Lymphadenitis caused by infection with Mycobacterium leprae.

About 5.5 million people are estimated to have leprosy around the world, with the majority of cases occurring in the tropics (1). Sixty percent of cases occur in Asia (2,3), and 3.5 million of these are in India (2). The prevalence is not high—1% to 5%, even in endemic areas. In the United States, the number of cases has increased, and leprosy is reported among immigrants from countries where the disease is endemic (2,3). Approximately 6,000 cases exist in the United States, with 200 to 300 new cases added annually. Small endemic foci exist in Hawaii, Louisiana, and Texas (1).

Morphologically, M. leprae is almost indistinguishable from M. tuberculosis but is slightly less acid-resistant in its staining properties (4). The human leprosy bacillus is an obligate intracellular parasite that is notable for its limited host range, its unique ability to enter nerves, and its failure to grow in vitro, which has prevented progress in research and the development of vaccines. A leprosy-like disease was reproduced in armadillos, and the leprosy bacillus was grown in the mouse footpad (5). In humans, the transmission of leprosy is by direct contact through skin abrasions or mucous membranes of the mouth or nose. Infectivity is not high, even among close family contacts (2).

The clinicopathologic manifestations of leprosy directly reflect the relationship between infectious agent and host. The host response is determined by the host immune status, which appears to be affected in leprosy by a diminished number of circulating T cells (6). The disease takes two principal forms, lepromatous and tuberculoid, which represent opposite extremes of the immune response; a spectrum of intermediate lesions and manifestations is seen (7). Studies in which HLA typing is used indicate that individual receptivity for leprosy is genetically determined and that the tuberculoid form is related to an HLA-linked recessive gene (8). A critical problem in leprosy is the relative deficiency of antigen-specific, T-cell–mediated immunity. These T cells are generated in the lymph nodes, but are not present in the peripheral blood of patients with lepromatous leprosy (9).

Persons of any age may become infected. Leprosy has a bimodal age distribution with peaks at 10 to 14 years and 35 to 44 years. The male to female ratio is 2:1, and infants are rarely affected (1). In lepromatous leprosy, cellular resistance against M. leprae is minimal, and patients may present with erythematous or hypopigmented skin macules, papules, nodules, plaques, or diffuse infiltrations. The lepromatous lesions are poorly defined and tend to become confluent. In this, the progressive phase of the disease, the lesions contain numerous lepra cells and large numbers of acid-fast bacilli. There is a deficiency of helper T cells specific for M. leprae antigens, and of suppressor T cells specific for M. leprae (10). The lepromin test result is generally negative. In tuberculoid leprosy, specific cellular immunity is pronounced, and skin lesions are sharply defined and localized. They include few lepra cells and rare bacilli; a proliferation of fibrous tissues is seen, and the lepromin test result is positive.

A distinctive feature of leprosy is involvement of nerves, which may occur during any phase of the disease. M. leprae is the only mycobacterium that regularly invades nerves (4). M. leprae multiplies in the skin histiocytes and the Schwann cells of nerves. Thus, the most characteristic clinical feature of leprosy is the association of skin and nerve lesions. Another distinctive feature of leprosy is the distribution of lesions in cooler parts of the body, particularly the skin, mucous membranes of the upper respiratory tract (turbinates, nasal septum, larynx), anterior part of the eye, testes, nerve trunks that are close to the skin surface, and lymph nodes that drain dermal lesions (11). Occasionally, lymphadenopathies may be the initial manifestation of the disease in the absence of skin and nerve lesions (12). When tests for syphilis are performed on the sera of patients with moderate or advanced lepromatous leprosy, the results are frequently false-positive (4).