In this article we will discuss about the morphology and life-history of plasmodium vivax.
Morphology of Plasmodium Vivax:
Plasmodium vivax causes benign tertian malaria, so called, because the paioxysm of fever occurs every other day and the disease is rarely fatal. Plasmodium is an endoparasite which lives within the red blood corpuscles of man to whom it is transmitted by the bite of an infected female mosquito.
Thus the parasite has two hosts—man and Anopheles—and there is an alternation of the hosts in its life-cycle. Laveran discovered P. vivax in 1890 in the blood of man but its connection with the insect-host and its mode of transmission was worked out in Calcutta by Ross in 1899. Plasmodium is the most important sporozoon which infects man.
It causes wide-spread havoc in tropical and subtropical regions, especially West Bengal, Bangladesh, Behar and Assam. There are three species of malarial parasite—P. vivax, P. malariae and P. falciparum. Their life-cycles closely resemble one another but the symptoms which they produce vary.
Thus P. vivax is responsible for benign tertian malaria, the meaning of which has already been explained. P. malariae causes quartan malaria, so called, because the outbreak of fever sets in every fourth day. P. falciparum produces malignant malaria, in which the fever is irregular, and the disease proves fatal unless the victim is promptly treated by antimalarial drugs. In untreated cases the parasites choke up the capillaries of the brain.
Life-History of Plasmodium Vivax:
There are three main phases or cycles in the life-history of Plasmodium:
(1) An exo-erythrocytic phase of growth and asexual multiplication within the liver cells of man lasting for ten to fourteen days;
(2) A phase of growth and asexual reproduction by multiple fission (schizogony) in the red blood cells of man which is completed in about forty-eight hours;
(3) A sexual phase which starts in man and is continued within the stomach of the mosquito and is followed by asexual reproduction by multiple fission (sporogony)—the cycle being completed in about two weeks.
Life-Cycle in Man:
The parasite is transmitted to man by the female Anopheles. The male insect does not bite and therefore plays no part in transmission. Infection starts by the injection into the blood stream of the victim of a few spindle-shaped individuals along with the saliva of the infected female mosquito. The parasite, known at this stage as sporozoites, circulates in the blood stream for about half an hour.
At the end of this period they disappear from the blood and enter the liver cells. Here each sporozoite grows rapidly and then divides by multiple fission to produce about one thousand small individuals called cryptozoites.
The cryptozoites are released into the liver sinusoids by rupture of the liver cell. This phase of rapid growth and reproduction in the human liver has been discovered only recently and is known as the pre-erythrocytic or exo-erythrocytic phase.
It is completed in about ten to fourteen day during which the man usually does not suffer from fever. It is, therefore, known as the incubation period of the disease. The exo-erythrocytic phase may continue and the liver may thus be converted into a reservoir of parasites which can prolong the disease in a latent form for indefinite period.
From the liver sinusoids the parasites pass either into the general circulation or re-infect fresh liver cells. Those which pass in the general blood stream attack and penetrate into red blood corpuscles where each is converted into a trophozoite.
The young trophozoite is ring-like in shape. It grows at the expense of the erythrocyte, absorbing fluid nutrition by the surface of its body, and ingesting haemoglobin particles by throwing out pseudopodia like an Amoeba.
The ingested haemoglobin is converted into a brown pigment called melanin which is deposited in the cytoplasm of the trophozoite. In about 48 hours the trophozoite is full-grown. It is now called a schizont, which soon reproduces asexually by schizogony and divides into about twenty-four merozoites.
The infected red blood corpuscle now bursts, liberating the merozoites. Each escaping merozoite attacks a fresh red cell of the host and is again converted into a young trophozoite. Soon after it has settled down in the red cell, a vacuole appears in its centre, pushing the small nucleus to one side.
The parasite now looks like a signet ring. This ring-form is a resting stage of the parasite, because in its active state it is amoeboid and constantly changes shape by throwing out pseudopodia. Most of the merozoites become schizonts, which repeat the sexual cycle; a few, however, develops into male and female gametocytes which remain latent till an Anopheles sucks them up along with the blood of the host.
Unless this occurs within a reasonable time the gametocytes degenerate and perish. Formation of gametocytes marks the onset of the sexual phase in the life-history of the parasite.
A merozoite, which is destined to produce a gametocyte, rounds up and grows slowly within a red blood corpuscle. It is converted into a compact spherical body without a vacuole and is loaded with pigment granules. In P. falciparum the gametocytes appear to be crescentic in outline.
The male gametocyte is smaller than the female gametocyte.
Life-Cycle in Mosquito:
When the female Anopheles sucks blood from a malaria-victim, the gametocytes continue further development in her stomach while the schizonts are destroyed and digested. The male gametocyte is converted into what is called a flagellating body.
Its nucleus breaks up into a number of chromatin granules each of which collects a long whip-like mass of cytoplasm and projects out from the surface. The flagella-like structures are the actual microgametes, and they soon break away from the parent body and swim about actively in search of a macro- gamete. The female gametocyte matures into a macrogamete within the stomach of the mosquito.
A macrogamete is non-motile and spherical in shape. One of the microgametes now fuses with a mature macrogamete—the process is known as fertilisation and the product of fertilisation is a zygote. This is a rounded body containing two nuclei, the male and female, which mix up intimately. The zygote puts out a pseudopodium and is converted into the ookinete which starts penetrating the gut-wall of its host.
Ultimately the ookinete protrudes into the body cavity of the mosquito and encysts. The resulting oocyst gradually expands and the zygote reproduces asexually by sporogony. Its nucleus breaks up into a number of fragments by multiple fission of the cytoplasm becomes vacuolated. Eventually a large number of spindle-shaped sporozoites are formed within the expanding oocyst which now bursts and liberates its contents into the coelom.
The sporozoites find their way into the salivary glands and the mosquito is now ready to infect other human hosts. The cycle in the mosquito is completed in about two weeks. The sporozoites are injected into the wound along with saliva when the mosquito bites man. They are carried, through the blood stream to the liver where they start the pre-erythrocytic phase, and the entire life-cycle is repeated in the new host.