Leishmaniasis

Leishmaniasis is a complex group of clinical diseases caused by protozoal parasites. These parasites belong to the genus Leishmania within the Trypanosomatida group.

The disease is a major health concern in tropical and subtropical regions. These areas include parts of Africa, Asia, the Americas, and southern Europe. Because it often lacks sufficient attention and resources, the World Health Organization classifies it as a neglected tropical disease.

The disease is significant because it affects millions of people and can lead to serious health complications.

The biological mechanism of infection follows a specific cycle between insects and hosts. It begins when an infected female phlebotomine sandfly bites a host to take a blood meal. During this process, the fly injects the infective stage of the parasite, known as metacyclic promastigotes.

Once inside the host, these parasites are phagocytized, or swallowed, by specialized immune cells called macrophages. Inside these cells, the parasites transform into a different form called amastigotes. These amastigotes then multiply within the infected cells. The specific tissues they affect depend on the host and the Leishmania species involved.

The cycle continues when a sandfly ingests macrophages containing amastigotes during a blood meal. In the sandfly's midgut, the parasites differentiate into promastigotes and eventually migrate to the proboscis to infect the next host.

Leishmaniasis presents in three primary clinical forms based on how the parasites affect the body. Cutaneous leishmaniasis is the most common type and causes skin ulcers at the site of the bite. These sores may heal over several months or a year and a half, but they often leave permanent scars.

Some versions, like diffuse cutaneous leishmaniasis, produce widespread skin lesions that may not heal on their own. Mucocutaneous leishmaniasis involves ulcers on both the skin and the mucosal membranes of the nose and mouth. The third type is visceral leishmaniasis, also known as kala-azar or "black fever." This is the most severe form and is generally fatal if it is not treated. It causes systemic issues like fever, anemia, and enlargement of the spleen and liver.

Diagnosis requires different methods depending on the type of infection. Doctors can diagnose all three forms by using microscopy to see the parasites directly.

By examining samples from the blood, spleen, or skin lesions, they can find the small, round amastigotes. These parasites are typically 2–4 μm in diameter and possess a nucleus and a rod-shaped kinetoplast. For visceral disease, doctors may also use blood tests or more advanced methods like PCR. Polymerase chain reaction (PCR) tests detect the DNA of the parasite, including its kinetoplast DNA. This method is highly sensitive and can detect the disease even when the parasite load is very low.

Several factors increase the risk of contracting or spreading the disease. Socioeconomic conditions, such as poverty and overcrowded housing, can increase exposure to sandflies. Malnutrition is a major risk factor because deficiencies in protein, iron, and vitamins weaken the immune system. Environmental changes also play a role in the spread of the disease. Deforestation, urbanization, and climate change affect sandfly habitats and survival rates. Rising temperatures may even allow sandflies to move into new regions like North America. Human population mobility, such as migration or displacement, also contributes to the spread of the infection.

The scale of leishmaniasis is quite large in terms of both infection and mortality. Currently, between 4 and 12 million people are infected across approximately 98 countries. Every year, there are about 2 million new cases reported globally. These infections result in between 20,000 and 50,000 deaths annually. Furthermore, about 200 million people live in areas where the disease is common, including much of Asia, Africa, and South and Central America.

This high number of people at risk makes prevention and control efforts vital.

Prevention strategies focus on controlling the sandfly vectors and the animal reservoirs. People can protect themselves by using insect repellents containing DEET or sleeping under insecticide-treated nets.

Because many sandfly attacks happen at sunset, covering doors and windows is also helpful. For controlling the insect population, authorities use Integrated Vector Management, which includes spraying insecticides. Controlling animal reservoirs is more complex, as dogs are major reservoirs for Leishmania infantum. Instead of culling dogs, many use insecticide-impregnated collars to protect them. Scientists are also researching new tools, such as genetically modifying sandflies or developing vaccines to provide long-term control.