Structure life-cycle pathogenicity and Control Method of Leishmania donovani.

Habit and habitat:

In man, L.Donovani lives as an intercellular parasite in leucocytes or cells of liver, spleen, bone marrow, lymphatic glands, etc. It is the causative agent of the disease known as kala-azar resulting in fever, enlargement of the spleen and a reduction in the number of white corpuscles in the blood. It is transmitted through the bile of sandfly.

Structure:

1. Shape and size:The genus Leishmania occurs only in two forms or stages, leishmanial and leptomonad, which alternate between a vertebrate and invertebrate host. The two forms are recognized on the basis of their kinetoplast and blepharoplast and the course taken by the flagellum

(A) Leishmanial or amastigote form:It occurs intercellularly in blood cells or reticulo-epithelial cells of the vertebrate host or man. It is microscopic, rounded or oval, with a central or eccentric nucleus, blepharoplast and kinetoplast, but no free flagella.It measures 2 microns to 4 microns in diameter.

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(B) Leptomonad or promastigote form:It is found in the midgut of the invertebrate host or sandfly. It is elongated, slender and spindle-shaped with a large centrally placed nucleus, blepharoplast, kinetoplast and a long free flagellum. A fully formed promastigote measure 15-20 microns in length and 1-2 microns in width.

2. Cell membrane: The whole body is externally covered by a very thin, delicate, elastic and firm covering or pellicle. It gives the definite shape to the body.

3. Flagellum:Leishmania is uniflagellate, bearing a single flagellum. In the leptomonad form of parasite in sandfly flagellum is long and free. It is closely associated blepharoplast lies a disc-shaped parabasal or kinetoplast. There is no cytoplasm. In the leishmanial form of parasite in man, there is no free flagellum, as it is greatly reduced, fibril-like and lies embedded in cytoplasm.

4. Cytoplasm:Underneath pellicle, the body cytoplasm is colorless homogeneous ,and not differentiated into ectoplasm and endoplasm. Electron microscope studies that cytoplasm is marked by longitudinal striations or microtubules which may be contractile. Other structures present in cytoplasm are nucleus Golgi body, vacuole.

5. Nucleus: A single large spherical nucleus lies eccentrically or in the middle of the body. It is vesicular and with distinct karyosome or nucleus. It is covered by a double unit membrane with pores and measure about 1 microns in diameter.

Life cycle of leishmania donovani:

Leishmania is a digenetic parasite and requires two hosts for completion of its life cycle. Primary host is a invertebrates or man, in which the parasite feeds and multiplies asexually. Secondary (intermediate) host or vector is a sand fly (Phlebotomus sp.), which transmits L.Donovani to the man. Salivary gland of the vector is not involved. When such sandfly bites a man, it liberates the parasites in the skin wound caused by the proboscis.

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[I] Life-cycle in man:The parasites when to enter human body are in promastigote os leptomonad form. Some of them get immediately destroyed, some of these that enter liver, spleen, bone marrow and lymph nodes change into amastigote or leishmanial forms.These undergo slow multiplication by binary fission so that the host cells become greatly charged. When the parasites reach to a number of about 200, the host cell ruptures. The liberated parasites infect new host cells and repeat multiplication cycle hence, the reticule endothelial system becomes more and more infected. Some of the free amastigotes become phagocysted by neutrophils and monocytes.These heavily parasitized cells wander through the general blood circulation leading to a general infection.

[II] Life- cycle in sandfly:When the sandfly sucks the blood of an infected human, it obtains free amastigotes as well as parasitized neutrophils and monocytes along with the blood meal. In the midgut of the sandfly, the mastigote forms become elongated and acquire a free flagellum, by longitudinal binary fission. In 6 to 9 days, the number of parasites becomes enormous and they spread into the pharynx and buccal cavity. The salivary gland is not infected.Transmission into a new host occurs when such a heavily infected P sandfly bites the host.

Pathogenesis and symptoms:L.donovani causes a serious disease in a man called kala-azar. The incubation period is long, from 3 to 6 months, and symptoms may appear even after 2 years. Early symptoms include swelling, high fever and enlargement of spleen and liver. It is followed by general weakness, emaciation, anemia, and a peculiar darkening of the skin. In the advanced stage, skin becomes very dry, rough and dark and pigmented; hair becomes brittle and falls out. If left untreated, the patient dies within 2 years. Death is generally due to the secondary infections by bacteria or viruses as the defense mechanism of the body becomes so weak that the patient can't resist them.

Treatment control and prevention:

For treatment of Kala-azar, two groups of drugs are used. Pentavalent antimony compounds extensively used are sodium-antimony tartrate and gluconate, urea stibine, amino tib urea, neo stiboan,etc. Pentamidine isethionate is also used.

Prevention measure include:

(a)Eradication of the insect vector, i.e., sandfly.

In the endemic areas, low trees and bushes etc should be cleared out. Periodic fumigation and spray of insecticides of sleeping quarters should be proper treatment campaign.

(b) An attack on the parasite.

In areas where dogs act as reservoirs hosts, all street dogs should be killed. Control measure in India should be proper treatment campaign.

(c) Personal defense.

For avoiding the bite of sandflies, use mosquito nets or screens and avoid sleeping on ground floors.

Reference:

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L., Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

Aggarwal Sarita. A Text Book of Biology,New Delhi.: Madhuban Educational Books, 2011.

http://www.biologynoteshelp.com/leishmanialeishmaniasis/

http://www.yourarticlelibrary.com/zoology/parasite-leishmania-donovani-life-cycle-mode-of-infection-and-treatment/24263/

https://en.wikipedia.org/wiki/Leishmania_donovani