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Leishmania are kinetoplastid protozoan parasites responsible for a spectrum of zoonotic, and less commonly anthroponotic, diseases of man and other mammals. The parasite is transmitted by a blood-feeding dipteran vector of the subfamily Phlebotominae, and undergoes radical morphological and metabolic changes as it passes from the insect vector to the mammalian host. In the latter, it is found as an obligately intracellular, small oval cell, 2–3 μm in diameter, with a truncated flagellum that barely extends beyond the plasma membrane. The intracellular, or amastigote, stage of the parasite specifically infects cells of the mononuclear phagocyte system, in which it lives and divides within the acidified phagolysosomes at pH 4.7-5.3 (Antoine et al., 1990). The parasites disseminate within the mammal as amastigotes, released as a result of the destruction of the host cell, and are bound and engulfed by neighboring uninfected macrophages. Transmission from one mammal to another occurs through the sandfly vector: amastigotes within mononuclear cells are ingested during the blood meal of the female sandfly and differentiate within the presumably neutral-to-alkaline environment of the sandfly midgut to form elongated, motile, flagellate cells called promastigotes, which are 12–16 μm long and 1.5–3.5 μm wide. The promastigotes remain free in the midgut lumen or attach by means of their flagella to the microvillous epithelial cells of the insect midgut. The parasites divide actively, then colonize the chitinous foregut of the insect, where the morphologically distinct paramastigotes adhere by means of hemidesmosomes at the tips of their modified flagella (Killick-Kendrick et al., 1974; reviewed in Molyneux and Killick-Kendrick, 1987). |
