Leishmaniasis is a group of diseases caused by protozoan parasites from the genus Leishmania and is transmitted to humans by the bite of an infected female sandfly. Classified by the World Health Organization as a neglected tropical disease, it primarily affects some of the world’s poorest populations. Between 700,000 and one million new cases occur annually, with over one billion people living in endemic areas. The disease presents a public health challenge due to its diverse clinical manifestations, ranging from self-healing skin sores to a severe, life-threatening systemic illness.
The Parasite and Transmission Cycle
The causative agents of Leishmaniasis are more than 20 different species of Leishmania, which are single-celled protozoan parasites. Transmission occurs exclusively through the bite of the female phlebotomine sandfly, a tiny insect. When the sandfly takes a blood meal from an infected host, such as a human or an animal reservoir, it ingests the parasites.
Inside the sandfly’s gut, the parasites multiply and develop into an infectious stage called promastigotes. When the infected sandfly subsequently bites a new host, it injects these promastigotes into the skin along with its saliva. Reservoir hosts vary by geographic region and include rodents, dogs, and other mammals.
The transmission cycle is maintained through this interaction between the sandfly vector, the parasite, and a mammalian host. Once inside the human host, the parasites are engulfed by immune cells, specifically macrophages, where they multiply and spread throughout the body.
The Three Distinct Forms of Leishmaniasis
Leishmaniasis manifests in three primary clinical forms, defined by the area of the body the parasite affects.
Cutaneous Leishmaniasis (CL)
The most common presentation is Cutaneous Leishmaniasis (CL), which causes lesions on the skin at the site of the sandfly bite. While generally not life-threatening, CL can lead to lifelong scarring and social stigma.
Visceral Leishmaniasis (VL)
Visceral Leishmaniasis (VL), also known as kala-azar, is the most severe and systemic form. This form involves the parasite disseminating internally to organs like the spleen, liver, and bone marrow. Untreated, VL has a fatality rate that can exceed 90%.
Mucocutaneous Leishmaniasis (MCL)
The third form is Mucocutaneous Leishmaniasis (MCL), often considered a complication of the cutaneous form. MCL involves the destructive spread of the parasite to the mucous membranes of the nose, mouth, and throat. This form results in severe tissue damage and facial disfigurement.
Recognizing the Manifestations
The signs of Visceral Leishmaniasis (VL) usually develop gradually, sometimes weeks to months after infection. Patients experience irregular bouts of fever, weight loss, and fatigue. Internally, the parasite causes the enlargement of the spleen and liver (splenomegaly and hepatomegaly).
VL also impacts the blood system, leading to pancytopenia—a reduction in red blood cells, white blood cells, and platelets. Post-kala-azar dermal leishmaniasis (PKDL) is a complication that can appear as a bumpy rash on the face and body, sometimes occurring after successful VL treatment.
Cutaneous Leishmaniasis begins with a well-defined lesion at the bite site, often starting as a small bump or papule that slowly enlarges. This lesion develops into a painless, open sore or ulcer with a raised border. These ulcers can persist for months to years, healing spontaneously but often leaving disfiguring scars.
Mucocutaneous Leishmaniasis is characterized by tissue destruction in the nasopharyngeal region. Symptoms include a stuffy nose, frequent nosebleeds, and painful sores inside the mouth or nose. If left untreated, the infection can cause mutilation and destruction of the nasal cartilage and palate.
Diagnosis and Therapeutic Strategies
Diagnosis involves clinical assessment and laboratory confirmation, as symptoms can mimic other illnesses.
Diagnosis
For Visceral Leishmaniasis (VL), diagnosis is commonly achieved by identifying the parasite in tissue samples obtained through a needle biopsy of the spleen, lymph nodes, or bone marrow. Serological tests, which detect antibodies against the parasite, are also helpful for diagnosing VL.
For cutaneous and mucocutaneous forms, diagnosis relies on examining a tissue sample taken from the ulcer or lesion under a microscope to visualize the parasites. Polymerase Chain Reaction (PCR)-based assays are increasingly used for sensitive identification of the parasite species. Accurate species identification is important because treatment protocols vary based on the parasite and geographic region.
Treatment
The therapeutic strategy depends on the clinical form, the infecting species, and the patient’s immune status. Treatment for VL involves systemic anti-parasitic medications. Liposomal amphotericin B is frequently the drug of choice due to high cure rates. Oral miltefosine is another effective systemic option used for VL and some forms of cutaneous disease.
Cutaneous Leishmaniasis may be treated with local therapies for simple lesions, such as:
- Topical paromomycin ointment.
- Cryotherapy.
- Heat therapy.
More widespread or complex CL, as well as Mucocutaneous Leishmaniasis, requires aggressive systemic treatment, often with liposomal amphotericin B or miltefosine, to prevent disfigurement. Historically, pentavalent antimonials, such as sodium stibogluconate, were used but are less common now due to toxicity and drug resistance.
Geographic Distribution and Prevention
Leishmaniasis is found in over 90 countries, primarily in tropical and subtropical regions across four continents. The disease is distributed across parts of Latin America, Africa, the Middle East, and South and Central Asia. Over 90% of Visceral Leishmaniasis cases occur in just a few countries, including Brazil, Ethiopia, Kenya, and Sudan.
Prevention focuses on vector control and minimizing exposure to the sandfly bite, as no human vaccine is currently available. Sandflies are most active from dusk until dawn, so avoiding outdoor activities during these hours is an effective protective measure. Using insecticide-treated bed nets while sleeping greatly reduces the risk of being bitten.
Other prevention strategies include:
- Applying insect repellent to exposed skin and clothing.
- Controlling animal reservoirs, such as domestic dogs in some regions.
- Using residual insecticides to spray homes and animal shelters.
- Ensuring early diagnosis and prompt treatment of infected individuals to reduce transmission.