Comprehensive Guide to the Pathophysiology of Leprosy

Leprosy is among the least contagious infectious diseases known. Despite centuries of stigma, the vast majority of people are naturally immune to Mycobacterium leprae — estimates suggest 95% of the global population will never develop leprosy even after prolonged exposure. Transmission requires prolonged, close, repeated contact with an untreated lepromatous (multibacillary) patient — not brief or casual contact. The main transmission route is respiratory droplets and nasal secretions from untreated patients with high bacterial loads; skin-to-skin contact is a much less efficient route. The incubation period is extraordinarily long — typically 3–5 years, but ranging from 6 months to over 20 years — which is why the disease was historically mysterious and hard to trace. Once a patient starts multidrug therapy (MDT), they become non-infectious within a few days, as the treatment rapidly reduces bacterial shedding. No isolation of leprosy patients is required or recommended — it causes unnecessary social harm and discourages people from seeking care. Brief encounters — sharing a vehicle, being in the same room, touching hands — pose essentially no risk. Children of untreated lepromatous patients face the highest household transmission risk, which is why contact screening and chemoprophylaxis (single-dose rifampicin for close contacts) is now recommended in India.

The nerve damage that causes sensory loss in leprosy is the most clinically significant consequence of the disease and the primary driver of disability — loss of sensation leads to unnoticed injuries, burns, and secondary infections that ultimately cause deformity. M. leprae has a unique preference for peripheral nervous tissue: it specifically targets Schwann cells, which form the myelin sheath around peripheral nerve axons. The bacteria invade Schwann cells and multiply within them, causing demyelination (loss of the protective myelin coating) and axonal degeneration. Two mechanisms compound this damage: direct bacterial invasion of nerves, and the immune-mediated granulomatous inflammation around infected nerves — macrophages and lymphocytes gather around infected Schwann cells and form granulomas that compress and choke the nerve from outside. The nerves most commonly affected are those that run close to the skin surface and in cooler body areas (where M. leprae thrives — it grows best at temperatures below core body temperature): ulnar nerve at the elbow (causing claw hand), median nerve at the wrist, common peroneal nerve at the knee (foot drop), posterior tibial nerve (causing plantar anaesthesia and predisposing to plantar ulcers), facial nerve (lagophthalmos — inability to close the eye, leading to corneal damage), and great auricular nerve (visible thickening at the neck). Reversal of nerve damage depends entirely on how early treatment starts — early, mild neuropathy responds well to MDT and corticosteroids; established axonal degeneration is permanent. This is why early diagnosis before nerve damage occurs is the single most important goal of leprosy control.

The two poles of leprosy represent opposite ends of the immune response spectrum to M. leprae, and this difference determines everything — symptoms, infectiousness, treatment duration, and risk of disability. Tuberculoid leprosy (paucibacillary, TT/BT on the Ridley-Jopling scale): the patient mounts a strong cell-mediated immune response (Th1 type), which keeps the bacteria in check. There are few (1–5) well-defined skin lesions, with distinct edges, hypopigmentation, and complete loss of sensation within the lesion. Nerve involvement is prominent but localised. Bacterial load is very low — skin smears are negative. Patients are minimally infectious and require 6 months of MDT (rifampicin + dapsone). Lepromatous leprosy (multibacillary, LL/BL): the immune response is Th2-dominated and fails to clear the bacteria, which proliferate extensively. Skin lesions are numerous, poorly defined, widespread, and symmetric. The face becomes thickened and nodular (leonine facies). Nasal mucosa is infiltrated and collapses, causing the saddle-nose deformity. Eyebrows and eyelashes thin and fall out (madarosis). Nerve damage is diffuse and bilateral. Bacterial load is extremely high — skin smears show millions of bacilli per gram of tissue. Patients are the primary source of community transmission. Treatment is 12 months of triple therapy (rifampicin + dapsone + clofazimine). Between the poles sit borderline forms (BB, BT, BL) that are immunologically unstable and at highest risk of lepra reactions — acute inflammatory episodes that can cause sudden nerve damage even during or after treatment.

Yes — leprosy is fully curable with multidrug therapy (MDT), and treatment is free through the National Leprosy Eradication Programme (NLEP) at all government health facilities in India. MDT was introduced by WHO in 1981 to prevent the emergence of drug resistance, which was becoming a serious problem with single-drug dapsone therapy. Standard WHO MDT regimens: paucibacillary (PB) leprosy — 6 months of daily dapsone 100mg + monthly supervised rifampicin 600mg; multibacillary (MB) leprosy — 12 months of daily dapsone 100mg + daily clofazimine 50mg + monthly supervised rifampicin 600mg + clofazimine 300mg. Rifampicin is bactericidal against M. leprae — it kills 99.9% of viable bacteria within the first 3–5 days of treatment, which is why patients become non-infectious almost immediately. Dapsone and clofazimine are bacteriostatic and clear remaining bacteria over the treatment course. After completing the full course, the patient is declared cured and discharged from treatment. Skin lesions gradually fade over months to years after treatment — full resolution depends on lesion type and duration of disease before diagnosis. Nerve damage that has already occurred does not reverse with antibiotics alone — established disability requires physiotherapy, protective footwear, self-care education to prevent secondary injuries, and sometimes reconstructive surgery. Lepra reactions (Type 1 reversal reaction and Type 2 erythema nodosum leprosum) are immune-mediated inflammatory episodes that can occur before, during, or after MDT and must be treated with corticosteroids (Type 1) or thalidomide/clofazimine (Type 2) to prevent acute nerve damage — they are not treatment failure.