Biology and Morphological Forms
Leishmania reproduce by binary fission and alternate between two developmental forms in their life cycle depending on the host.
- Promastigote (flagellated form). Found in the gut of the sandfly vector or in artificial culture media. It is an elongated, spindle-shaped form (10–20 µm long) with a flagellum at the anterior end that provides motility. It contains a nucleus, cytoplasm, kinetoplast, and volutin granules. Amastigotes ingested with blood by the vector transform into promastigotes within 6–8 days.
- Amastigote (non-flagellated form). Found in the mammalian host (including humans). It is an obligate intracellular parasite residing within macrophages and the reticuloendothelial system (RES) of the liver, spleen, and lymph nodes. These are small (2–5 µm) round or oval cells lacking a flagellum. Upon Giemsa staining, the cytoplasm appears grayish-blue, while the nucleus and kinetoplast stain reddish-purple.
Clinical Forms of Leishmaniasis
Depending on the species of the causative agent, infections are classified into visceral and cutaneous or mucocutaneous forms.
Visceral Leishmaniasis Characterized by infection of the RES (liver, spleen, bone marrow, lymph nodes) and the GI tract.
- Indian variant (Kala-azar) is caused by the L. donovani complex. It is an anthroponosis (reservoir is human). It presents with hypergammaglobulinemia, skin darkening due to adrenal involvement, and secondary skin lesions (post-kala-azar dermal leishmanoid).
- Mediterranean-Central Asian variant is caused by L. infantum. The reservoir includes canids and rodents, and children are predominantly affected. The clinical picture resembles Kala-azar, but the skin appears pale.
Cutaneous and Mucocutaneous Leishmaniasis
- Anthroponotic cutaneous leishmaniasis (urban/dry form) is caused by L. tropica. The incubation period lasts for months, with a slow-growing papule forming on the face and hands that ulcerates after 3–4 months and scars over about a year.
- Zoonotic cutaneous leishmaniasis (rural/wet form) is caused by L. major (reservoir: rodents). The incubation period is short (2–4 weeks), the clinical course is acute, and weeping ulcers develop (typically on the lower limbs).
- Mucocutaneous leishmaniasis (Espundia) is caused by L. braziliensis. Found in South and Central America, it features a granulomatous process with deep destruction of the skin and mucous membranes (e.g., "tapir nose" deformity).
Diagnostic Methods
Diagnosis relies on identifying the pathogen or detecting specific antibodies.
- Microscopy (primary method): Examination of smears from ulcers, bone marrow aspirates, or lymph node punctures. Giemsa staining reveals intracellular amastigotes.
- Culture method: Inoculation of clinical material onto NNN medium (containing defibrinated rabbit blood) and incubation for about 3 weeks.
- Animal inoculation: Inoculation of laboratory animals (hamsters, white mice).
- Serology: IFA, ELISA (though limited by potential cross-reactivity).
- Skin test: Montenegro test (leishmanin skin test). Based on a delayed-type hypersensitivity reaction, primarily used for epidemiological surveys.