Plasmodium

Phylum - Protozoa

Class - Sporozoa

Order - Haemosporidia

Members of the genus Plasmodium are collectively known as ma­larial parasites. They cause a febrile disease called malaria. Malaria as a chill and fever disease is known to mankind for a long time. Eradication of malaria is an important problem in public health. For a long time it was believed that malaria was caused by harmful vapours produced in marshy land (Gr. Malo-bad+air). Charles Laveran, a French military surgeon, for the first time, noticed Plasmodium in the blood of a malarial patient, in 1880. Its connection with the intermediate host and the modes of transmission were experimentally worked out in Calcutta by Sir Ronald Ross in 1889. For this discovery he was awarded the Nobel Prize for medicine in 1902. Grassi (1890) provided absolute scientific proof for the specific relationship between Anopheles mosquito and the human malarial parasite.

Plasmodium : The plasmodium is an intracellular sporozoan blood parasite. For the completion of life cycle it requires two hosts, a vertebrate and a blood sucking invertebrate. Transference of the parasite is effected by the inverte­brate host. In man, the infection takes place by the inoculation of the slender, sickle shaped nucleated sporozoite in the blood by the bite of an infected female mosquito belonging to the genus Anopheles. At least four species of Plasmodium, P. vivax, P. falciparum, P. malariae and P. ovale, are known to attack man causing different kinds of malaria.

The life cycle of the malarial parasite involves two hosts, the man and the mosquito. The modes of development in these two hosts are different. In man the mode of reproduction is asexual and in mosquito it is sexual. Man is the intermediate host and the mosquito is the definitive host.

Life cycle in Man - Schizogony

There are two phases in the life cycle of malarial parasite in man. They are (1) Pre erythrocytic cycle or Exoerythrocytic cycle (in liver cells) and (2). Erythrocytic cycle or Endo-erythrocytic cycle (inside the red blood corpuscles)

Pre-erythrocytic cycle:

The pre-erythrocytic cycle comprises the asexual reproduction of the parasite in the liver. When an infected female Anopheles mosquito bites a person, thousands of slender, sickle shaped nucleated sporozoites are in­jected in the blood. The sporozoites first enter the capillary vessels of the skin and then enter the general circulation. These parasites circulate in the blood for about 30 minutes and enter into the pre-erythrocytic cycle in the reticu­loendothelial cells of the liver.

Fig. 1.3.1 Life cycle of malarial parasite

The sporozoites penetrate the liver cells and develop into forms known as cryptozoites. A cryptozoite has a compact nucleus and no pigment or vacuoles. Cryptozoites rapidly grow feeding on the liver cells. When a cryptozoite has reached its full growth it fills the entire cell. In this stage it is known as the cryptoschizont. It undergoes schizogony and the resulting cells known as crypto-merozoites are set free in the blood by the rupture of the liver cells. The released crypto-merozoites invade fresh liver cells or red blood corpuscles. This cycle is considered as a period of incubation before the para­sites could start the erythrocytic cycle. During this period of 7 - 17 days, the parasites are not seen in the blood stream.

Erythrocytic or Endo-erythrocytic cycle.

Each cryptomerozoite makes its way into a red blood corpuscle and feeds on its contents. After some time, the parasite gets an amoeboid shape. This growing stage is known as the trophozoite stage. Soon it develops a vacuole which gradually increases in size. Thus the nucleus is pushed to one side. This stage is called the signet ring stage. With further growth the vacu­ole disappears and the amoebula occupies the entire interior of the corpuscle. This stage is known as the schizont stage.

In the schizont, the nucleus breaks up into bits (6-24) and each be­comes surrounded by a small amount of cytoplasm. These cells are known as merozoites. By the rupture of the wall of the red blood corpuscles the merozoites along with wastes(haemozoin) are released into the blood. This causes the malarial fever. The liberated merozoites attack another set of corpuscles and start the life cycle anew. This method of infection is known as autoinfec­tion. The life cycle in the blood of man is called the cycle of Golgi or schizogony or endoerythrocytic cycle.

Schizogony keeps up the multiplication of the parasites and their main­tenance in the blood.

After schizogony has taken place for several generations some of the merozoites which invade the red corpuscles, instead of developing into tro­phozoites and schizogonts, develop into gametocytes. The gametocytes are of two types - marcogametocytes and micro-gametocytes. The macro- gametocyte has a small nucleus and a dense food laden cytoplasm. The microgametocyte has a relatively large nucleus and clear cytoplasm. Their fur­ther development depends on their entry into the stomach of a female anoph­eles. If it does not take place they disintegrate.

Life cycle in the mosquito - sporogony

When a female anopheles mosquito bites an infected person, it sucks blood along with all the stages of parasite. But in the gut of the mosquito, only the mature gametocytes survive and the rest of the stages are destroyed. From the gametocytes develop gametes. The process of development of ga­metes from gametocytes is known as gametogony.

Gametogony:

The nucleus of the micro-gametocyte divides into many fragments and the cytoplasm is thrown into flagellated structures. There may be as many cytoplasmic structures as there are nuclei. This process is known as exflag­ellation. The resultant cells are called the microgametes. The nucleus of the macrogametocycte divides equally into two. The cytoplasm divides un­equally. So among the resulting cells one is bigger and the other is smaller. The small cell is thrown out. This process is known as maturation. The re­sulting bigger cell is known as female gamete or macrogamete.

Syngamy and sporogony :

Inside the stomach of the mosquito the microgamete and the macro­gametes come into union and nuclear fusion takes place. This kind of union is called syngamy and the resultant form is known as zygote.

The zygote assumes an elongated form and is capable of movement. It is known as ookinete. It pierces the wall of the stomach and comes to lie under the outer layer of stomach wall. There, it ceases to move, becomes round and forms a membranous cyst-wall. This stationary zygote enclosed in a cyst-wall is known as oocyst. It grows in size absorbing the nourishment from the host.

The nucleus of the oocyst divides repeatedly, each being surrounded by a fragment of cytoplasm. Thus inside the oocyst, a large number of cells develop into minute, slender, sickle shaped bodies called sporozoites. The cyst wall breaks, liberating the sporozoites into the body cavity (haemocoel) of the host. They wriggle forward and enter the salivary gland. When such an infected female anopheles mosquito bites a healthy person, it injects into his blood a stream of sporozoites. This kind of transmission is called inoculation.

Types of Malaria:

The disease caused by Plasmodium is known as malarial fever. It is characterized by recurring bouts of fever, each lasting several hours. The febrile condition in man is due to toxins liberated into the blood along with the merozoites when the corpuscle is ruptured at the end of schizogony.

There are four species of Plasmodium known to cause malaria in man. The commonest and most widely distributed species is P. vivax. It causes benign tertian malaria in which the fever recurs every third day (every 48 hours). P.falciparum is largely limited to the tropics and subtropics and causes the malignant tertian or subtertian malaria. This type of malaria has a high death rate. Blood corpuscle parasitized by this species tend to clump together and block up small blood vessels and damage the essential organs. It is a dangerous species and the disease often appears in an epidemic scale. P. malariae causes quartan malaria with feverish fits every fourth day (every 72 hours). The fourth species is P. ovale. It is principally found in West Africa but occasionally in S. America, Russia and Palestine. It causes benign tertian malaria in which the fever recurs every third day (every 48 hours).

These four species differ from each other in the details of structure, time needed to complete the Schizogony, the incubation period, number of merozoites released and duration of sexual cycle.

Control of Malaria

The control measures fall under the following three categories.

Treatment of infected patient

          (1) Plasmodium does not produce antitoxins or antibodies in human blood. Therefore malaria cannot be treated by inoculation or vaccination with immune sera. It can only be treated with drugs that may kill all stages of the parasite without poisoning the patient. Quinine, which is extracted from the bark of cinchona trees, had been used effectively for the past 300 years to cure malaria. The various synthetic drugs, such as Paludrine, Atabrin, Camoquin, Chloroquine, Resochin, Pamaquin etc. are used as suppressants of various stages of the parasites.

          (2) Prevention of infection

It can be effected in two ways.

i) Using protective measures such as mosquito nets, anti-mosquito creams (repellants) and coils.

ii) Use of the prophylactic drugs; small daily dose of anti-malarial drugs will kill the parasite either in the sporozoite or merozoite stage.

          (3) Control of vector

It is perfectly clear that if the vector is completely exterminated the infection cannot be transmitted from one person to another. It is the most effective and surest way of controlling malaria. It is achieved by using effective insecticides and by draining swamps. It destroys the breeding places of mosquitoes.

Adult mosquito can be most effectively controlled by spraying DDT, malathion or any other insecticide in the houses; fumigating pyrethrum cresol and other compounds of naptha; sterilization of male mosquitoes. The young stages of mosquito can be controlled by introducing larvivorous fishes like Gambusia and Lebistes in ponds, lakes, canals and tanks.

References

T. Sargunam Stephen, Biology (Zoology). First Edition – 2005, © Government of Tamilnadu.

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