Leishmania
Summary
Leishmania is a genus of protozoan parasites transmitted by the bite of female sandflies (Phlebotomus in Old World, Lutzomyia in New World), causing cutaneous, mucocutaneous, or visceral leishmaniasis. Visceral leishmaniasis (kala-azar), caused mainly by L. donovani, presents with fever, hepatosplenomegaly, pancytopenia, and hypergammaglobulinemia. Diagnosis is via visualization of amastigotes (Donovan bodies) in macrophages on tissue biopsy or bone marrow aspirate.
Detail
Leishmania species are obligate intracellular protozoan parasites that exist in two forms: the promastigote (flagellated, found in sandfly vector and culture) and the amastigote (non-flagellated, found intracellularly within host macrophages). Transmission occurs via the bite of infected female sandflies, which inject promastigotes into the skin; these are phagocytosed by macrophages and transform into amastigotes, replicating within phagolysosomes.
Clinical syndromes depend on species and host immune response: - Cutaneous leishmaniasis (L. major, L. tropica): self-limited skin ulcers at the bite site, often with raised borders, can heal with scarring. - Mucocutaneous leishmaniasis (L. braziliensis): destructive lesions of nasal/oral mucosa, can occur years after cutaneous infection, due to exaggerated immune response. - Visceral leishmaniasis/kala-azar (L. donovani, L. infantum/chagasi): parasites disseminate to reticuloendothelial system (spleen, liver, bone marrow), causing massive hepatosplenomegaly, fever, weight loss, pancytopenia (due to hypersplenism and marrow infiltration), and polyclonal hypergammaglobulinemia. Can be fatal if untreated, especially in immunocompromised or malnourished patients.
Immunology: Control of infection depends on a Th1-mediated cellular immune response (IFN-gamma, macrophage activation); Th2 response is associated with disease progression. This is a classic exam concept paralleling Th1/Th2 paradigm with Mycobacterium leprae.
Diagnosis: Definitive diagnosis via microscopic identification of amastigotes (Leishman-Donovan bodies) within macrophages on splenic aspirate, bone marrow biopsy, or skin lesion biopsy using Giemsa stain. Serology (rK39 antigen) and PCR are also used.
Treatment: Amphotericin B (liposomal formulation preferred for visceral disease) or pentavalent antimonials (sodium stibogluconate) are first-line therapies depending on region and species.
Epidemiology: Endemic in parts of the Mediterranean, Middle East, Africa, India, and Latin America. HIV coinfection increases risk of visceral disease reactivation.
Sources
- First Aid for the USMLE Step 1
- Sherris Medical Microbiology
- CDC DPDx - Leishmaniasis
- Harrison's Principles of Internal Medicine
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