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[Microbiology](https://durgical.com/en/microbiology/) · [Mycobacteria](https://durgical.com/en/microbiology/groups/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

## Key points

- Numb patch + thickened nerve = suspect leprosy
- Tuberculoid (Th1, few bacilli) versus lepromatous (Th2, many bacilli)
- MDT: rifampicin + dapsone + clofazimine (the latter darkens the skin)
- It does not grow in vitro; prefers cool body areas

## 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) →](https://durgical.com/en/conditions/leprosy/)
- 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:

- [Rifampicin](https://durgical.com/en/pharmacology/rifampicin/)
- [Dapsone](https://durgical.com/en/pharmacology/dapsone/)
- [Clofazimine](https://durgical.com/en/pharmacology/clofazimine/)

## 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.

## In Colombia

Colombia still diagnoses new cases every year (MinSalud reports a stable figure of about 400 a year, most of them multibacillary), with most cases in Norte de Santander and Valle del Cauca and high rates in Arauca. MDT has been free since 1985, and leprosy is a SIVIGILA surveillance event.

## Memory aid

Leprosy «switches off» what it touches: the skin's colour, the sense of touch, and it seeks the body's cool spots like someone escaping the heat.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/microbiology/mycobacterium-leprae/)

## Related

- [Mycobacterium tuberculosis](https://durgical.com/en/microbiology/mycobacterium-tuberculosis/)
- [Mycobacterium avium complex and other non-tuberculous mycobacteria](https://durgical.com/en/microbiology/mycobacterium-avium-complex-and-other-non-tuberculous-mycobacteria/)

## Related conditions

- [Leprosy (Hansen disease)](https://durgical.com/en/conditions/leprosy/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

[← Back to the group: Mycobacteria](https://durgical.com/en/microbiology/groups/mycobacteria/)

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

Sources: [Leprosy](https://www.who.int/news-room/fact-sheets/detail/leprosy) (World Health Organization (WHO), 2025); [Lepra o enfermedad de Hansen](https://www.minsalud.gov.co/salud/publica/PET/Paginas/Lepra.aspx) (Ministerio de Salud y Protección Social de Colombia, 2017); [About Leprosy (Hansen's Disease) | Leprosy (Hansen's Disease) | CDC](https://www.cdc.gov/leprosy/about/index.html) (Centers for Disease Control and Prevention (CDC), 2024).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/mycobacterium-leprae/