Leishmania parasites are transmitted through the bites of infected female phlebotomine sandflies. The epidemiology of leishmaniasis depends on the characteristics of the parasite species, the local ecological characteristics of the transmission sites, current and past exposure of the human population to the parasite, and human behaviour. Some 70 animal species, including humans, have been found as natural reservoir hosts of Leishmania parasites.

Mediterranean basin

In the Mediterranean basin, visceral leishmaniasis is the main form of the disease. It occurs in rural areas, villages in mountainous regions, and also some periurban areas where Leishmania parasites live mainly on dogs.

South-East Asia

In South-East Asia, visceral leishmaniasis is the main form of the disease. Transmission generally occurs in rural areas with a heavy annual rainfall, a mean humidity above 70%, a temperature range of 15–38 °C, abundant vegetation, subsoil water and alluvial soil. The disease is most common in agricultural villages where houses are frequently constructed with mud walls and earthen floors, and cattle and other livestock live close to humans. People are considered to be the sole reservoir of the Leishmania parasites in this region.

East Africa

In East Africa, there are frequent outbreaks of visceral leishmaniasis in the northern acacia–balanite savanna and the southern savanna and forest areas where sandflies live around termite mounds. Humans are considered the main reservoir of the Leishmania parasites causing visceral leishmaniasis in this part of Africa. Cutaneous leishmaniasis occurs in the highlands of Ethiopia and other places in East Africa, where increased human–fly contact occurs in villages built on rock hills or river banks, which are the natural habitat of hyraxes.

North Africa-Eurasia

In North Africa-Eurasia, cutaneous leishmaniasis is the main form of the disease. Agricultural projects and irrigation schemes can increase the prevalence of cutaneous leishmaniasis as people who have no immunity to the disease move in to work on the projects. Large outbreaks in densely populated cities also occur, especially during war and large-scale population migration. The parasites causing cutaneous leishmaniasis live mainly on humans or rodents.

Americas

Visceral leishmaniasis in the Americas is very similar to that found in the Mediterranean basin. The habit of keeping dogs and other domestic animals inside the house is thought to promote human infection. The epidemiology of cutaneous leishmaniasis in the Americas is very complex, with variations in transmission cycles, reservoir hosts, sandfly vectors, clinical manifestations and response to therapy, and multiple circulating Leishmania species in the same geographical area.

Post-kala-azar dermal leishmaniasis (PKDL) is a sequel of visceral leishmaniasis that appears as macular, papular or nodular rash usually on face, upper arms, trunks and other parts of the body. It occurs mainly in East Africa and on the Indian subcontinent, where 5–10% of patients with kala-azar develop the condition. It usually appears 6 months to 1 or more years after kala-azar has apparently been cured, but can occur earlier. People with PKDL are considered to be a potential source of kala-azar infection.

Leishmania-HIV coinfected people have high chance of developing the full-blown clinical disease, and high relapse and mortality rates. Antiretroviral treatment reduces the development of the disease, delays relapses and increases the survival of the coinfected patients. High Leishmania-HIV coinfection rates are reported from Brazil, Ethiopia and the state of Bihar in India.

Socioeconomic conditions
Poverty increases the risk for leishmaniasis. Poor housing and domestic sanitary conditions (such as a lack of waste management or open sewerage) may increase sandfly breeding and resting sites, as well as their access to humans. Sandflies are attracted to crowded housing as these provide a good source of blood-meals. Human behaviour, such as sleeping outside or on the ground, may increase risk. The use of insecticide-treated bednets reduces risk.

Malnutrition
Diets lacking protein-energy, iron, vitamin A and zinc increase the risk that an infection will progress to kala-azar.

Population mobility
Epidemics of both cutaneous and visceral leishmaniasis are often associated with migration and the movement of non-immune people into areas with existing transmission cycles. Occupational exposure as well as widespread deforestation remain important factors. For example, people settling in areas that used to be forests may be moving near sandflies’ habitat. This can lead to a rapid increase in cases.

Environmental changes
Environmental changes that can affect the incidence of leishmaniasis include urbanization, domestication of the transmission cycle and the incursion of agricultural farms and settlements into forested areas.

Climate change
Leishmaniasis is climate-sensitive, and strongly affected by changes in rainfall, temperature and humidity. Global warming and land degradation together affect the epidemiology of leishmaniasis in a number of ways:

• changes in temperature, rainfall and humidity can have strong effects on vectors and reservoir hosts by altering their distribution and influencing their survival and population sizes;
• small fluctuations in temperature can have a profound effect on the developmental cycle of Leishmania promastigotes in sandflies, allowing transmission of the parasite in areas not previously endemic for the disease;
• drought, famine and flood resulting from climate change can lead to massive displacement and migration of people to areas with transmission of Leishmania, and poor nutrition could compromise their immunity.

In visceral leishmaniasis, diagnosis is made by combining clinical signs with parasitological, or serological tests (such as rapid diagnostic tests). In cutaneous and mucocutaneous leishmaniasis serological tests have limited value. In cutaneous leishmaniasis, clinical manifestation with parasitological tests confirms the diagnosis.

The treatment of leishmaniasis depends on several factors including type of disease, concomitant pathologies, parasite species and geographic location. Leishmaniasis is a treatable and curable disease, which requires an immunocompetent system because medicines will not get rid of the parasite from the body, thus the risk of relapse if immunosuppression occurs. All patients diagnosed as with visceral leishmaniasis require prompt and complete treatment. Detailed information on treatment of the various forms of the disease by geographic location is available in the WHO technical report series 949, "Control of leishmaniasis".

Prevention and control of leishmaniasis requires a combination of intervention strategies because transmission occurs in a complex biological system involving the human host, parasite, sandfly vector and in some causes an animal reservoir host. Key strategies for prevention are listed below:

• Early diagnosis and effective case management reduces the prevalence of the disease and prevents disabilities and death. Early detection and prompt treatment of cases help to reduce transmission and to monitor the spread and burden of disease. Currently there are highly effective and safe anti-leishmanial medicines particularly for visceral leishmaniasis. Access to these medicines has significantly improved thanks to a WHO-negotiated price scheme and a medicine donation programme through WHO.
• Vector control helps to reduce or interrupt transmission of disease by controlling sandflies, especially in domestic conditions. Control methods include insecticide spray, use of insecticide–treated nets, environmental management and personal protection.
• Effective disease surveillance is important. Prompt data reporting is key to monitor and take action during epidemics and situations with high case fatality rates under treatment.
• Control of animal reservoir hosts is complex and should be tailored to the local situation.
• Social mobilization and strengthening partnerships – mobilization and education of the community with effective behavioural change interventions must always use locally tailored communication strategies. Partnership and collaboration with various stakeholders and other vector-borne disease control programmes is critical.

WHO's work on leishmaniasis control involves:

• supporting national leishmaniasis control programmes to produce updated guidelines and make disease control plans;
• raising awareness and advocacy on the global burden of leishmaniasis, and promoting equitable access to health services for disease prevention and case management;
• developing evidence-based policy guidelines, strategies and standards for leishmaniasis prevention and control, and monitoring their implementation;
• providing technical support to Member States to build sustainable, effective surveillance system and epidemic preparedness and response systems;
• strengthening collaboration and coordination among partners, stakeholders and other bodies;
• monitoring the global leishmaniasis situation, trends, progress in the disease control, and financing;
• providing diagnostic tests and antileishmanial medicines as last resort source;
• promoting research on effective leishmaniasis control including in the areas of safe, effective and affordable medicines, as well as diagnostic tools and vaccines; and
• facilitating the dissemination of research findings.

4 April 2014 – Mosquitoes, flies, ticks and bugs may be a threat to your health – and that of your family - at home and when travelling. This is the message of this year’s World Health Day, on 7 April. This short video highlights simple measures we can take to protect ourselves.