Mycobacterium leprae (Leprosy)

Key points

• • Mycobacterium leprae is an acid-fast bacillus, like the tuberculosis organism, but it grows very slowly and cannot be grown on artificial laboratory media.
• • It has a special liking for the cooler parts of the body and for nerves, so it damages the skin and the nerves supplying the hands, feet and face.
• • The form the disease takes depends on the strength of the cell-mediated immune response: a strong response gives a mild, few-patch tuberculoid form, a weak response a widespread lepromatous form.
• • In the lepromatous form the bacteria are very numerous and shed from the nose, so this form is more infectious; spread needs prolonged close contact, probably by the respiratory route.
• • Nerve damage causes loss of feeling, so painless injuries and burns lead to the ulcers and deformities long associated with the disease.
• • It is diagnosed by the skin and nerve signs, by finding acid-fast bacilli in a slit-skin smear, and by the pattern on a skin biopsy.