Leprosy

Organism

M. leprae. Acid-fast, obligate intracellular. LONGEST generation time of any human bacterial pathogen (~12-13 days) → long incubation (3-5yr, up to decades), long treatment courses. NEVER cultured on artificial media. Propagation: mouse footpad (limited) or nine-banded armadillo (research, cool body temp suits tropism).

Tropism: COOL body sites (skin, peripheral nerves, anterior eye chamber, testes, nasal mucosa) — below core temp. EXPLAINS entire clinical distribution (can’t thrive in warm internal organs).

Transmission/Pathogenesis

Transmission: mainly NASAL DROPLETS from untreated multibacillary case, prolonged close contact usually needed. One of LEAST contagious mycobacterial diseases. Most exposed never develop disease.

KEY CONCEPT: HOST CMI response determines clinical form >> bacterial factors. Spectrum from robust-CMI pole to CMI-absent pole.

Clinical Spectrum (Ridley-Jopling)

Tuberculoid (TT) pole: strong CMI (brisk TH1/DTH) → contains organism. Paucibacillary. Few, well-defined, hypopigmented/anesthetic patches, sharp raised edge. Nerve involvement DISPROPORTIONATELY prominent (immune response itself damages nerve). Lepromin test STRONGLY POSITIVE.

Lepromatous (LL) pole: CMI vs M. leprae ABSENT (specific anergy, not global immunosuppression). Unchecked multiplication. Numerous, symmetric, poorly-defined lesions/nodules. Multibacillary. Diffuse nerve involvement. Late findings: leonine facies, madarosis (brow/lash loss), saddle-nose deformity, diffuse infiltration. Lepromin test NEGATIVE (DTH response missing).

Borderline (BT, BB, BL): between poles, IMMUNOLOGICALLY UNSTABLE — can shift toward either pole, esp. under treatment stress (reactions).

Lepra Reactions (Complications)

Acute immune reactions, often TREATMENT-PRECIPITATED (dying bacilli release antigen):

Type 1 (reversal) reaction: TYPE IV (delayed) hypersensitivity. Mainly borderline disease. CMI abruptly ↑ (shift toward tuberculoid). Existing lesions acutely inflamed/swollen/tender. NERVE DAMAGE — sudden, severe, potentially IRREVERSIBLE. MEDICAL EMERGENCY for nerve function — prompt corticosteroids prevent permanent damage.

Type 2 (ENL, erythema nodosum leprosum): TYPE III (immune complex) hypersensitivity. Lepromatous/BL disease. Ag-Ab complex deposition (bacterial antigen release). Painful subcutaneous nodules + fever + systemic (arthralgia, iritis, neuritis, orchitis). Rx: corticosteroids or THALIDOMIDE (effective, contraindicated pregnancy — teratogenic).

Laboratory Diagnosis

Slit-skin smear: tissue fluid (earlobe+active lesion edges), modified ZN stain, AFB. BI (Bacteriological Index) = bacillary load (log scale). MI (Morphological Index) = viable(solid)/dead(fragmented) ratio. Together: classify + monitor Rx (↓BI = confirms killing).

Skin biopsy + histopath = DEFINITIVE Ridley-Jopling classification (granuloma pattern, nerve involvement).

Lepromin test: DTH skin test. NOT diagnostic for active infection (+ in healthy contacts, − in active LL). Real use: CLASSIFY already-diagnosed case (+ at TT pole, − at LL pole).

PCR: M. leprae-specific genes, useful in paucibacillary disease (poor smear sensitivity).

WHO field classification (treatment purposes):

PB (paucibacillary): ≤5 skin lesions, smear-negative
MB (multibacillary): >5 lesions OR any smear-positive

Treatment — MDT (Multidrug Therapy)

WHO regimen, 3 drugs (prevents rapid monotherapy resistance — historical dapsone-only failure):

PB: Rifampicin (monthly, supervised) + Dapsone (daily) × 6 months
MB: Rifampicin (monthly, supervised) + Clofazimine (monthly supervised + daily) + Dapsone (daily) × 12 months

Rifampicin = MOST bactericidal drug in regimen. Infectivity drops dramatically within days — patient considered NON-INFECTIOUS after ONE dose (matters for stigma reduction).

Lepra reactions managed SEPARATELY/ALONGSIDE MDT — MDT CONTINUES through reaction, not interrupted.

Prevention

Early detection + prompt MDT (interrupts transmission from infectious MB cases), household contact screening + single-dose rifampicin PEP (WHO recommendation), BCG vaccination (partial cross-protection, secondary to primary TB indication), destigmatization (social burden historically > physical burden).

The Organism

Mycobacterium leprae is an acid-fast, obligate intracellular bacillus with the longest generation time of any known human bacterial pathogen — roughly 12–13 days, which alone explains why leprosy has such a prolonged incubation period (commonly 3–5 years, sometimes decades) and why treatment courses run for many months. It has never been successfully cultured on artificial media; laboratory propagation depends on the mouse footpad (limited, slow multiplication) or, more practically for research purposes, the nine-banded armadillo, whose low body temperature happens to suit the organism’s cool-tissue tropism.

That tropism — a strong preference for cooler body sites (skin, peripheral nerves, the anterior chamber of the eye, testes, and the nasal mucosa, all below core body temperature) — is the single fact that explains the disease’s entire clinical distribution: M. leprae essentially cannot thrive in warm internal organs, which is exactly why leprosy manifests where it does and nowhere else.

Transmission and Pathogenesis

Transmission is thought to occur mainly via nasal droplets from an untreated multibacillary (lepromatous) case, with prolonged close contact generally required — leprosy is, in practice, one of the least contagious of the classic mycobacterial diseases, and the great majority of exposed individuals never develop disease at all, since host cell-mediated immune response determines the clinical form far more than bacterial factors do. This host-response-dependence is the central organizing idea of the whole disease: leprosy is best understood as a spectrum defined by how vigorously (or poorly) an individual’s CMI responds to the organism, running from a robust-CMI pole to a CMI-absent pole.

The Clinical Spectrum (Ridley-Jopling Classification)

At the tuberculoid (TT) pole, strong cell-mediated immunity (a brisk TH1/DTH response) tightly contains the organism — few bacilli are present (paucibacillary), lesions are few, well-defined, hypopigmented/anaesthetic patches with a sharp, raised edge, and peripheral nerve involvement (thickened, tender nerves) is often disproportionately prominent relative to the modest skin findings, since the vigorous immune response itself damages the nerve. Lepromin skin test (a DTH read-out) is strongly positive.

At the lepromatous (LL) pole, CMI against M. leprae is essentially absent (though other immune function is intact — LL patients are not globally immunosuppressed, just specifically anergic to this one organism), so the bacillus multiplies unchecked — numerous, symmetric, poorly-defined skin lesions/nodules, extremely high bacillary load (multibacillary), diffuse nerve involvement, and the classic disfiguring late findings: leonine facies (thickened facial skin), madarosis (eyebrow/eyelash loss), saddle-nose deformity (cartilage destruction), and diffuse infiltration. Lepromin test is negative here, precisely because the DTH response it measures is exactly what’s missing.

Borderline forms (BT, BB, BL) sit between these poles and are, importantly, immunologically unstable — a patient can shift toward either pole over time, particularly under the stress of treatment itself (see reactions, below).

Complications: Lepra Reactions

Leprosy’s most feared complications are not steady disease progression but acute immunological “reactions,” often precipitated by treatment itself (as dying bacilli release antigen) — recognizing and managing these correctly matters as much as treating the underlying infection:

Type 1 (reversal) reaction — a type IV (delayed) hypersensitivity reaction, occurring mainly in borderline disease, where CMI abruptly increases (a shift toward the tuberculoid pole) — existing lesions become acutely inflamed, swollen, and tender, and peripheral nerves can suffer sudden, severe, potentially irreversible damage; this is a genuine medical emergency for nerve function specifically, since prompt corticosteroid treatment can prevent permanent nerve damage that delay will not reverse.
Type 2 reaction (erythema nodosum leprosum, ENL) — a type III (immune complex) hypersensitivity reaction, occurring in lepromatous/borderline-lepromatous disease, from antigen-antibody complex deposition as large amounts of bacterial antigen are released — painful subcutaneous nodules, fever, and systemic symptoms (arthralgia, iritis, neuritis, orchitis). Managed with corticosteroids or thalidomide (notably effective here, though obviously contraindicated in pregnancy given its teratogenicity).

Laboratory Diagnosis

Slit-skin smear: tissue fluid expressed from an incised skin lesion (typically earlobe and active lesion edges), stained by modified Ziehl-Neelsen, examined for acid-fast bacilli; the bacteriological index (BI) quantifies bacillary load logarithmically and the morphological index (MI) estimates the proportion of viable (solidly-staining) versus dead (fragmented/granular) bacilli — together used to classify disease and monitor treatment response, since a falling BI over treatment confirms bacterial killing.

Skin biopsy with histopathology provides the definitive Ridley-Jopling classification, examining the granuloma pattern and degree of nerve involvement directly.

Lepromin test is a DTH skin test (not a diagnostic test for active infection, since it can be positive in healthy contacts and negative in active lepromatous disease) — its real clinical use is classifying an already-diagnosed case along the immunological spectrum, strongly positive at the tuberculoid pole, negative at the lepromatous pole.

PCR (targeting M. leprae-specific gene sequences) offers a molecular alternative, useful particularly in paucibacillary disease where smear sensitivity is poor.

WHO’s practical field classification for treatment purposes is simpler than the full Ridley-Jopling spectrum: paucibacillary (PB) — 5 or fewer skin lesions, smear-negative; multibacillary (MB) — more than 5 lesions, or any smear-positive case.

Treatment: Multidrug Therapy (MDT)

WHO-recommended multidrug therapy combines three drugs, deliberately preventing the rapid monotherapy resistance that undermined dapsone-only treatment historically:

Paucibacillary: rifampicin (monthly, supervised) + dapsone (daily), for 6 months.
Multibacillary: rifampicin (monthly, supervised) + clofazimine (monthly supervised + daily) + dapsone (daily), for 12 months.

Rifampicin is the single most bactericidal agent in the regimen and drives the dramatic drop in infectivity within days of starting treatment — a patient is generally considered non-infectious after just one dose, which matters enormously for stigma-reduction and public health messaging. Lepra reactions (above) are managed separately from, and alongside, MDT — MDT itself continues through a reaction rather than being interrupted.

Prevention

Early case detection and prompt MDT (interrupting transmission from infectious multibacillary cases), household contact screening and single-dose rifampicin post-exposure prophylaxis (recommended by WHO for eligible contacts), BCG vaccination (offers partial, variable cross-protection against leprosy as a secondary effect, alongside its primary TB indication), and destigmatization efforts, since leprosy’s social burden has historically exceeded even its physical disease burden.