Kala-azar, also known as visceral leishmaniasis, is caused by a microscopic parasite called *Leishmania*. This parasite is transmitted to humans through the bite of infected female sand flies, tiny insects that thrive in certain environmental conditions. When a sand fly carrying the parasite bites a person, it injects the *Leishmania* parasites into the bloodstream, where they invade and multiply inside specific immune cells called macrophages. Over time, this leads to the symptoms of kala-azar, which affect internal organs like the spleen, liver, and bone marrow.
The root cause of kala-azar is this complex interaction between the *Leishmania* parasite, the sand fly vector, and the human host. The parasite species most commonly responsible for kala-azar are *Leishmania donovani* in the Indian subcontinent and East Africa, and *Leishmania infantum* in the Mediterranean region and Latin America. These parasites have evolved to survive inside the sand fly and the human immune system, making the disease difficult to control.
Several factors influence the spread and occurrence of kala-azar. The sand flies that transmit the disease prefer warm, humid climates, often found in tropical and subtropical regions. Changes in climate, such as rising temperatures and altered rainfall patterns, have expanded the geographical range of these sand flies, allowing the disease to appear in new areas, including higher altitudes and previously non-endemic regions. Human activities like deforestation, urbanization, and migration also contribute by creating favorable environments for sand flies and increasing human exposure to them.
Poor sanitation and waste management can increase sand fly breeding sites, while poverty and population growth exacerbate the problem by limiting access to preventive measures and healthcare. Additionally, the presence of animal reservoirs, such as dogs and rodents, can maintain the parasite in the environment, facilitating ongoing transmission to humans.
Inside the human body, once infected, the parasite targets macrophages, evading the immune system and multiplying within these cells. This leads to widespread infection of vital organs, causing symptoms like prolonged fever, weight loss, anemia, and enlargement of the spleen and liver. If untreated, kala-azar is almost always fatal due to the severe damage caused by the parasite.
In some cases, after treatment, the disease can reappear in a skin form called post-kala-azar dermal leishmaniasis (PKDL), which also plays a role in maintaining the parasite in the community. Co-infections with diseases like HIV can worsen kala-azar by weakening the immune system further.
Efforts to control kala-azar focus on reducing sand fly populations through insecticide spraying, improving housing and sanitation, and early diagnosis and treatment of infected individuals to prevent transmission. Understanding the environmental, biological, and social factors that cause kala-azar is crucial for effective prevention and control strategies.
