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The following points highlight the two hosts for completion of life cycle of plasmodium. The hosts are: 1. Life Cycle of Plasmodium in Man 2. Life Cycle of Plasmodium in Female Anopheles Mosquito.
Host # 1. Life Cycle of Plasmodium in Man:
Infection of man:
The infective stage of Plasmodium is a minute organism called sporozoite. When the female Anopheles mosquito bites a man, sporozoites present in the salivary gland of the mosquito are injected into the blood of the man.
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Pre-erythrocyte Schizogony:
Cryptozoites (crypto = hidden) are formed.
Exoerythrocytic Schizogony:
The cryptozoites enter new liver cells where they divide to form metacryptozoites. Exoerythrocytic schizogony is absent in Plasmodium falciparum.
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Erythrocyte Schizogony:
Metacryptozoite enters human RBC.
(i) Young Trophozoite and Signet-ring Stage:
After entering into an erythrocyte (RBC), the metacryptozoite becomes a rounded young trophozoite. As it grows, a large non-contractile vacuole appears in its centre, pushing the cytoplasm and nucleus to a thin peripheral layer to give a signet-ring appearance to the trophozoite.
(ii) Amoeboid Stage:
The vacuole disappears and the trophozoite becomes somewhat amoeba-shaped. The trophozoite acquires brownish-black haemozoin granules. The haemozoin granules are in-fact the products of decomposition of haemoglobin derived from the cytoplasm of the erythrocytes (RBCs).
Hemoglobin is broken up into globin (protein) and haematin (iron part). A number of orange or yellow granules of unknown nature, called Schuffner’s dots (= granules), appear in the cytoplasm of the host RBC.
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(iii) Formation of Erythrocyte Merozoites:
The trophozoite undergoes mitotic divisions to form the merozoites.
Incubation period:
The RBC gets ruptured to release merozoites. At this stage malaria fever is felt. The interval between the entry of the sporozoite into human blood and first appearance of the fever is called incubation period.
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It is about 14 days in Plasmodium vivax and P. ovale, 12 days in P. falciparum, 28 days in P. malariae. During this period, the parasites multiply to increase their number so that they can produce enough toxins to cause malaria.
Symptoms of Malaria:
Haemozoin causes chilliness and fever. Other symptoms are muscular pains and headache. In response to chills the body temperature starts rising. The patient sweats a lot and the temperature steadily goes down to normal, till the next attack takes place after 48 hours.
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(iv) Formation of Gametocytes:
Some erythrocytic merozoites enter fresh RBCs and form rounded gametocytes. The gametocytes are of two types— (a) Smaller male gametocytes or microgametocytes, and (b) Larger female gametocytes or microgametocytes.
Host # 2. Life Cycle of Plasmodium in Female Anopheles Mosquito:
Ingestion by Mosquito:
When a female Anopheles mosquito sucks the blood of an infected human host, it receives RBCs containing different stages of erythrocytic cycle, including gametocytes. In stomach of the female Anopheles mosquito, all stages except the gametocytes are digested. The gametocytes come out of the RBCs into the lumen (cavity) of the stomach of the mosquito.
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Gametogony:
The formation of gametes is called gametogony. The male and female gametocytes, liberated in the lumen of stomach of female Anopheles mosquito, form male and female gametes.
Fertilization:
A microgamete penetrates a macrogamete through its cone of reception and fertilization (syngamy) takes place, resulting in the formation of a zygote. The syngamy is anisogamous as the uniting male and female gametes are dissimilar.
Ookinete:
The zygote elongates and becomes worm like motile organism called ookinete.
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Penetration and Encystment:
The ookinete moves and bores through the wall of the stomach of female Anopheles mosquito. The encysted zygote is called oocyst or sporont. Oocysts appear upon the surface of the stomach of an infected female Anopheles bulging as tiny nodules. The cyst wall of the oocyst is partly secreted by the zygote and partly secreted by the stomach of the mosquito.
Sporogony:
The nucleus of oocyst divides first by meiosis and subsequently by mitosis, forming large number of small haploid nuclei. The tiny nuclei and cytoplasmic masses form elongated and spindle shaped bodies called sporozoites.
When mature oocysts rupture, the sporozoites are liberated into the haemocoel (body cavity filled with blood) of the mosquito. Being motile, the sporozoites move to different organs in the body cavity of the mosquito, but many of them penetrate the salivary glands of the mosquito.
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When the female Anopheles mosquito bites a healthy person, the sporozoites are injected in his/her blood along with saliva. These sporozoites start the cycle again in human body.
Control of Malaria:
Malaria is widely spread disease in India. There is separate anti-malaria department of the government which controls malaria through National Malaria Eradication Programme (NMEP). Two types of measures are undertaken to control malaria: elimination of mosquitoes and their larvae and pupae and prophylaxis (prevention of infection).
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1. Elimination of Mosquitoes and their larvae and pupae:
(i) Destruction of adult mosquitoes.
Mosquitoes can be killed by spraying insecticides (D.D.T., flit, etc.) in the habitated areas.
(ii) Destruction of larvae and pupae of mosquitoes:
(a) Larvae and pupae can be destroyed by spraying certain chemical larvicides, such as DDT, DDD, BHC (Benzene hexachloride).
(b) Ducks, larvivorous fish like Gambusia, some adult insects like dragon flies, insectivorous plants such as Utricularia, are the natural enemies of mosquito larvae and pupae as they feed upon them. These may be introduced in the water containing the larvae and pupae.
(iii) Elimination of breeding places:
Breeding grounds of larva and pupae, such as standing waters, should be drained, so that it does not become stagnant.
2. Prophylaxis:
(i) Protection against mosquito bites:
Mosquito nets should be used. The exposed body parts may be protected by using mosquito repellant creams, oil, etc.
(ii) Use of anti-malarial drugs:
The persons living in malarious regions are advised to take small regular dose of preventive medicines such as quinine, paludrine etc.
Treatment:
Quinine, the oldest drug for malaria, and other drugs are also used for this purpose. Quinine is extracted from the bark of the cinchona tree which is mostly growing in West Indies, India, Sri Lanka, Java and Peru. Other anti-malarial drugs are Paludrine, Primaquin, Chloroquinine, Camoquin Comoprima and Mepacrine.
