What is leprosy and how is it diagnosed?
Leprosy (Hansen disease) is a chronic granulomatous infection by the Mycobacterium leprae complex (M. leprae and M. lepromatosis) that mainly affects the skin, peripheral nerves, upper respiratory mucosa and eyes. The bacillus grows best at about 27–33 °C, which explains its liking for cool areas — skin and superficial nerves — and it preferentially infects Schwann cells.
WHO reports around 200,000 new cases a year from more than 120 countries; India, Brazil and Indonesia each still report over 10,000 new cases annually. Spread is believed to be mainly by respiratory droplets during prolonged close contact with an untreated patient, and most people (WHO: up to 95%) have natural immunity.
Men account for about 63% of diagnoses. The main risk factor is prolonged close contact with an untreated patient; others are living in an endemic area, impaired cell-mediated immunity, malnutrition, overcrowding and genetic susceptibility. In the southern United States, the nine-banded armadillo is a natural reservoir and a source of zoonotic infection.
Sensory loss in a patch typically affects temperature perception first. The lepromin test has no role in diagnosis — it only shows whether a person can mount a granuloma to M. leprae antigens — and anti-PGL-1 serology is not sensitive enough to diagnose leprosy.
What is the Ridley–Jopling classification?
The Ridley–Jopling system places patients on an immunological spectrum using skin or nerve biopsy and bacillary load. At one pole, strong T-cell (cell-mediated) immunity walls the bacilli into compact epithelioid granulomas; at the other, absent cell-mediated immunity lets bacilli multiply freely inside foamy macrophages.
| Type | Immunity and bacilli | Skin lesions | WHO group |
|---|---|---|---|
| TT — tuberculoid | Strong CMI; few or no bacilli, AFB stain usually negative | Few large, well-demarcated hypopigmented or erythematous plaques with raised margins; anaesthetic | PB |
| BT — borderline tuberculoid | Good CMI, few bacilli | More lesions than TT, well defined, often on one side of the body | PB |
| BB — mid-borderline | Unstable | Punched-out lesions with anaesthetic centres | MB |
| BL — borderline lepromatous | Weak CMI, many bacilli | Many ill-defined macules, papules, nodules, widely distributed | MB |
| LL — lepromatous | No effective CMI; huge numbers of bacilli in globi | Generalised symmetric macules, papules, nodules; loss of eyebrows, ear-lobe thickening, septal perforation | MB |
| Indeterminate | Earliest phase | One or few flat, faint, poorly defined hypopigmented spots | Usually PB |
Histology mirrors the spectrum. Tuberculoid lesions show well-formed granulomas of epithelioid cells and Langhans giant cells that destroy nerve bundles. Lepromatous lesions show sheets of lipid-laden foamy macrophages (Virchow or lepra cells) packed with bacilli, often in clumps called globi.
How does WHO classify leprosy as PB or MB?
For treatment in the field, WHO uses a simpler operational classification based on lesion count and smear result.
| Group | Definition |
|---|---|
| Paucibacillary (PB) | 1–5 skin lesions, without bacilli in a skin smear |
| Multibacillary (MB) | More than 5 skin lesions; OR nerve involvement (pure neuritis, or any number of lesions with neuritis); OR bacilli in a slit-skin smear, whatever the lesion count |
What are the slit-skin smear and bacteriological index?
A slit-skin smear is taken by a superficial incision and scrape from standard sites — commonly the ear lobes, plus sites such as the elbows, forehead or chin, and one active lesion — then stained for acid-fast bacilli. The bacteriological index (BI) counts bacilli on Ridley's logarithmic scale and the indices from all sites are averaged.
| BI | Bacilli seen |
|---|---|
| 0 | No bacilli in 100 fields |
| 1+ | 1–10 bacilli in 100 fields |
| 2+ | 1–10 bacilli in 10 fields |
| 3+ | 1–10 bacilli per field (average) |
| 4+ | 10–100 bacilli per field |
| 5+ | 100–1,000 bacilli per field |
| 6+ | More than 1,000 bacilli per field |
A full-thickness biopsy from the active edge of a lesion allows Ridley–Jopling typing. PCR for M. leprae DNA is highly specific; its sensitivity is above 90% in MB disease but only about 30–80% in PB disease.
Which nerves are involved in leprosy?
Nerve damage is what makes leprosy disabling. Thickened, sometimes tender, nerves are palpated at their superficial sites, and loss of sensation comes first, then weakness and deformity.
| Nerve | Deficit |
|---|---|
| Ulnar (most commonly involved) | Claw hand, loss of sensation in the ring and little fingers |
| Common peroneal (fibular) | Foot drop, weak dorsiflexion |
| Median | Thenar weakness, loss of sensation on the thumb side of the palm |
| Radial | Wrist drop |
| Facial | Facial paralysis; the eyes are a recognised target of the disease |
For how each nerve lesion looks in the hand, see upper limb nerve injuries and hand muscles and nerve supply. Advanced disease also brings loss of eyebrows and eyelashes and perforation of the nasal septum.
What are the current MDT regimens for leprosy?
Dapsone monotherapy led to resistance, so leprosy is always treated with multidrug therapy (MDT). WHO's current recommendation is one three-drug regimen — rifampicin, dapsone and clofazimine — for all patients, with only the duration differing: 6 months for PB, 12 months for MB. The WHO guideline development group gave this uniform regimen a conditional recommendation.
| Age group | Rifampicin | Clofazimine | Dapsone |
|---|---|---|---|
| Adults (over 14 years) | 600 mg once a month | 300 mg once a month + 50 mg daily | 100 mg daily |
| Children 10–14 years | 450 mg once a month | 150 mg once a month + 50 mg on alternate days | 50 mg daily |
| Children under 10 years or under 40 kg | 10 mg/kg once a month | 100 mg once a month + 50 mg twice weekly | 2 mg/kg daily |
- Clofazimine side effects include skin pigmentation and dry skin (xerosis), which is one criticism of adding it for PB patients.
- Prevention: WHO recommends contact screening and single-dose rifampicin as post-exposure prophylaxis (SDR-PEP) for contacts.
- Some experts (and the US programme, using rifampicin, moxifloxacin and minocycline) prefer longer courses; no trials compare these with the WHO regimen.
How do type 1 and type 2 lepra reactions differ?
Lepra reactions are acute immune flares that can occur before, during or after MDT and cause sudden nerve damage. They are emergencies for the nerves, not signs of drug failure, so MDT is continued throughout.
| Feature | Type 1 — reversal reaction | Type 2 — erythema nodosum leprosum (ENL) |
|---|---|---|
| Hypersensitivity type | Type IV (delayed, cell-mediated) — sudden rise in CMI | Type III — immune-complex deposition |
| Who | Borderline patients — BT, BB, BL | BL and LL (high bacillary load) |
| Skin | Existing lesions become red, swollen, tender | Crops of new painful red nodules, which may pustulate or ulcerate |
| Nerves | Acute neuritis — pain and new loss of function | Neuritis can occur |
| Systemic | Fever, malaise, swelling of hands and feet | Fever, malaise, arthralgia, myalgia — a systemic illness |
| Histology | Oedema, granulomas invaded by lymphocytes and giant cells | Intense neutrophil infiltrate with vasculitis |
| Treatment of severe reaction | Corticosteroids (prednisolone); cyclosporine or methotrexate as steroid-sparing agents | Thalidomide (avoid in women who could become pregnant); alternatives corticosteroids, clofazimine, methotrexate |
How is leprosy asked in NEET PG and INI-CET?
- Cardinal signs — any one of: anaesthetic patch, thickened nerve with deficit, positive slit-skin smear.
- PB vs MB — 1–5 lesions and smear-negative = PB; >5 lesions, nerve involvement or smear-positive = MB.
- Duration — 6 months PB, 12 months MB; same three drugs for both under current WHO and NLEP guidance.
- Adult monthly supervised dose — rifampicin 600 mg + clofazimine 300 mg; daily dapsone 100 mg + clofazimine 50 mg.
- Most commonly involved nerve — ulnar; foot drop from common peroneal involvement.
- ENL — type III hypersensitivity in BL/LL; drug of choice for severe ENL — thalidomide.
- Globi and foamy (Virchow) cells — lepromatous leprosy; BI 6+ — more than 1,000 bacilli per field.
- Lepromin test — not diagnostic.