Cutaneous and mucocutaneous leishmaniasis

CUTANEOUS AND MUCOCUTANEOUS LEISHMANIASIS

Leishmaniasis is the term used for diseases caused by species of the genus Leishmania that are transmitted by the bite of infected sand flies. The lesions of cutaneous and mucocutaneous leishmaniasis are localized to the skin and mucous membranes. Visceral leishmaniasis is a much more severe disease, which involves the entire reticuloendothelial system, and is discussed in section 2.2.4 of this chapter. Cutaneous leishmaniasis appears 2–3 weeks after the bite of an infected sand fly as a small cutaneous papule; this slowly develops and often becomes ulcerated and develops secondary infections. Secondary or diffuse lesions may develop. The disease is usually chronic but may occasionally be selflimiting. Leishmaniasis from a primary skin lesion may involve the oral and nasopharyngeal mucosa. Leishmania species that infect humans are all morphologically similar and only exhibit one form, the intracellular amastigotes (3–6 μm long and 1.5–3 μm in diameter). Promastigotes (Figure 6.2b) are found in the sand fly.

In mammalian hosts amastigotes are phagocytosed by macrophages, but resist digestion and divide actively in the phagolysosome (Figure 6.4). The female sand fly ingests parasites in the blood meal from an infected person or animal and these pass into the stomach where they transform into promastigotes, and multiply actively. The parasites attach to the walls of the oesophagus, midgut and hindgut of the fly, and some eventually reach the proboscis and are inoculated into a new host.

The obvious symptoms of this infection are caused by the uptake of parasites by local macrophages. Host response to infection produces tuberclelike structures designed to limit the spread of infected cells. Some lesions may resolve spontaneously after a few months but other types of lesion may become chronic, sometimes with lymphatic and bloodstream dissemination. In infections due to L. braziliensis there is a highly destructive spread of infected macrophages to the oral or nasal mucosa. In L. mexicana, L. amazonensis and L. aethiopica infections the disease becomes more disseminated. The immunological response of the host plays an important factor in determining the precise pathology of the disease and this is apparent from the more severe type of infection seen in individuals with HIV. In Europe and Africa several rodents may act as reservoirs of the disease, but in countries such as India, transmission can occur in a human–sand fly– human cycle without rodent intervention. In rural semiarid zones of Latin America, both wild and domestic dogs enter the epidemiological chain and the vector is a common sand fly, Lutzomyia longipalpis, abundant in and around houses. The disease is more common in children in both Latin America and the Mediterranean area.