Secondary transmission from imported cases has not been recognized. Consequently, leprosy in humans typically evolves very slowly and primarily affects the cooler peripheral tissues. The principal lesions are in the skin and peripheral nerves. Because of the long and variable incubation period of the disease, its epidemiology is incompletely understood. Early leprosy and indeterminate leprosy present as isolated nondescript skin lesions.
Skin biopsy is required to make a diagnosis.
Although many patients with indeterminate leprosy heal spontaneously, all such patients should receive chemotherapy. The cell-mediated immunity of patients with tuberculoid leprosy is intact. Patients who have lepromatous leprosy are anergic and show severely impaired cell-mediated immunity. Skin lesions of patients with tuberculoid leprosy contain predominantly helper T cells, well-formed granulomas, and few organisms, whereas the lesions of lepromatous leprosy are practically devoid of granulomas and helper T cells, containing instead suppressor T cells and numerous bacilli. The presence of a few large, asymmetrical, well-defined skin lesions characterizes tuberculoid leprosy.