffective humoral response and widespread infection.
Chemical Mediators	Tumor necrosis factor-alpha (TNF-α) contributes to granuloma formation and nerve inflammation in leprosy.
	Interferon-gamma (IFN-γ) activates macrophages to kill Mycobacterium leprae in tuberculoid leprosy.
	Interleukin-10 (IL-10) suppresses cell-mediated immunity, promoting bacterial proliferation in lepromatous leprosy.

Treatments

Pharmacological Treatments

Dapsone
- Mechanism:
  - Inhibits dihydropteroate synthase, blocking folate synthesis in Mycobacterium leprae.
- Side effects:
  - Hemolytic anemia
  - Methemoglobinemia
  - Hypersensitivity reactions
- Clinical role:
  - First-line
Rifampin
- Mechanism:
  - Inhibits bacterial DNA-dependent RNA polymerase, suppressing RNA synthesis in Mycobacterium leprae.
- Side effects:
  - Hepatotoxicity
  - Orange discoloration of body fluids
  - Flu-like syndrome
- Clinical role:
  - First-line
Clofazimine
- Mechanism:
  - Binds to mycobacterial DNA, interfering with replication and has anti-inflammatory properties.
- Side effects:
  - Skin discoloration
  - Gastrointestinal upset
  - Photosensitivity
- Clinical role:
  - First-line
Prednisone
- Mechanism:
  - Suppresses immune-mediated inflammation during lepra reactions.
- Side effects:
  - Hyperglycemia
  - Immunosuppression
  - Osteoporosis
- Clinical role:
  - Adjunctive

Non-pharmacological Treatments

Physical therapy to prevent disability and maintain joint mobility in affected limbs.
Surgical correction of deformities and nerve decompression in advanced cases.
Protective measures to prevent trauma and secondary infections in anesthetic skin areas.

Prevention

Pharmacological Prevention

Single-dose rifampin chemoprophylaxis for close contacts to reduce transmission
Multidrug therapy (MDT) with dapsone, rifampin, and clofazimine to prevent disease progression
Early treatment initiation to prevent nerve damage and disability
Corticosteroids to prevent nerve damage during lepra reactions
Thalidomide for severe Type 2 lepra reactions to reduce inflammation

Non-pharmacological Prevention

Avoidance of prolonged close contact with untreated patients to reduce transmission
Regular screening of household contacts for early detection
Protective footwear and skin care to prevent trauma and secondary infections
Health education to reduce stigma and encourage early treatment
Improved sanitation and living conditions to decrease spread in endemic areas

Outcome & Complications

Complications

Permanent nerve damage leading to sensory and motor deficits
Disfiguring deformities such as claw hand and foot drop
Chronic ulcers and secondary infections increasing morbidity
Blindness from ocular nerve involvement and corneal damage
Lepromatous leprosy can cause widespread skin nodules and systemic involvement

Short-term Sequelae	Long-term Sequelae
Lepra reactions (Type 1 and Type 2) causing acute neuritis and inflammation Acute nerve pain and tenderness during reaction episodes Edema and erythema of skin lesions during inflammatory flares Fever and malaise associated with systemic inflammatory response Transient worsening of neurological symptoms during treatment initiation	Permanent sensory loss leading to repeated injuries and ulcers Muscle atrophy and contractures causing disability and deformity Chronic neuropathic ulcers prone to secondary infection and osteomyelitis Facial disfigurement from nasal cartilage destruction and skin changes Social stigma and psychological impact due to visible deformities

Differential Diagnoses

Leprosy (Hansen's Disease) (Mycobacterium leprae) versus Tuberculoid Leprosy

Leprosy (Hansen's Disease) (Mycobacterium leprae)	Tuberculoid Leprosy
Weak cell-mediated immunity with numerous acid-fast bacilli in lesions	Strong cell-mediated immunity with granulomatous inflammation and few bacilli
Multibacillary disease with widespread skin lesions and nerve involvement	Localized, paucibacillary disease with few skin lesions
Positive acid-fast bacilli on skin smear	Negative or scant acid-fast bacilli on skin smear

Leprosy (Hansen's Disease) (Mycobacterium leprae) versus Cutaneous Tuberculosis

Leprosy (Hansen's Disease) (Mycobacterium leprae)	Cutaneous Tuberculosis
Infection with Mycobacterium leprae	Infection with Mycobacterium tuberculosis
Non-caseating granulomas with foamy macrophages (Virchow cells)	Caseating granulomas with Langhans giant cells
History of prolonged close contact with untreated leprosy patient	History of exposure to active tuberculosis or endemic area

Leprosy (Hansen's Disease) (Mycobacterium leprae) versus Peripheral Neuropathy from Diabetes Mellitus

Leprosy (Hansen's Disease) (Mycobacterium leprae)	Peripheral Neuropathy from Diabetes Mellitus
Asymmetric nerve involvement with skin hypopigmented or erythematous lesions	Symmetric distal polyneuropathy without skin lesions
Normal blood glucose and HbA1c	Elevated blood glucose and HbA1c levels
Granulomatous inflammation with acid-fast bacilli in nerves	Axonal degeneration and demyelination without granulomas

Leprosy (Hansen's Disease) (Mycobacterium leprae) versus Sarcoidosis

Leprosy (Hansen's Disease) (Mycobacterium leprae)	Sarcoidosis
Non-caseating granulomas with acid-fast bacilli present	Non-caseating granulomas without acid-fast bacilli
Primarily skin and peripheral nerve involvement	Multisystem involvement including lungs, lymph nodes, and skin
Positive acid-fast bacilli stain and PCR for M. leprae	Negative acid-fast bacilli stain and negative PCR for M. leprae

Leprosy (Hansen's Disease) (Mycobacterium leprae) versus Leishmaniasis

Leprosy (Hansen's Disease) (Mycobacterium leprae)	Leishmaniasis
Infection with Mycobacterium leprae (bacterium)	Infection with Leishmania species (protozoan)
Foamy macrophages with acid-fast bacilli	Macrophages containing amastigotes (Leishman-Donovan bodies)
Prolonged close contact with untreated leprosy patients	Exposure to sandfly bites in endemic areas