Mycobacterium leprae: Types of Leprosy, Pathogenesis & Epidemiology of leprosy

Leprosy is a chronic bacterial disease and it is caused by Mycobacterium leprae. Leprosy was first observed by Hansen in 1868 so also known as Hansen disease. It mainly affects the skin, eyes, nose and peripheral nerves.

It is a gram-positive acid-fast bacteria for which 5% percent sulphuric acid is used for the decolorization after staining with carbolfuchsin.

Antigenic structure

Inner most cell wall of bacteria is made of peptidoglycan. External to this is the lipoarabinomanan-B (LAM-B) layer, attached to mycolic acid. Outermost layer is composed of mycosodes which is composed of phenolic glycolipids-1 (PGL-1).

LAM-1 is highly immunogenic and is used in the serodiagnosis of leprosy.


  • Source of infection is patient in case of Leprosy.
  • Source of infection are nasal discharge and skin lesion of patient.

Types of Leprosy

  • A. Based on CMI
  • B. Based on clinical pathological and immunological findings
  • C. Based on the number of lesions and involvement of nerve trunks (WHO)

A. Based on CMI:

The type of leprosy in an individual is determined by the status of cell-mediated immunity. Infection with mycobacterium leprae produces both humoral and cell-mediated immunity. Patients with deficient cell-mediated immunity develop the lepromatous type of disease and when cell-mediated immunity is adequate, the lesions are of tuberculoid type.

There are 4 type of leprosy

1. Lepromatous Leprosy

2. Tuberculoid Leprosy

3. Dimorphous Leprosy

4. Indeterminate Leprosy

Lepromatous and tuberculoid type of leprosy are two extreme forms of leprosy.

FeaturesLepromatous leprosyTuberculoid leprosy
Form of diseaseGeneralized formLocalized form
M. leprae in lesionsNumerous/manyScanty/little
InfectivityHighly infectious
most infectious
Usually Non-infectious
Cell-Mediated ImmunityDeficient/absentAdequate
Lepromin testNegativePositive
lesionsNodular skin lesions know as Lepromatahypo or hyperpigmented macular patches
AntibodiesAbundantRarely produced
Difference between Lepromatous & Tuberculoid leprosy

B. Based on clinical pathological and immunological findings

On the basis of clinical pathological and immunological findings, Ridley & Jopling’s classifying leprosy into five groups.

  1. Tuberculoid (TT)
  2. Borderline tuberculoid (BT)
  3. Borderline (BB)
  4. Borderline lepromatous (BL) and
  5. Lepromatous (LL)
CharacterTuberculoid (TT)Borderline-Tiberculoid (BT) Borderline (BB) Borderline Lepromatous (BL) Lepromatous (LL)
Lepromin test+++++/-
Antibodies to M. laprae+/- +/-++++++
Bacilli in skin +/-++++++
Bacilli in nasal secretion++++
Granuloma formation++++++
Five types of Leprosy

C. According to WHO, type of leprosy

Leprosy is classified into two classes, This classification is based on the number of skin lesions & involvement of the nerve.

a. Paucibacillary Leprosy

b. Multibacillary Leprosy

Leprosynumber of skin lesionInvolvement of nerve
Paucibacillaryfive or less than fiveno or one nerve trunk
Muiltibacillary Six or more than sixmore than one nerve trunk
Paucibacillary & Multibacillary leprosy

Immune reactions in leprosy may cause additional tissue damage. These are two types

  1. Type 1 (reversal reaction type)
  2. Type 2 (Erythema nodosum leprosum, ENL)
Type 1 (reversal reaction type) Type 2 (Erythema nodosum leprosum, ENL)
Occurs in Borderline LeprosyOccurs in Lepromatous leprosy taking treatment of Leprosy
Due to Delay type Hypersensitivity (CMI)Due to the formation of Ag:Ab reaction
may lead to permanent nerve damageRed nodules appear in the skin
Lepra reaction

Epidemiology of leprosy

Source of infection of Mycobacterium leprae is the patient. nasal secretions and discharges from superficial lesions are the most likely infectious materials. The mood of entry may be either through the respiratory tract or to the skin.

The incubation period is very long and averages two to five years it may vary up to 30 year.

Lepromin test

it is not a diagnostic test but it is used to access the resistance of a person to Mycobacterium leprae infection. This reaction was first described by Mitsuda in 1919. He was used lepromin as Antigen.

Nowadays, two types of lepromins antigens are used:

  1. Lepromin-H (Human origin)
  2. Lepromin- A (Armadillo derived)

A lepromin test is carried out by the intradermal injection of 0.1 ml of lepromin.

Read more Lepromin Test click here

Laboratory diagnosis of Leprosy

1. Specimens

2. Acid-fast staining

3. Skin and nerve biopsy

4. Animal inoculation

5. Serological test

Prevention of leprosy

  1. Early early diagnosis and treatment
  2. Surveillance surveillance of contacts
  3. Health health education
  4. Vaccination

There are no effective vaccine against leprosy but a number of vaccines have been tried.

Following list of various candidate vaccine trade

  • BCG vaccine
  • Armadillo derived killed M.laprae
  • Combination of BCG & KILLED M.leprae
  • ICRC bacillus
  • Mycobacterium ‘w’

Treatment of LEPROSY

Pharmacological treatment of Leprosy: Click here

Leprosy: Important key points

  • Mycobacterium leprae produces leprosy in human and only source of infection is patient.
  • Lepromin test is delayed type of hypersensitivity reaction. This test can be used to classify leprosy assessment of prognosis and assess the resistance of a individual to leprosy.
  • Laboratory diagnosis of electricity depends on demonstration of bacilli in specimen by microscopy.
  • The specimen sample from skin erupted by slit and scrap method.
  • Bacteriological index is defined as number of Total bacilli in a tissue.
  • Morphological index is defined as the percentage of live bacilli out of the total number of bacilli.
  • Mycobacterium leprae has not yet been given on artificial culture media for tissue culture.

Multiple Choice Questions (MCQ): Leprosy- Mycobacterium leprae

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