Describe Mycobacterium leprae and the classification, diagnosis and treatment of leprosy.

The Organism

• Mycobacterium leprae — an acid-fast bacillus, straight or slightly curved, occurring in parallel bundles ("cigar bundles") and in rounded masses called globi within foamy macrophages (virchow or lepra cells)
• Acid-fast to only 5% sulphuric acid (M. Tuberculosis resists 20%), so a modified Ziehl-Neelsen stain is used
• It has never been cultivated in artificial media — a defining practical fact. It is grown in the mouse footpad and in the nine-banded armadillo, which has a low body temperature and permits heavy multiplication
• The slowest-growing bacterium known — a generation time of 12 to 14 days, which explains the very long incubation period of 2 to 5 years or more, and the prolonged treatment
• Prefers cooler parts of the body (about 30°C) — skin, superficial nerves, nose, testis, eye and the extremities; it spares the warm axilla, groin and scalp, which is why the distribution of lesions is characteristic
• An obligate intracellular parasite of macrophages and schwann cells; invasion of Schwann cells is what makes it uniquely neurotropic among bacteria

Transmission and Immunology

• Transmission is chiefly by nasal droplets from untreated multibacillary patients, who may shed millions of bacilli daily; prolonged close contact is usually required
• Most exposed people never develop disease — the outcome depends almost entirely on the host's cell-mediated immunity
• A strong Th1 response gives tuberculoid disease — few organisms, few lesions, and much nerve damage from the vigorous granulomatous reaction
• A weak or Th2-dominated response gives lepromatous disease — abundant organisms, widespread lesions, and less intense but more extensive damage

Classification

• The RIDLEY-jopling spectrum places disease as TT – BT – BB – BL – LL, with the borderline forms being immunologically unstable and liable to shift, which is what produces reactions
• The WHO operational classification, used for treatment, is simpler: paucibacillary (up to 5 skin lesions, no bacilli on smear) and multibacillary (more than 5 lesions, or any positive smear)
• Indeterminate leprosy — the earliest form, a single ill-defined hypopigmented macule which may heal spontaneously or progress

Diagnosis and Treatment

• The WHO cardinal signs — any one establishes the diagnosis: (1) a hypopigmented or reddish skin lesion with definite loss OF sensation; (2) a thickened or tender peripheral nerve with loss of sensation or weakness in its distribution; (3) demonstration of acid-fast bacilli in a slit-skin smear
• Slit-skin smear — taken from the ear lobe and active lesions; reported as the bacteriological index (density of bacilli, on a logarithmic 0 to 6+ scale) and the morphological index (percentage of solidly staining, and therefore viable, bacilli), which falls rapidly with effective treatment
• Nerves to examine — ulnar at the elbow, median, radial cutaneous, common peroneal at the neck of the fibula, posterior tibial, great auricular and supraorbital
• Lepromin test — an intradermal injection of killed bacilli; the Fernandez reaction is read at 48 hours and the mitsuda reaction at 3 to 4 weeks. It is not a diagnostic test — it assesses cell-mediated immunity and is therefore of prognostic and classificatory value
• Multidrug therapy (MDT) — supplied free through the national programme: paucibacillary, rifampicin monthly with daily dapsone for 6 months; multibacillary, rifampicin and clofazimine monthly with daily dapsone and clofazimine for 12 months
• Patients become non-infectious within days of starting rifampicin, so isolation is unnecessary and stigma is unjustified

Deformities and Complications

Applied Aspects

• Test sensation in every hypopigmented patch; this single bedside step distinguishes leprosy from the far commoner pityriasis alba, vitiligo and tinea versicolor, and costs nothing
• Palpate the peripheral nerves routinely, particularly the ulnar and common peroneal; a thickened nerve in a patient with a skin lesion is diagnostic
• Disability is caused by nerve damage, not by the bacillus, and most of it is preventable by early diagnosis and prompt treatment of reactions; deformity appearing during treatment usually means an unrecognised reaction
• Teach self-care of anaesthetic hands and feet — daily inspection, protective footwear, soaking and oiling; ulcers and their consequences arise from unnoticed injury, not from continuing infection
• India accounts for a large share of new cases worldwide despite elimination as a public health problem having been declared; case detection, contact tracing and single-dose rifampicin prophylaxis for contacts remain important
• Stigma remains the greatest obstacle; explaining that treated patients are not infectious, and that the disease is curable and free to treat, changes presentation times more than any laboratory advance
• Examine the eyes at every visit; lagophthalmos and corneal anaesthesia cause blindness silently, and simple measures such as spectacles, lubricants and lid exercises prevent it
• Deformity is not a sign of active infection, and a patient who has completed treatment needs rehabilitation and reconstructive surgery rather than more antibiotics
• Absorption of digits results from repeated unnoticed trauma, not from the organism destroying bone; explaining this makes self-care instructions comprehensible rather than arbitrary
• Single-dose rifampicin prophylaxis for contacts reduces the risk of developing leprosy and is now part of national policy in several countries including India
• Examine all household contacts at diagnosis and annually; early detection in a contact prevents both disability and further transmission
• Clofazimine causes reversible skin darkening, which is distressing and a common reason for stopping treatment; warning patients in advance markedly improves completion
• Leprosy is curable and treatment is free, and saying so plainly at the first consultation does more for adherence and for early presentation of contacts than any other single intervention