Haemoflagellates has two stages in its life cycle:
A. the amastigote form, occurring in man and
B. the promastigote form occurring in sand-fly.
A. Life cycle in Man:
L. donovani is transmitted to man by the sand-fly, Phlebotomus argentipes. In case of this parasite sand-fly acts as vector. The insect vector which has fed on some suitable fruit or plant juice after feeding on infected human blood meal, shows an enormous number of parasites in its buccal cavity and pharynx.
When such a vector bites a man, it introduces the parasites in the skin wound by its proboscis. Some authors are of opinion that the Indian vector (Sand-fly) does not bite but spreads infection by being crushed possibly by slapping.
The parasites administered by the vector into human body are the promastigote or leptomonad form. Some of them, entering the blood circulation directly become destroyed while those entering the cells of R. E. system (liver, bone marrow, lymph node, spleen) change into amastigote or leishmanial forms.
Multiplication by binary fission goes on continuously till the cells become packed with the parasites. The host cell is thereby enlarged and eventually ruptures (50 to 200 or even more may be found embedded in the enlarged host cell) (Fig. 3.3).
When the number of parasites reaches upto 200 or even more, the host cell ruptures. The parasites liberated as a result of the rupture into the circulation are again either taken up by or invade fresh cells and the multiplication cycle is repeated so that the entire R. E. system becomes progressively infected. In the blood stream, some of the free amastigotes are phagocytosed by the neutrophil granulocytes and monocytes.
A blood sucking insect draws these free amastigote forms as well as those within the monocyte during its blood meal. These heavily parasitized cells wander through the general blood circulation leading to a general infection.
B. Life cycle in vector (Sand-fly):
(a) Transfer to vector:
When the vector sucks blood of an infected person, it receives free amastigotes as well as parasitized neutrophils and monocytes along with the blood meal.
(b) Development in the vector:
In the midgut of sand-fly, the amastigote form becomes elongated and acquires a free flagellum, thus developing into promastigote form which again multiplies by binary fission producing an enormous number of flagellates. This multiplication proceeds in the midgut of this insect vector and the flagellates tend to spread forwards to the anterior part of digestive tract of (pharynx and buccal cavity) sand-fly.
A heavy pharyngeal infection of this insect vector is usually found between 6 to 9 days of its infective blood meal. This type of development is called anterior station development. The transmission into a new host is thereby effected when such a heavily infested sand-fly bites the host (but salivary glands are not infected).