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Microbiology · Mycobacteria

Mycobacterium leprae !

  • Bacterium
  • Acid-fast
  • Exam priority: medium

The leprosy bacterium: it damages skin and peripheral nerves, spreads poorly and is cured with several free antibiotics.

Updated 2026-11-01

Also called: Hansen's bacillus

At a glance

Gram
Acid-fast
Shape
Rod; clusters in «globi» within tissues
Oxygen
Aerobe; prefers cool temperatures (skin, nerves, nose, testes)
Family
Mycobacteriaceae
Stain
Ziehl-Neelsen or Fite-Faraco: acid-fast bacilli
Size
About 1–8 µm long

Morphology

  • Acid-fast rod, intracellular in macrophages and Schwann cells
  • Multiplies very slowly: incubation can last years
  • Does not grow on artificial media

Transmission

  • Nasal and oral droplets from an untreated person, after close and prolonged contact
  • Not spread by casual contact; the patient stops transmitting once treatment starts
  • Most people are naturally resistant

Reservoir: Humans; also nine-banded armadillos

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Virulence factors

  • Invades Schwann cells and destroys nerve fibres: numbness, weakness and deformities
  • Strong Th1 response: tuberculoid form, few lesions and few bacilli
  • Weak Th2-type response: lepromatous form, widespread lesions and many bacilli

Diseases it causes

  • Leprosy (Hansen disease): paucibacillary and multibacillary Leprosy (Hansen disease) →
  • Leprosy reactions: type 1 (reversal) and type 2 (erythema nodosum leprosum)

Clinical picture

  • Hypopigmented or reddish patches with loss of sensation
  • Thickened peripheral nerves (ulnar, peroneal, great auricular)
  • Lepromatous: leonine facies, loss of eyebrows, nasal damage
  • Ulcers and amputations from numb hands and feet

Diagnosis

  • Clinical: skin lesion with loss of sensation or a thickened nerve
  • Slit-skin smear (earlobe and lesions) with Ziehl-Neelsen
  • Skin biopsy with Fite-Faraco stain
  • WHO classification: paucibacillary (few lesions, negative smear) or multibacillary

Treatment

WHO multidrug therapy (MDT) with rifampicin, dapsone and clofazimine; the length depends on whether it is paucibacillary or multibacillary. Leprosy reactions are managed separately with anti-inflammatory drugs such as corticosteroids.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Early diagnosis and treatment to stop transmission and prevent disability
  • Examination of household contacts
  • Post-exposure prophylaxis of contacts with rifampicin, per programme
  • Care of numb hands and feet to prevent ulcers

Vaccine: There is no specific vaccine; the BCG of the PAI schedule gives some protection.

Test yourself

Think of the answer, then tap the question to see it.

Which two clinical signs point to leprosy?

A skin lesion with loss of sensation and a thickened peripheral nerve.

Which immune response predominates in lepromatous leprosy?

A weak cell-mediated response (Th2 profile), with many lesions and a high bacillary load.

What are the three drugs of WHO multidrug therapy?

Rifampicin, dapsone and clofazimine.

Which isolation precautions does a hospitalised leprosy patient need?

Standard precautions only; it spreads poorly and stops once MDT begins.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Leprosy (World Health Organization (WHO), 2025); Lepra o enfermedad de Hansen (Ministerio de Salud y Protección Social de Colombia, 2017); About Leprosy (Hansen's Disease) | Leprosy (Hansen's Disease) | CDC (Centers for Disease Control and Prevention (CDC), 2024).

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Representative model, for educational purposes only.