Lymphadenitis caused by infection with protozoa of the genus Leishmania.

Leishmaniasis is endemic in Asia, South America, Africa, where it is caused by L. donovani, and in the Mediterranean regions by L. infantum. The geographic distribution depends on the insect vectors. Travelers from nonendemic regions may acquire the disease (1,2). It affects as many as 12 million worldwide, with 1.5 to 2 million new cases each year (3).

Leishmania donovani, a protozoan, is transmitted by sandflies (Phlebotomus species) to several mammals, including humans. The Leishmania promastigotes are extracellular flagellated forms in the insect that gain access to the host through sandfly bites (4). In human tissues, Leishmania are transformed into amastigotes, which are phagocytosed by macrophages. They appear as small, ovoid or round organisms, 1.5 to 3 μm in diameter with a thin cell membrane, a large nucleus, and a rod-shaped kinetoplast. The parasites multiply by binary fission within histiocytes; these eventually burst and release the amastigotes, which then infect new histocytes (4,5). The natural reservoirs of Leishmania organisms are various animals, including dogs and cats.

Leishmania induce a large spectrum of diseases in humans from cutaneous lesions to visceral fatal diseases (4). The various clinical forms of leishmaniasis depend on the protozoan species, the immune response of the host, and the geographic site (1). Visceral leishmaniasis, or kala-azar, the most severe and extensive form of the disease, prevails in areas where the climate favors the propagation of sandflies, such as the Indian subcontinent (5). Cutaneous leishmaniasis, manifesting as ulcers of the skin with scarring at the site of sandfly bites, is more common in South America. Both the visceral and cutaneous forms of leishmaniasis are accompanied by regional lymphadenitis. In Iran, visceral leishmaniasis is common in children and associated with palpable lymph nodes in 5% of cases (6). In addition, leishmaniasis may be solely localized to the lymph nodes, without any evidence of visceral or cutaneous involvement, such as cases of isolated lymphadenitis reported from Spain, Tunisia, Iran, and Saudi Arabia (6,7,8,9). Cutaneous lesions may be present, but overlooked (6). In a study of 66 cases with lymph node involvement, the epitrochlear lymph nodes were the most commonly affected in 68% of patients, followed by inguinal (18%) and axillary (15%) (8). Leishmania lymphadenitis was reported in American service personnel overseas and in patients in other countries under the name of localized lymph node leishmaniasis without any other clinical manifestations (2,6). The lymph nodes are firm, mobile, nontender, and moderately enlarged (8).

In one study of 19 cases of localized Leishmania lymphadenitis, peripheral lymph nodes in all areas, from submandibular to inguinal, were involved (5,6). The lymphoid follicles are hyperplastic with reactive germinal centers. Multiple small collections of histiocytes and epithelioid cells are scattered in the cortex and medulla, creating a “starry sky” pattern (Fig. 30.1) and often involving the lymphoid follicles; the pattern is similar to the distribution of epithelioid cells in toxoplasmic lymphadenitis. Larger collections of histiocytes may accumulate under and even within the lymph node capsule. Some granuloma-like aggregates of epithelioid cells (Fig. 30.2) also include a few multinucleated giant cells (Fig. 30.3). Areas of necrosis may be present in the center of granulomas with bodies that stain with hematoxylin, some of which are dead organisms (6) (Fig. 30.2). A few histiocytes in the cortex contain abundant intracytoplasmic, small (1 to 2 μm), basophilic organisms—the amastigotes or Leishman-Donovan bodies, which stain strongly with hematoxylin (Fig. 30.4). Plasma cells are inconspicuous. As the immunity develops, the granulomatous reaction predominates, and the Leishmania organisms are difficult to find (5). Granulomas with giant cells are usually devoid of parasites (1). In some patients, an anergic response is characterized by numerous large histiocytes (Fig. 30.4) and giant cells (Figs. 30.5 and 30.6); these contain multiple intracellular amastigotes (Fig. 30.7), and produce a starry-sky pattern (6). In others, necrosis is present, possibly the result of a strong antigen–antibody interaction (6,10,11). In some cases, the amastigotes were not present in the lymph node cells although leishmania antigen was demonstrable (8).