Description

• A chronic, inflammatory granulomatous infection (caused by Mycobacterium leprae, a slow-growing, acid-fast bacillus), preferentially affecting cooler regions of the body (e.g., skin, peripheral nerves). Initial presentation and disease progression are determined by cell-mediated host immune response.
• Classification (World Health Organization [WHO])
  • Single-lesion paucibacillary (SLPB): one skin lesion; no detectable bacilli on skin smears
  • Paucibacillary (PB): two to five skin lesions; no detectable bacilli on skin smears
  • Multibacillary (MB): ≥6 lesions; may be skin smear positive
• Classification (Ridley-Jopling) based on skin/neurologic changes and biopsy:
  • Indeterminate, tuberculoid (TT), borderline tuberculoid (BT), midborderline, borderline lepromatous (BL), lepromatous (LL)
• System(s) affected: endocrine/metabolic, hemic/lymphatic/immunologic, musculoskeletal, nervous, pulmonary, reproductive, skin/exocrine
• Synonym(s): Hansen disease

Epidemiology

Incidence

• 211,973 new cases detected worldwide in 2015; becoming limited to a small number of countries:
  • 94% of new leprosy cases were reported from 14 countries (WHO).
  • 80% of new cases are in three countries: India, Brazil, and Indonesia (WHO).
• Global incidence decreasing steadily
  • New cases fell 59% from 2003 to 2015 (WHO).
• Rare in the United States:
  • 178 new cases in 2015 (WHO, 2015)
  • Highest rates over last decade: Louisiana > Hawaii > Texas > New York > California > Florida

Prevalence

• 176,176 registered cases worldwide (WHO, 2015)
• Countries with high-rate “pockets” of leprosy (WHO):
  • Angola, Brazil, Central African Republic, Democratic Republic of Congo, India, Madagascar, Mozambique, Nepal, and Tanzania
• May present at any age; cases in infants <1 year of age are extremely rare.
• Male > female = 1.5:1 (in adults)

Pediatric Considerations
Rare in infants <1 year of age
Pregnancy Considerations
Women with leprosy who become pregnant are more likely to develop types I and II reactions and disease relapse postpartum, during the 3rd trimester and with lactation, respectively.

Etiology and Pathophysiology

• Widespread dissemination occurs (in susceptible individuals) once respiratory tract is infected.
• Vigorous cellular immune response results in TT form (PB form).
• Minimal cellular immune response results in LL form (MB form).
• M. leprae: Incubation period is 2 to 5 years for MB cases and 8 to 12 years for PB cases.
• Spread via respiratory transmission and (likely) through broken skin

Genetics

• Leprosy pathogenesis appears to be a three-step process: (i) One group of genes confers susceptibility to infection, (ii) different genes impact the clinical manifestation of disease, and (iii) a third set of genes influences leprosy reversal reaction (1).
• 95% of humans are not susceptible to leprosy.
• Vitamin D deficiency is correlated with genetic susceptibility to leprosy (2).

Risk Factors

• Close family contacts of untreated leprosy patients (8-fold risk); higher risk if patient has MB leprosy
• Impaired cell-mediated immunity/use of “biologic agents” for autoimmune disease (tumor necrosis factor [TNF] antagonists)
• Poor socioeconomic status
• Contact with infected animals, in particular, armadillos (Texas and Louisiana)
• Military service or travel in endemic areas

General Prevention

• Early-case detection and treatment to control spread:
  • Emphasize self-reporting.
• Bacillus Calmette-Guérin (BCG) vaccination in certain locations worldwide may aid in disease prevention.

Commonly Associated Conditions

There is a somewhat higher incidence of leprosy in HIV patients but with concurrent infection:

• HIV-positive patients with early or subclinical leprosy are somewhat more likely to develop overt disease.
• Concurrent leprosy may accelerate HIV disease course; the interaction between leprosy and HIV is less clear than between HIV and TB.