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- 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
- 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
- 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)
Rare in infants <1 year of age
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
- 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).
- 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
- 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.