Morphology: Acid-fast bacillus (AFB) on Fite stain, similar to MTB. **Uncultivable in standard lab media** — grows only in armadillos + mouse footpad model. Extremely slow doubling time (~12–14 days). Tropism for cooler tissues (skin, peripheral nerves, anterior eye, upper airway).
Typical drugs
- #1Dapsone— 100 mg PO daily. Backbone of WHO MDT for both PB + MB. Check G6PD before start.
- #2Rifampin— **600 mg PO once monthly (supervised)** in WHO MDT. Bactericidal — single dose reduces transmission risk substantially.
- #3Clofazimine— **300 mg monthly + 50 mg daily** for MB regimen. Causes skin discoloration.
Empiric therapy when resistant
Rifampin-resistant leprosy: fluoroquinolone (moxifloxacin / ofloxacin) + clarithromycin + minocycline regimens — specialist-managed.
Resistance mechanisms
altered-target
rpoB mutations → rifampin resistance
Example: Documented in untreated relapse cases; rare in MDT-adherent patients.
Resistance notes
Resistance to rifampin / dapsone / ofloxacin is monitored globally; remains uncommon with adherent MDT.
Pearls
Ridley-Jopling spectrum: TT (tuberculoid — paucibacillary, strong cell-mediated immunity, few skin lesions, anesthetic) → BT → BB → BL → LL (lepromatous — multibacillary, weak CMI, many lesions, AFB abundant). WHO PB vs MB classification (simplified): ≤5 skin lesions + negative smear = PB; ≥6 lesions or positive smear = MB. Treatment (WHO MDT 2018+): PB = rifampin monthly + dapsone daily × 6 months; MB = rifampin monthly + dapsone daily + clofazimine (300 monthly + 50 daily) × 12 months. Lepra reactions (immune-mediated, can occur during or after treatment): Type 1 (reversal) → corticosteroids; Type 2 (ENL, erythema nodosum leprosum) → corticosteroids ± thalidomide ± clofazimine (anti-inflammatory effect at higher doses). Don't stop MDT for reactions — treat the reaction separately. Notifiable disease in US — report to state health + Hansen's Disease Program. Derm relevance: armadillo exposure + hypopigmented anesthetic patch + nerve thickening = leprosy until proven otherwise.