Leishmania is a major pathogenic flagellate protozoan parasite found in reticulo- endothelial system (liver, spleen, bone marrow) of human. It causes a serious disease called visceral leishmsaniasis, commonly known as kala-azar or dum dum fever.

It is named after the discoverers, Leishman and Donovan, both of them reported the organism simultaneously; Leishman from London in May 1903 and Donovan from Madras of India in July 1903. Sir William Leishman in 1900 observed the parasite in spleen smear of a soldier who died of "Dum dum fever" or kala-azar contracted at Dum Dum, Calcutta. Leishman reported this finding from London in 1903. Donovan also reported the same parasite in spleen smear of patient in Madras.

It is endemic in many places in China, India, Africa, Southern Europe, South America and Russia. It is also reported from Nepal.

The natural habitat of L. Donovani in man reticulo-endothelial system such internal organs especially spleen, liver, bone marrow, lymphnodes. The parasite may be found in endothelial cells of the kidney, suprarenal capsules, lungs, meninges and cerebrospinal fluid. It is transmitted through biting of sand fly.

L.donovani is transmitted by the sand fly (Phlebotomus argentipes). When an infected sandfly bites a healthy person, promagstigote forms of parasites are introduced in blood.

These promagstigote forms develop into amagstigote forms which reach to the cells of reticulo-endothelial system and multiply by binary fission. Multiplication goes on continuously till the cell becomes packed with the parasites. The reticulo-endothelial cells enlarge and enventually ruptured. The parasites liberated as a result of the rupture into the blood circulation are again either taken up by sandfly or invade fresh cells and the cycle is repeated. In this way the entire reticulo-endothelial system becomes progressively infected.

Method of transmission
The transmission of L.donovani from one person to the other person is carried out by a species of sand fly of the genera Phelebotomus argentipes through biting.

Incubation period
Incubation is long, from 3 to 6 months, and symptoms may appear even after 2 years.

Pathogenesis
It causes a dreadful disease visceral leishmaniasis or kalazar or black fever. Early symptoms of kala-azar include swelling and enlargement of spleen and liver. It is followed by high fever, general weakness, emaciation, anaemia due to reduction in number of blood cells, and a peculiar darkening of skin. The word "kala-azar" has been derived from two Indian words, kala and azar, meaning "black sickness". The clinical illness begins with fever, which may be continuous, remittent or irregular. The spleen is the organ most affected. The enlargement of spleen (splenomegaly) starts early and is progressive and massive.

Diagnosis
Kala-azar can be diagnosed by microscopical examination of blood film or biopsy material taken from spleen or bone marrow of patient, for the presence of amastigote forms of L. donovani. Examination of W.B.C. count shows decrease of neutrophils but increase of lymphocytes and monocytes. Number of erythrocytes (R.B.C.) is also decreased.

Treatment
Pentavalent antimony compounds are the drugs used for the treatment of Kala-azar. Antimony compounds are stibamine, aminostiburea, neostibosan, neostam, and sodium-antimony-gluconate. However, antimony resistance has become a serious problem. The aminoglycoside antibiotic aminosidine (Paromomycin) is useful.

- Treat all human cases harbouring L. donovani adequately with antimony.

- Eradiction of sandfly by periodic fumigation and spray of insecticides.

- Avoiding bite of sandfly by using mosquito nets.

- Avoid houses surrounded by dense vegetation.

· L. tropica – found central and south America, cause muco-cutaneous leishmaniasis.

· L. brasilensis – found in Arabia, Syberia, shores of Mediterranean, cause cutaneous leishmaniasis or oriental sore.