Mycobacterium leprae

Mycobacterium leprae is an obligate intracellular acid-fast bacillus causing leprosy (Hansen's disease) — a chronic granulomatous disease primarily of the skin and peripheral nerves. It is unique among medical bacteria for being non-cultivable in vitro, having the longest generation time of any bacterium, and producing a disease whose entire clinical spectrum is decided by the host's cell-mediated immunity (CMI).

Definition & microbiological identity

Leprosy is a chronic mycobacterial infection with predilection for cooler body sites (skin, superficial peripheral nerves, anterior eye, upper respiratory mucosa, testes) because the organism grows optimally at 27–33°C, not core body temperature. This temperature preference explains the classic distribution of lesions over ears, nose, malar region, and the dorsal/extensor cooler surfaces of limbs.

Key microbiological features of M. leprae:

• Obligate intracellular, weakly acid-fast bacillus (acid-fast with 5% sulphuric acid in modified Ziehl-Neelsen, i.e. less acid than M. tuberculosis which uses 20–25% H₂SO₄ via Fite-Faraco stain).
• Cannot be grown on any artificial culture medium or cell culture — the only human-pathogenic mycobacterium with this property. This is the single most-tested microbiology fact.
• Generation time of ~12–14 days (longest of any known bacterium; M. tuberculosis ~20 hours), hence the disease's chronicity and prolonged therapy.
• Reduced genome (~half of its genes are non-functional pseudogenes), explaining its dependence on the host and inability to be cultured.
• Forms "globi" — clumps of bacilli bound by a lipid (gloea) — and arranges in parallel bundles like "cigarettes in a packet."
• Invades Schwann cells (binds the G domain of laminin-2 in basal lamina via α-dystroglycan and ErbB2 receptor) → demyelination and nerve damage.
• Has PGL-1 (phenolic glycolipid-1), a virulence factor unique to M. leprae, used in serology (anti-PGL-1 antibody).

High-yield: M. leprae is the only mycobacterium that cannot be cultured in vitro. It is grown for research in the mouse footpad (Shepard's model) and the nine-banded armadillo (its low body temperature ~34°C and large reticuloendothelial mass make it ideal). Mycobacterium lepromatosis is a separate species causing diffuse lepromatous leprosy / Lucio phenomenon (Mexico, Caribbean).

Transmission: Mainly via nasal droplets / respiratory route from untreated multibacillary cases (a single sneeze of a lepromatous patient can shed millions of bacilli). Prolonged close contact is usually required. Incubation period is very long: 2–5 years (tuberculoid), up to 8–10 years or more (lepromatous). Natural reservoir other than humans is the armadillo.

Pathophysiology — the immunological spectrum

The clinical form depends entirely on the Th1 vs Th2 balance of the host response:

• Th1 response (IL-2, IFN-γ, IL-12) → strong CMI → few organisms → tuberculoid (paucibacillary) end with well-formed granulomas.
• Th2 response (IL-4, IL-5, IL-10) → weak CMI, antibody-driven → uncontrolled bacterial multiplication → lepromatous (multibacillary) end with abundant bacilli inside foamy macrophages.

The lepromin (Mitsuda) test measures CMI: strongly positive in tuberculoid, negative in lepromatous. It is a marker of host resistance and prognosis, not a diagnostic test (everyone with prior exposure may react).

Ridley–Jopling classification

This is the immunopathological gold standard, based on clinical, bacteriological, immunological and histological criteria across a five-group spectrum.

Feature	TT (Tuberculoid)	BT	BB (Mid-borderline)	BL	LL (Lepromatous)
CMI	High	High–moderate	Intermediate	Low	Absent
Number of lesions	Single/few	Few	Several	Many	Innumerable, symmetrical
Bacillary load (BI)	0	0–1+	2–3+	4–5+	5–6+
Lepromin test	Strong +	+	±	−	−
Nerve involvement	Early, marked, asymmetric	Marked	Moderate	Late, symmetric	Late, symmetric, glove-stocking
Granuloma	Epithelioid + Langhans giant cells, no bacilli	Epithelioid	Mixed	Foamy macrophages	Virchow/lepra (foamy) cells, globi, Grenz zone
Stability	Stable	Unstable	Most unstable	Unstable	Stable

High-yield: The two polar forms (TT and LL) are immunologically stable; the three borderline forms (BT, BB, BL) are unstable and may upgrade or downgrade. BB is the most unstable of all. The Grenz zone (a clear subepidermal band of uninvolved papillary dermis) is characteristic of lepromatous histology.

WHO operational classification (for field treatment)

Because Ridley–Jopling needs biopsy, WHO uses a simple skin-lesion count to decide the MDT regimen:

Category	Skin lesions	Nerves involved	Skin smear
Paucibacillary (PB)	1–5	Only one nerve trunk	Negative
Multibacillary (MB)	>5	More than one nerve	Positive (or any smear-positive case)

A single-lesion PB category also exists, treated with single-dose ROM (rifampicin + ofloxacin + minocycline).

Clinical features

Cardinal signs of leprosy (WHO diagnostic triad — any ONE confirms diagnosis):

1. Hypopigmented or reddish skin patch with definite loss of sensation.
2. Thickened peripheral nerve with sensory/motor loss in its distribution.
3. Slit-skin smear positive for acid-fast bacilli.

Sequence of sensory loss in a leprosy patch: temperature → pain (light touch) → touch is preserved longest. (Mnemonic: lepra hits Temperature first.)

Tuberculoid (TT/BT): few, large, well-defined anaesthetic plaques with raised erythematous margins, dry and hairless (loss of sweating due to autonomic nerve damage), early and severe nerve thickening (often a single nerve, asymmetrical).

Lepromatous (LL): numerous small, symmetrical, ill-defined macules/papules/nodules; "leonine facies" (nodular thickening of forehead/cheeks/ears); madarosis (loss of lateral eyebrows); saddle-nose; nasal stuffiness/epistaxis; glove-and-stocking sensory loss developing late; gynaecomastia and testicular atrophy (sterility). The patient is highly infectious.

Most commonly involved peripheral nerves (high-yield):

• Ulnar nerve — the most commonly involved nerve overall → claw hand (ring + little finger), wasting of hypothenar/interossei.
• Common peroneal (lateral popliteal) — most common cause of foot drop.
• Posterior tibial — plantar anaesthesia → trophic ulcers of sole; most commonly damaged nerve in the lower limb leading to disability.
• Great auricular nerve — most easily palpable/visible thickened nerve.
• Facial (zygomatic branch) → lagophthalmos; radial cutaneous, median nerves also affected.

High-yield: Leprosy is the commonest cause of peripheral neuritis worldwide and the commonest treatable cause of peripheral neuropathy in India. Nerve thickening with sensory loss is the most characteristic feature.

Reactional states (immunological emergencies)

Reactions are acute episodes of immune activity superimposed on the chronic course — the main cause of nerve damage and disability — and are medical emergencies requiring urgent treatment without stopping MDT.

Feature	Type 1 (Reversal / RR)	Type 2 (ENL)
Immune mechanism	Type IV delayed hypersensitivity (cell-mediated, ↑CMI)	Type III immune-complex (Arthus-like)
Occurs in	Borderline (BT, BB, BL)	Lepromatous & BL (high bacterial load)
Skin	Existing lesions become red, swollen, oedematous	Crops of tender erythematous nodules (erythema nodosum leprosum)
Nerves	Acute painful neuritis, sudden palsy	Neuritis (less than Type 1)
Systemic	Usually none	Fever, malaise, iridocyclitis, orchitis, arthritis, glomerulonephritis, lymphadenitis
Direction	Usually upgrading (towards tuberculoid)	—
Treatment	Systemic corticosteroids (prednisolone)	Thalidomide (drug of choice); steroids; clofazimine

High-yield: Type 1 reaction is treated with corticosteroids (NOT thalidomide). Type 2 reaction (ENL) responds dramatically to thalidomide (contraindicated in women of childbearing age — teratogenic phocomelia). Lucio phenomenon is a severe necrotising reaction seen in diffuse lepromatous leprosy (M. lepromatosis).

Flow of a reaction emergency: Recognise reaction → continue MDT → assess nerve function → start prednisolone (Type 1) or thalidomide/steroids (Type 2) → splint affected limb → reassess.

Diagnosis & investigation of choice

Slit-skin smear (SSS): The standard bacteriological test. Skin is pinched (to make it bloodless), incised, and dermal scrapings are stained by modified Ziehl–Neelsen (Fite-Faraco). Sites: both ear lobes + lesion margins. Results expressed as:

• Bacterial Index (BI): logarithmic 0–6+ scale (Ridley's scale) = density of bacilli.
• Morphological Index (MI): percentage of solidly (uniformly) stained = viable bacilli. Fragmented/granular forms are dead. MI falling to zero indicates effective treatment.

High-yield: BI = number/load; MI = viability. A fall in MI is the earliest indicator of treatment response; BI falls by roughly 1+ per year.

Histopathology (skin/nerve biopsy): Confirms classification — epithelioid granulomas with Langhans cells (tuberculoid) vs foamy Virchow cells with Grenz zone and globi (lepromatous).

Other tests:

• Lepromin (Mitsuda) test — prognostic, NOT diagnostic.
• Anti-PGL-1 serology — high titres in MB, low in PB.
• PCR — useful in early/paucibacillary or pure neuritic cases (highest sensitivity for low loads).

Diagnosis is essentially CLINICAL (cardinal signs) supported by SSS.

Management — WHO Multidrug Therapy (MDT)

MDT is given free worldwide and is fully supervised for the monthly dose. As of the current WHO guideline, all confirmed cases (PB and MB) receive a three-drug regimen (rifampicin + dapsone + clofazimine), simplifying field use:

Drug	Mechanism	Dose (adult, MB)	Key adverse effect
Rifampicin	Inhibits RNA polymerase; most potent bactericidal anti-leprosy drug	600 mg once monthly (supervised)	Hepatotoxicity, orange secretions
Dapsone	Folate synthesis inhibitor (PABA analogue); bacteriostatic	100 mg daily	Haemolysis (esp. G6PD deficiency), methaemoglobinaemia, dapsone hypersensitivity syndrome
Clofazimine	Binds DNA; also anti-inflammatory	300 mg monthly + 50 mg daily	Reddish-black skin pigmentation, ichthyosis, GI

Duration: MB = 12 months; PB = 6 months.

High-yield: Rifampicin is the single most bactericidal drug in leprosy — even a single monthly dose renders an infectious patient non-contagious within days. Dapsone causes haemolysis, so screen for G6PD deficiency. Clofazimine causes skin discoloration (a common reason for non-compliance) but is also therapeutic in Type 2 reactions.

Single-lesion PB: single-dose ROM = Rifampicin 600 + Ofloxacin 400 + Minocycline 100 mg.

Chemoprophylaxis: Single-dose rifampicin (SDR) to close contacts reduces risk by ~50–60% (current WHO recommendation as post-exposure prophylaxis, PEP).

Vaccine: BCG provides partial protection against leprosy (more than against TB in some studies); not a dedicated leprosy vaccine.

Mnemonic for MDT drugs: "Real Doctors Cure leprosy" → Rifampicin, Dapsone, Clofazimine.

Complications & deformities

• Disability is graded by WHO (Grade 0 = none; Grade 1 = anaesthesia of hands/feet/eyes without visible deformity; Grade 2 = visible deformity/damage e.g. claw hand, foot drop, lagophthalmos, corneal opacity).
• Hand: claw hand (ulnar then median), ape thumb, main-en-griffe.
• Foot: foot drop (peroneal), plantar trophic/neuropathic ulcers (posterior tibial), and resorption of digits.
• Eye: lagophthalmos, exposure keratitis, iridocyclitis → blindness (leprosy is a leading infective cause of blindness in endemic areas).
• Renal: amyloidosis (AA type) from chronic ENL; glomerulonephritis.
• Systemic (LL): sterility, gynaecomastia, nasal collapse.

Key differentials

• Hypopigmented anaesthetic patch: vitiligo (no sensory loss), pityriasis alba, tinea versicolor, post-inflammatory hypopigmentation — leprosy is distinguished by definite sensory loss + nerve thickening.
• Annular plaques: tinea corporis, granuloma annulare, sarcoidosis (granulomatous but no sensory loss).
• Peripheral neuropathy with thick nerves: hereditary motor-sensory neuropathy (Charcot-Marie-Tooth), amyloidosis, acromegaly — none give the classic patchy anaesthesia.
• Nodular facial disease: leishmaniasis, lymphoma, neurofibromatosis.

Recently asked / exam angle

• M. leprae is the only mycobacterium that cannot be cultured in artificial media — repeatedly asked single-best-answer.
• Animal models: mouse footpad (drug sensitivity studies) and nine-banded armadillo (bulk antigen / vaccine).
• Longest generation time of any bacterium (~12–14 days).
• Ulnar nerve = most commonly involved; lateral popliteal/common peroneal = foot drop.
• Lepromin test is prognostic, not diagnostic; positive in tuberculoid, negative in lepromatous.
• Type 2 reaction (ENL) — thalidomide is drug of choice; Type 1 — steroids.
• Grenz zone and foamy Virchow cells with globi = lepromatous histology.
• MI (morphological index) reflects viability/treatment response; BI reflects load.
• Rifampicin is the most bactericidal MDT drug; given once monthly supervised.
• Single-dose rifampicin (SDR) as post-exposure prophylaxis for contacts.
• WHO leprosy elimination target = prevalence <1 case per 10,000 population (achieved nationally in India in 2005, but pockets persist).
• PGL-1 is the species-specific antigen/virulence factor.

Rapid revision

1. M. leprae — obligate intracellular, weakly acid-fast (5% H₂SO₄), non-cultivable in vitro.
2. Grown only in mouse footpad and nine-banded armadillo; longest generation time (~13 days).
3. Invades Schwann cells via laminin-2/α-dystroglycan; prefers cooler sites (27–33°C).
4. Ridley–Jopling: TT–BT–BB–BL–LL; TT & LL polar/stable, BB most unstable.
5. Clinical form decided by Th1 (tuberculoid) vs Th2 (lepromatous) immunity.
6. Cardinal signs: anaesthetic hypopigmented patch, thickened nerve, AFB-positive smear (any one).
7. Sensory loss order: temperature → pain → touch.
8. Lepromin (Mitsuda): prognostic; + in tuberculoid, − in lepromatous.
9. Type 1 reaction → steroids; Type 2 (ENL) → thalidomide.
10. MDT = rifampicin (monthly, most bactericidal) + dapsone (haemolysis) + clofazimine (skin pigment); MB 12 mo, PB 6 mo.
11. BI = bacterial load; MI = viability (falls first with treatment).
12. WHO elimination target = <1/10,000; BCG offers partial protection; SDR for contacts.