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Pathology

Mycobacterium leprae is an acid- and alcohol-fast, gram-positive, rod-shaped intracellular bacillus which resembles M. tuberculosis. It is the least infectious of infectious diseases, with an incubation period which varies from a minimum of 3 months to 40 years; 2-4 years is the norm. The bacillus multiplies slowly, probably in 18-42 days and leprosy develops slowly clinically, over months and years compared with the hours or days of acute bacterial infections. M. leprae is an obligatory intracellular parasite. The organism multiplies best in the cooler parts of the body, the skin, ears, upper respiratory tract, the anterior chamber of the eye, the superficial nerves, and the testes. The earliest clinically detectable lesions are in the skin of the face and limbs or in the more superficial nerves. There are no established prodromal symptoms, although the skin lesions may itch and have focal paresthesiae. Invasion of deep organs takes place in lepromatous leprosy but the eyes, testes, and muscles are the most commonly affected. The bacillus multiplies inside macrophages and the host response is predominantly cellular. In fact, the way the disease develops clinically depends on whether the macrophages can kill the M. leprae bacilli. If successful, little or no disease develops but, if not, there will be widely disseminated lesions. The organisms are absorbed either by histiocytes in the skin, or in the nerves by Schwann cells. The initial skin lesions (known as the "indeterminate" lesions because there is no indication as to how it will develop) usually spontaneously resolve and the majority of skin lesions are never noticed.

The pathological, and therefore the clinical, pattern of the outcome of the disease depends on the nature and extent of the host's response to the organism. When there is well-developed cell-mediated immunity, the pattern is tuberculoid leprosy, but if cell-mediated immunity fails to develop, the pattern is that of lepromatous leprosy.

Classification

Leprosy presents in many different ways, both clinically and pathologically. There have been many different classifications but the most frequently used was developed at an international leprosy congress in 1953, which divided leprosy into two different polar groups: tuberculoid leprosy and lepromatous leprosy. All the variations between these two groups became "borderline or indeterminate". Even this was too complicated for field workers, so in 1982 a WHO Expert Committee on Leprosy simplified the system based on the number of M. leprae in the patient. If the skin smears are negative for M. leprae, the patient is classified as paucibacillary (PB). If the skin smears are positive, they are multibacillary (MB). Any patients who have only three or less skin lesions and without any induration or neuritis are considered paucibacillary regardless of the skin smear results. All the others are multibacillary. There are, as far as is known, no variations in the strain of M. leprae, except for drug sensitivity. The many differences in the clinical and pathological presentation are due to the extent of the cell-mediated response of the patient.

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Copyright: Palmer and Reeder