Life History of Leshmania | Zoology Optional Notes for UPSC PDF Download

Systematic position

• Phylum: Protozoa
• Subphylum: Sacromastigophora
• Superclass: Mastigophora
• Class: Zoomastigophora
• Order: Kinetoplastida
• Genus: Leishmania
• Species: donovani

Discovery and Distribution

Discovery of Leishmania donovani

• The species was identified simultaneously by Leishmania in London and Donovan in Madras in 1903, leading to the naming of the species as Leishmania donovani.

Disease Caused

• L. donovani is responsible for causing a malaria-like fever known as kala-azar, Dumdum fever, or Black fever in humans.

Geographical Distribution

• The organism causes Visceral leishmaniasis (kala-azar) in several regions, including Africa, the Middle East, the Mediterranean coasts, Asia, and South America.
• It is endemic in continents such as Asia, Africa, the Americas, and regions around the Mediterranean.

Habits and Habitat of Leishmania donovani

Nature of Leishmania donovani

• It acts as an intracellular parasite affecting humans and other mammals.

Intracellular Presence in Humans

• The amastigote form of L. donovani resides within reticuloendothelial cells, notably in:
  • The spleen
  • Bone marrow
  • Leucocytes or liver cells
  • Intestinal mucosa
  • Mesenteric lymph nodes

Presence in Vectors and Culture

• The promastigote form is observed in:
  • The midgut of the sandfly vector
  • Artificial culture environments

Morphology of Leishmania donovani

Parasitic Forms of Leishmania donovani

• Amastigote Stage (Leishmanial Form)
  • Found within vertebrate hosts, including humans.
• Promastigote Stage (Leptomonad Form)
  • Exists within invertebrate hosts, specifically sandflies.

1. Leishmanial or Amastigote stage

Characteristics of the Amastigote Stage

• Location: Intracellularly within blood cells or reticuloendothelial cells of vertebrate hosts, including humans.
• Size: Measures 2-4 micrometers in length, presenting a microscopic, rounded, or oval shape.
• Flagellum: Lacks a free flagellum; it is significantly reduced to a fibril-like structure within the cytoplasm.
• Identification: Known as LD bodies in the flagellar stage of the amastigote.
• Nucleus: Positioned centrally or eccentrically.
• Cell Membrane: Delicate and observable only in fresh specimens.
• Kinetoplast: Rod-shaped or dot-like, positioned at a right angle to the nucleus.
• Axoneme (Rhizoplat): A delicate filament stretching from the kinetoplast to the body's margin, representing the flagellum's base.
• Staining: Effectively stained with Giemsa or Wright stain, displaying a pale blue cytoplasm with a red-stained larger nucleus and a deep red kinetoplast.
• Reproduction: Undergoes binary fission at 37°C.
  

2. Leptomonad or promastigote stage

Characteristics of the Promastigote Stage

• Habitat: Resides in the midgut of the invertebrate host, particularly the sandfly.
• Shape and Size: Elongated, slender, and spindle-shaped, measuring 15-20µm in length and 1-2µm in width.
• Flagellum: Features a long flagellum (15-28µm), which is free and originates from a small basal body or blepharoplast near the anterior end.
• Flagellum and Body: The flagellum does not encircle the parasite's body, eliminating the presence of an undulating membrane.
• Nucleus: Positioned centrally within the cell.
• Kinetoplast: Located transversely near the anterior end.
• Vacuole: Present near the flagellum's root.
• Staining: Cytoplasm appears blue, the nucleus pink or violet, and the kinetoplast bright red when stained with Leishman stain.
• Reproduction: Promastigote undergoes binary fission at 27°C.

Life cycle of Leishmania donovani

Hosts

• Digenetic Nature of Leishmania: Leishmania requires two hosts to complete its lifecycle, making it a digenetic parasite.
• Primary Host: The primary host is a vertebrate, including humans, where Leishmania feeds and multiplies asexually.
• Secondary Host/Vector: The secondary host or vector comprises invertebrates or blood-sucking insects, specifically the sandfly, belonging to the genus Phlebotomus.
• Reservoir Hosts: Certain mammals like dogs, jackals, gerbils, and squirrels act as reservoir hosts. In these hosts, the parasite remains without undergoing any change, awaiting transfer to the human host.
  

(I) Life cycle in Man

Life Cycle Stages
Leishmania donovani exhibits two stages in its life cycle:

• Amastigote form: Found in humans and mammals.
• Promastigote form: Occurs in the sandfly vector.

Transmission and Introduction to Host

• L. donovani is transmitted to humans or other vertebrates through the bite of the blood-sucking sandfly, Phlebotomus argentipes.
• Parasites introduced by the sandfly into the human body are in the promastigote form.

Transformation and Multiplication

• Upon entry, some promastigotes directly entering the bloodstream may be destroyed.
• Promastigotes entering the reticuloendothelial system (liver, spleen, bone marrow, and lymph nodes) transform into amastigotes or leishmanial forms.
• Amastigotes multiply via simple binary fission within the reticuloendothelial system, generating a large number of amastigotes.

Host Cell Rupture and Spread

• As the parasite population within host cells increases (typically reaching 50 to 200 or more), the host cell ruptures.
• Liberated parasites are then taken up by new host cells, perpetuating the multiplication cycle and progressively infecting the reticuloendothelial system.

Infection Spread

• Some free amastigotes may be phagocytosed by neutrophils and monocytes (macrophages) in the bloodstream.
• These heavily parasitized cells circulate through the blood, leading to a general infection.

(II) Life cycle in sandfly

Parasite Acquisition by Sandfly
When the sandfly feeds on the blood of an infected individual, it acquires:

• Free amastigotes
• Parasitized neutrophils and monocytes

Parasite Transformation in Sandfly
The obtained amastigotes undergo a transformation process:

• Amastigotes change into procyclic promastigotes.
• Procyclic promastigotes then transform into metacyclic promastigotes within the midgut of the sandfly.

Promastigote Multiplication

• Promastigotes multiply through longitudinal binary fusion, leading to the production of numerous promastigotes.
• The promastigotes proliferate extensively, filling the lumen of the sandfly's gut.

Spread in Sandfly

• Within 6 to 9 days, the parasite population becomes immense, heavily distributed throughout the sandfly's pharynx and buccal cavity.
• Notably, the salivary glands remain uninfected during this process.

Transmission to New Host

• Transmission occurs when a heavily infected sandfly, containing a vast number of parasites, bites a new host.