Leishmania donovani, Unveiling the Intricacies of a Parasite Responsible for Visceral Leishmaniasis and its Impact on Global Health, Pathogenesis, Transmission, and Emerging Treatment Strategies in Endemic Regions.

Leishmania donovani is a protozoan parasite responsible for causing visceral leishmaniasis, also known as kala-azar, the most severe form of leishmaniasis. It is transmitted to humans through the bite of infected female sandflies. Once in the human body, the parasite targets internal organs, particularly the liver, spleen, and bone marrow. Symptoms of visceral leishmaniasis include fever, weight loss, anemia, and enlargement of the spleen and liver. Without treatment, the disease can be fatal, but it can be managed with antiparasitic medications.

1. Leishmania donovani S.Nahidha begum II –M.Sc Microbiology Sacred Heart College

2. The species of  Leishmania donovani  was reported simultaneously by  Leishmania  from London and  Donovan  from Mardas in (1903) hence the name   Leishmania donovani . L .  donovani  causes a malaria-like fever-oriental disease in the man called kala-azar, Dumdum fever, or Black fever. L. donovani , the causative organism of Visceral leishmaniasis  (kala-azar) in Africa, the Middle East, Mediterranean coasts, Asia, and South America. It is endemic in Asia, Africa, the Americas, and the Mediterranean region. Leishmania donovani

3. In man, intracellular amastigote forms are found in reticuloendothelial cells of the spleen, bone marrow, leucocytes or cells of the liver, intestinal mucosa, and mesenteric lymph nodes. It is an intracellular parasite of man and other mammalian hosts. The promastigote form is found in the midgut of sandfly or in artificial culture. Habits and Habitat of Leishmania donovani

4. Morphology of  Leishmania donovani The parasite occurs in two forms or stages, leishmanial or amastigote and leptomonad or promastigote, which alternate between a vertebrate (man) and an invertebrate (sandfly) host.

5. (1). Leishmanial or Amastigote stage This stage occurs intracellularly in blood cells or reticuloendothelial cells of the vertebrate hosts or man. It is microscopic, rounded, or oval in shape measuring 2-4 micrometer in length. There is no free flagellum, it is greatly reduced, fibril-like, and lies embedded in the cytoplasm. A flagellar stage of amastigote is known as LD bodies. The nucleus is central or eccentric. The cell membrane is delicate and can be demonstrated only in a fresh specimen.  Kinetoplast is rod-shaped or dot-like and lies at the right angle to the nucleus.  The axoneme(rhizoplat) is a delicate filament extending from the kinetoplast to the margin of the body. It represents the foot of the flagellum. They are stained well with Giemsa or Wright stain. In a Giemsa stained preparation, the cytoplasm surrounded by a limiting membrane appears pale blue. The nucleus relatively is larger and stained red. The kinetoplast stained deep red. Amastigote divides by binary fission at 37°C.

6. Life cycle of  Leishmania donovani Hosts Leishmania  is also a digenetic parasite that requires 2 hosts for completion of its life cycle. The primary host is a vertebrate or man, in which the parasite feeds and multiplies asexually. The secondary host or vector is invertebrates or blood-sucking insects or sand-fly, belonging to the genus  Phlebotomus . Some mammals like dogs, jackals, gerbils, and squirrels also serve as reservoir hosts in which the parasite does not undergo any change but simply waits for its introduction into the human host.

7. (2). Leptomonad or promastigote stage It is found in the midgut of the invertebrates host or sandfly. It is elongated, slender, and spindle-shaped measuring 15-20µ in length and 1-2µ in width. A flagellum is long measures 15-28µ and free and arises from a minute basal body or blepharoplast situated near the anterior end. The flagellum does not curve around the body of the parasite and therefore there is no undulating membrane. The nucleus is centrally placed. The kinetoplast lies transversely near the anterior end.  A vacuole is present near the root of the flagellum With Leishman stain, the cytoplasm appears blue, the nucleus pink or violet, and the kinetoplast bright red. Promastigote multiplies by binary fission at 27°C.

8. (I). Life cycle in Man The parasite has two stages in its life cycle: Amastigote form  occurs in humans and mammals. Promastigote form   occurs in sandfly. L.   donovani  is transmitted to humans or other vertebrates by the bite of blood-sucking sandfly  Phlebotomus argentipes The parasites introduced by sandfly into the human body are in the promastigote form. Some of the promastigote entering the blood circulation directly become destroyed. while those entering the reticuloendothelial system(liver, spleen, bone marrow, and lymph nodes ) change into amastigote or leishmanial forms. The amastigotes multiply by simple binary fusion inside the Reticuloendothelial system to form a large number of amastigotes. When the number of parasite reaches 50 to 200 or even more, the host cell rupture. The liberated parasites are taken up by new host cells and the multiplication cycle is repeated so that the reticuloendothelial system becomes progressively infected. Some of the free amastigotes are phagocytosed by the neutrophils and monocytes(macrophages) in the bloodstream. These heavily parasitized cells wander through the general blood circulation leading to a general infection.

9. When the sandfly sucks the blood of an infected person, it obtains free amastigotes as well as the parasitized neutrophils and monocytes along with the blood-meal. The parasite begins a process of transformation and the amastigotes change to procyclic promastigotes and then to metacyclic promastigotes in the midgut of the sandfly. These promastigotes multiply by longitudinal binary fusion and produce large numbers of promastigotes completely filling the lumen of the gut. In 6 to 9 days, the number of parasites becomes enormous and heavily spread into the pharynx and buccal cavity. The salivary glands are not infected. Transmission into a new host occurs when such a heavily infected sandfly bites the host. (II) Life cycle in sandfly

10. Kala-Azar Symptoms Loss of interest and desire for everything, pale appearance, loss of weight with weakness. Abnormal enlargement of the spleen, usually nontender and soft. Dry, skin and scaly skin of hands, abdomen, feet and face gives the Indian name Kala-azar which means ‘Black Fever’. Anaemia(a condition of deficiency of haemoglobin or red cells in the blood) progresses rapidly. Anaemia along with emaciation(weakness) creates a typical appearance of the affected person.

11. How Kala-azar is Diagnosed? Clinical : A condition of fever lasting for more than two weeks and not responding to antibiotics and antimalarials. Clinical laboratory searches may include spreading leucopenia thrombocytopenia and anaemia. Laboratory : Serology tests: Different tests are available for the diagnosis of Kala-azar. The most used tests are based on relative sensitivity, while operational feasibility involves the Direct Agglutination Test (DAT), ELISA and rk39 dipstick. Aldehyde Test is usually applied but it is a nonspecific test. IgM (immunoglobulins M)tests are under development and not available for use. Confirmatory diagnosis is  Parasite demonstration in spleen/bone marrow or culture medium. However, sensitivity varies with the organ chosen for aspiration. Though the spleen has the most sensitivity and specificity, a skilled professional with appropriate precautions can perform well only at a well-equipped hospital .

12. Amphotericin. Miltefosine. Pentavalent antimonials, such as sodium stibogluconate. Paromomycin PREVENTION Avoiding Sandfly Bites : Since sandflies are most active during dawn and dusk, try to stay indoors during these times. When outdoors, wear long-sleeved shirts, long pants, and use insect repellents containing DEET on exposed skin. Using Bed Nets Environmental Control : Reduce the presence of sandflies by eliminating their breeding sites, such as stagnant water sources near the home. Regularly clean and dispose of organic waste where sandflies may breed. Protective Clothing Indoor Residual Spraying (IRS) : In high-risk areas, indoor spraying with insecticides can help reduce the population of sandflies, thereby decreasing the risk of transmission. Avoidance of Endemic Areas TREATMENT :

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