Leishmania

Introduction

• Ovoid organism within macrophage and possess rod shaped kinetoplast associated with rudimentary flagellum.
• Found as amastigote stage in cells of vertebrates host and as the promastigote stage in the intestine of sand fly.

Amastigote forms (Flageller stage)

• These forms are found in reticulo-endothelial cells (macrophages, skin, spleen, bone marrow, lymph nodes and mucosa), of dogs and rodents.
• Rounded or oval body measuring 2-4 µm long along the longitudinal axis.
• Have delicate cell membrane and demonstrated in fresh specimen only
• Nucleus measures less than 1 µm in diameter. It is oval or rounded and usually situated in the middle of cells or alongside of cell wall.
• Kinetoplast lies tangentially or at right angle to nucleus. It contains DNA containing body and a mitochondrial structure.
• Axonemes are delicate filament extending from the kinetoplast to the margin of the body.
• Vacuole is clear, unstained space lying alongside the axoneme.

Promastigote form (Flagellate stage)

• Found only in cultures and in insect vectors
• These are short, oval or pear-shaped bodies, measuring 9-10 µm in length and 2-3 µm in breadth.
• Fully developed forms are long, slender, spindle shaped bodies, measuring 15-20 µm x 1-2 µm.
• Nucleus is situated centrally
• Kinetoplast lies transversely near the anterior end.
• Vacuole is light staining area, lying in front of the kinetoplast over which root of flagellum runs.
• Flagellum may be of same length as body or even longer, projecting from front. Flagellum does not curve round the body of parasite and there is no undulating membrane.

Life cycle

• Life cycle is heteroxenous, occurring in two different host
• Vector: Sand fly (Phlebotomus sps.)

In definitive host

• Infected female sand flies inject the infective stage (i.e. promastigote) into definitive host during blood meal.
• Promastigotes injected in puncture wound are phagocytized by macrophages and other types of mononuclear phagocytic cell.
• Inside these phagocytic cells, they undergo transformation into amastigote. They multiply by simple division and proceed to infect another mononuclear phagocytic cell.
• Multiplication continues until the host cell can no longer hold and ruptures.

In sand flies

• These stages are then picked up by sand flies during blood feeding
• Amastigote ingested enter the mid gut or hind gut of sand fly depending on sub genus ( in hind gut for organism in viannia subgenus and in mid gut for organisms in leishmania sub genus).
• They undergo structural modification into flagellated promastigote, becomes larger and elongated. They undergo rapid multiplication by binary fission in gut epithelial linings.
• They migrate back to anterior part of digestive system such as pharynx and buccal cavity. Promastigote becomes infective at this stage and called metacyclic stage.
• Metacyclic promastigote then enter proboscis, where they accumulate and completely block the food passage.
• Immediately upon biting definitive host, parasites are released into host and host gets infection.

Transmission

• Transmission is through bite of sand fly.
• Infection may also occur when infected sand flies are crushed on the skin.

Three species of leishmania occurs in dogs