Leishmania donovani is a species of intracellular parasites that belong to the genus Leishmania, a group of haemoflagellate kinetoplastids that cause the disease leishmaniasis. It is a human blood parasite responsible for visceral leishmaniasis or kala-azar, the most severe form of leishmaniasis. It infects the mononuclear phagocyte system including spleen, liver and bone marrow. Infection is transmitted by species of sandfly belonging to the genus Phlebotomus in Old World and Lutzomyia in New World.

The systematic position of Leishmania donovani is as follows :

Domain: Eukaryota
Phylum: Euglenozoa
Class: Kinetoplastea
Order: Trypanosomatida
Genus: Leishmania
Species: L. donovani

The species complex it represents is prevalent throughout tropical and temperate regions including Africa (mostly in Sudan), China, India, Nepal, southern Europe, Russia and South America. The species complex is responsible for thousands of deaths every year and has spread to 88 countries, with 350 million people at constant risk of infection and 0.5 million new cases in a year.

L. donovani was independently discovered by two British medical officers William Boog Leishman in Netley, England, and Charles Donovan in Madras, India, in 1903. However, the correct taxonomy was provided by Ronald Ross. The parasite requires two different hosts for a complete life cycle, humans as the definitive host and sandflies as the intermediate host. In some parts of the world other mammals, especially canines, act as reservoir hosts.

• The species of Leishmania donovani was reported simultaneously by two British medical officers, William Boog Leishman in London and Charles Donovan in Madras, in 1903. However, the correct taxonomy was provided by Ronald Ross.
• L. donovani causes visceral leishmaniasis or kala-azar, the most severe form of leishmaniasis. It infects the mononuclear phagocyte system including spleen, liver and bone marrow.
• L. donovani is endemic in Asia, Africa, the Americas, and the Mediterranean region . It is found in North Africa, Southern Europe, and the Middle East through India into western China. The disease was brought to the America after Columbus and is now endemic in parts of South and Central America.
• The species complex is responsible for thousands of deaths every year and has spread to 88 countries, with 350 million people at constant risk of infection and 0.5 million new cases in a year.

Leishmania donovani is an intracellular parasite that infects humans and other mammals. It has two different forms or stages in its life cycle: the leishmanial or amastigote stage, which occurs inside the cells of the vertebrate host, and the leptomonad or promastigote stage, which occurs in the gut of the invertebrate vector .

The vertebrate hosts of Leishmania donovani include humans, dogs, jackals, gerbils, and squirrels. These animals serve as reservoirs of infection for the parasite. In humans, Leishmania donovani causes visceral leishmaniasis, also known as kala-azar, a potentially fatal disease that affects the spleen, liver, bone marrow, and lymph nodes. The parasite is transmitted to humans by the bite of infected female sand flies of the genus Phlebotomus.

The sand flies are the definitive hosts of Leishmania donovani, where the parasite undergoes sexual reproduction and development. The sand flies are found in tropical and subtropical regions of Africa, Asia, Europe, and South America. They breed in moist and shady places such as animal burrows, cracks in walls, tree holes, and piles of organic matter. They feed on blood at night and are attracted by heat, carbon dioxide, and odors from their hosts.

Leishmania donovani can also be cultured artificially in laboratory media such as blood agar or Novy-MacNeal-Nicolle (NNN) medium. This allows researchers to study the morphology, physiology, genetics, and immunology of the parasite.

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.

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.

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.

Life cycle of Leishmania donovani

Leishmania donovani is a digenetic parasite that requires two hosts for completion of its life cycle: a vertebrate host (human or other mammals) and an invertebrate host (sandfly). The parasite has two morphological forms: amastigote and promastigote, which alternate between the hosts.

In the vertebrate host

The infection begins when an infected female sandfly of the genus Phlebotomus bites a human or other mammal and injects promastigotes into the skin. The promastigotes are phagocytized by macrophages and other types of mononuclear phagocytic cells in the dermis. Inside these cells, the promastigotes transform into amastigotes, which are small, round, non-flagellated forms. The amastigotes multiply by simple binary fission and infect other phagocytic cells. The infection can spread to various organs and tissues, especially the spleen, liver, bone marrow, lymph nodes, and intestinal mucosa. This causes visceral leishmaniasis, also known as kala-azar, which is characterized by fever, weight loss, hepatosplenomegaly, anemia, and immunosuppression. If untreated, the disease can be fatal .

In the sandfly

When a female sandfly feeds on the blood of an infected vertebrate host, it ingests amastigotes along with the blood. In the midgut of the sandfly, the amastigotes transform into procyclic promastigotes, which are elongated, slender, flagellated forms. The procyclic promastigotes multiply by longitudinal binary fission and migrate to the anterior part of the midgut. There they differentiate into metacyclic promastigotes, which are infective forms that attach to the gut wall and the proboscis of the sandfly. The development of promastigotes in the sandfly takes about 6 to 9 days . The cycle is completed when the sandfly bites another vertebrate host and transmits the metacyclic promastigotes .

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