LEISHMANIA DONOVANI

Introduction

· Leishmania is a pathogenic protozoan endoparasite of the human being, causative organism of a dreadful disease called ‘Visceral leishmaniasis’ or ‘Kala-azar’ accompanied by swelling, fever, enlargement of liver and spleen, anaemia, and the skin color turning black.

· The vector is the female sandfly, Phlebotomus argentipes.

SYSTEMATIC POSITION
Phylum: Protozoa
Sub-phylum: Sarcomastigophora
Super-class: Mastigophora
Class: Zoomastigophorea
Order: Kinetoplastida
Genus: Leishmania
Species: donovani

Historical background

The genus Leishmania was created by Ross (1903 AD). The species Leishmania donovani was reported simultaneously by Leishman from London (May 1903 AD) and Donovan from Madras (July 1903 AD); hence the name Leishmania donovani.

Geographical distribution

Leishmania donovani infects man in many places in India, China, Africa, Southern Europe, South America, and Russia. In Nepal, it is widely distributed in the southern belt of the Terai region. (Saptari and Siraha districts more)

Habit and habitat

In man, Leishmania lives as an intracellular parasite in leucocytes or cells of the liver, spleen, bone marrow, lymphatic glands, etc. It causes the disease known as kala-azar. It is transmitted through the bite of the infected sand fly of the genus Phlebotomus.

Structure/Morphology

The Leishmania is a dimorphic parasite i.e. it exists in two stages. These stages alternate between a vertebrate (man) and an invertebrate (sandfly) host.

i. Amastigote stage (Formerly called ‘Leishmanial’ form): It occurs in man and in reservoir mammals like dogs, jackals, ground squirrels, etc.

ii. Promastigote stage (Formerly called ‘Leptomonad’ form): It occurs in the gut of the Sandfly.

Leishmania, A: Amastigote form & B: Mastigote form

Amastigote stage or aflagellar stage

This stage of the parasite is formed in the reticuloendothelial system of vertebrate hosts (man, dog, etc.)

a. Shape and size: It is a round or oval body measuring 2µ - 4µ in diameter.

b. Cell membrane: The whole body is covered by is very thin, delicate, elastic and firm covering or pellicle. It can be demonstrated in fresh specimen only. It gives definite shape to the body.

c. Nucleus: It measures a little less than 1µ in diameter. It is spherical and is usually situated in the middle of the cell or along the side of the cell.

d. Kinetoplast: It lies at the right angle to the nucleus. It comprises a DNA-containing body and mitochondrial structure.

e. Axoneme (Rhizoplast): It is the delicate filament extending from the basal body or kinetoplast to the margin of the body. It represents the root of the flagellum.

f. Vacuole: It is a clear unstained space lying along the side of the axoneme.

Promastigote stage or flagellar stage

This stage of the parasite is only encountered in cultures and in insect vectors (sandflies).

a. Shape and size: It is cylindrical in shape when they are matured but in the initial stage, it is pear-shaped measuring about 15-20µ in length and 1-2µ in breadth.

b. Nucleus is centrally located.

c. Kinetoplast lies transversely near the anterior region.

d. Eosinophilic vacuole is a large staining area lying in front of the kinetoplast over the root of the flagellum.

Life cycle

· Leishmania donovani is a digenetic parasite so requires two hosts to complete its life cycle. The primary host is a man and the secondary host is a sandfly.

· The aflagellar or leishmanial stage is found in man and in reservoir mammals and the leptomonad or flagellar stage is found in the sand fly and in the culture. The life cycle of Leishmania donovani in the mammalian host is a simple one, the organism is engulfed by the reticulo-endothelial cells which it multiplies by simple binary fission until the host cell is destroyed, where new macrophage cells are parasitized.

Life cycle in man

a. Infection: Leishmania donovani is transmitted to man by the sand fly- Phlebotomus argentipes.

After an infected human blood meal, shows an enormous number of parasites in its buccal cavity and pharynx. When such a sandfly bites a man, it liberates the parasites in the skin wound caused by its proboscis. However, the actual mode of transmission is not clear.

b. Multiplication: The parasites introduced by sand fly into the human body are in the form of promastigote or leptomonad forms. Some of them entering the blood circulation directly get destroyed, while those entering the cells of reticulo-endothelial system (liver, spleen, bone marrow, and lymph nodes) change into amastigote forms.

c. Spread of infection: When the number of parasites reaches 50-200 or even more, the host cell ruptures. Some of the free amastigotes become phagocytosed by neutrophils and monocytes.

Life cycle in sandfly

a. Transfer to sand fly: When sand fly sucks infected blood which forms amastigotes, as well as neutrophils and monocytes, are phagocytosed.

b. Development in the sandfly: In the mid-gut of the sandfly, the amastigote forms elongate and acquire a free flagellum, thus changing into promastigote forms. It multiplies by longitudinal binary fission in 6-9 days and spreads in the pharynx and buccal cavity.

Kala-azar or Dum-dum fever

Occurrence
Kala-azar also known as black fever or Dum-dum fever, is a serious oriental (fatal) disease to man. It occurs in Nepal, India, China, Mediterranean countries, parts of Africa, and South America. In Nepal, it is more common in Saptari and Siraha districts only. It was firstly found in dumdum of Kolkata.

Symptoms and pathogenesis
It has a long incubation period i.e. 3-6 months and sometimes more than two years. High fever, enlargement of liver and spleen which causes anemia, due to the lack of RBCs, WBCs, skin becomes dry, rough and hair becomes brittle and falls out. If not treated on time, the patient dies within 2 years. Death is generally due to secondary infections by bacteria or viruses.

Diagnosis
It is diagnosed by microscopical examination of blood, a biopsy was taken from the spleen or bone marrow for the presence of amastigote forms of Leishmania donovani.

In WBCs, there is a decrease in neutrophils but an increase in lymphocytes and monocytes. The number of RBCs is also decreased.

Treatment
For treatment, two groups of drugs are available.
Pentavalent antimony compounds like Sodium-antimony tartarate and gluconate, Urea stibamine, Aminostiburea, Neostibosan, etc.

Pentamidine isethionate is also used.

Prevention (Prophylaxis)
It includes
i. Eradication of insect vector (Sandfly): In endemic areas, low trees and bushes, etc. should be cleared out.
Periodic fumigation and spray of insecticides of sleeping quarters should be done.

ii. Attack on the parasite: Those areas where dogs act as reservoir hosts, all street dogs should be isolated and treated.

iii. Personal defense: Use mosquito -nets or screens and avoid sleeping on ground floors (avoiding the bite of sand fly).

iv. Have houses well lit and ventilated.

(In short, Eradication of insect vector sandfly by insecticides, elimination of reservoir hosts and avoiding the bite of sand fly)

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