Zheng Qi  2021-03-07

Leishmaniasis is a widespread, neglected tropical zoonotic disease caused by over 20 distinct species of the protozoan parasite Leishmania, transmitted almost exclusively through the bite of infected female phlebotomine sandflies. The tiny, 2 to 3 mm long sandflies are far smaller than common mosquitoes, making their bites easy to overlook, and over 90 different sandfly species globally are capable of carrying and spreading the parasite. Unlike many acute infectious diseases, a large proportion of immunocompetent people who are bitten never develop obvious clinical symptoms, remaining as asymptomatic carriers that silently sustain local transmission cycles.

Three Main Clinical Forms

The disease presents in three distinct, well-documented clinical variants, each with unique pathological features and health impacts.

‌Cutaneous Leishmaniasis (CL)‌: The most prevalent global form, it causes painless nodules, papules and chronic skin ulcers at the bite site, which often leave permanent, disfiguring scars after healing. Most cases are not life-threatening, but diffuse cutaneous leishmaniasis, a rare severe subtype, can trigger widespread non-ulcerative skin lesions that resemble leprosy and are extremely difficult to treat.

‌Visceral Leishmaniasis (VL)‌: Also known as kala-azar, this is the deadliest form of the disease. It targets the body’s reticuloendothelial system, causing persistent irregular fever, severe anemia, massive enlargement of the liver and spleen, and drastic weight loss. Without timely clinical intervention, the untreated mortality rate can reach as high as 95%, making it the second most lethal parasitic disease worldwide after malaria.

‌Mucocutaneous Leishmaniasis (MCL)‌: This destructive form invades the mucous membranes of the nose, mouth and throat, even decades after the initial skin lesion has healed. It progressively erodes soft tissues and cartilage, leading to severe facial disfigurement, and secondary bacterial infections often cause fatal complications such as pneumonia and severe malnutrition.

Global Public Health Burden

The disease remains a major public health threat across 98 countries, with an estimated 50,000 to 90,000 new cases of visceral leishmaniasis recorded every year. It disproportionately affects impoverished communities in tropical, subtropical and temperate regions, where poor sanitation, malnutrition and limited access to healthcare drastically increase infection risk. HIV co-infection raises the chance of developing full-blown leishmaniasis by more than 1000 times, and rare non-vector transmission routes including blood transfusion, organ transplantation and congenital mother-to-child spread have also been documented in clinical reports.

Edited by: Secretariat of the Asia-Pacific Network for Drug and Diagnostics Innovation
Produced by: Zheng Qi
Reviewed by: Zhang Haobing
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