Out of the various diseases which are caused by mosquito, the malaria is the most common and well known disease in our country. This disease is caused by a protozoan endoparasite known as Plasmodium. The genus Plasmodium includes all the blood parasitic protozoa causing a febrile disease known as malaria. P. vivax causes a special type of malaria fever known as Benign tertian malaria.
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 (Anopheles sp.).
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 Ronald Ross in 1898. Plasmodium is the most important sporozoan which infects man.
The life cycle of P. vivax is completed in two different hosts—definitive and intermediate (secondary) hosts. Definitive hosts are those hosts which provide the sexual stage of the parasite and the intermediate hosts are those hosts which provide the asexual stage as well as the developmental stage of the parasite.
As human host gives them (malarial parasite) shelter, food and as asexual multiplication occurs in this host so human host is regarded as intermediate host and as sexual reproduction of malarial parasite occurs within the female Anopheles mosquito, so it is regarded as definitive host.
Plasmodium is digenetic, as the life cycle is completed in two different hosts—the name of the cycles is given according to the host as human cycle and mosquito cycle.
A. Human Cycle (Asexual Phase):
The human cycle of malarial parasite (P. vivax) begins with the liberation of sporozoites in the peripheral blood of man by female Anopheles mosquito during the act of bitting.
Plasmodium completes human cycle through three main stages of development, such as:
(i) A pre-erythrocytic schizogony of growth and asexual multiplication within the hepatic cells (liver cells) of man lasting for ten to fourteen days;
(ii) An erythrocytic schizogony or 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 and
(iii) Gametogony—after the parasites have undergone erythrocytic schizogony for a certain period, some of the merozoites, instead of developing into trophozoites and schizonts, give rise to forms which are capable of sexual functions after leaving the human host.
They are known as gametocytes and develop in the red blood cells of the capillaries of internal organs like spleen and bone marrow. Only the mature gametocytes are seen in the peripheral blood and the maturation is completed in about 4 days. In other-words development of gamete from gametocyte is known as gametogony.
(a) Pre-Erythrocytic Schizogony:
The parasite is transmitted to the man by the female Anopheles. The male insect does not bite therefore plays no part in transmission of the parasite. 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 parasites, known at this stage as sporozoites, circulate in the blood stream for about half an hour.
At the end of this period they disappear from the blood and enter the hepatic cells of the liver. Here each sporozoite grows rapidly and then divides by multiple fission to produce about 12,000 small individuals called cryptozoites. The cryptozoites are released into the liver sinusoids by the rupture of the hepatic cells.
This phase of rapid growth and reproduction in the human liver has been discovered very recently and is known as the pre-erythrocytic schizogony or pre-erythrocytic phase of growth. It is completed in about 10-14 days during which the man usually does not suffer from fever. It is, therefore, known as the incubation period of the disease. This 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.
Exo-erythrocytic phase or schizogony:
Some of the cryptozoites instead of attacking RBC may reach the fresh hepatic cells via the liver sinusoids and by schizogony give rise to metacryptozoites or phenerozoites. This phase is known as exo-erythrocytic schizogony.
(b) Erythrocytic Schizogony or Phase of Growth:
From the liver sinusoids the cryptozoites or metacryptozoites pass either into the general blood circulation or re-infect fresh hepatic cells. Those which pass into the general blood stream attack and penetrate into RBC where each is converted into a trophozoite. The young trophozoite is ring like in shape (Fig. 5.3).
It grows at the expense of the erythrocyte and ingests haemoglobin particles by throwing out pseudopodia like an Amoeba. In about 48 hrs. the trophozoite in full-grown. It is now called a schizont, which soon reproduces asexually by schizogony and divides into about 24 merozoites (Fig. 5.5). The infected RBC now bursts, liberating the merozoites.
Each escaping merozoite attacks a fresh red blood cell of the host and is again converted into a young trophozoite. Soon after it has settled down in the RBC, 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 like an Amoeba.
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 gametocyte 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 RBC. It is converted into a compact spherical body without a vacuole and is loaded with pigment granules. The male gametocyte is smaller than the female gametocyte. The differences between a microgametocyte (male gametocyte) and a macro-gametocyte (female gametocyte) are given in Table 5.1.
Time taken to form mature gametocyte is 4 to 5 days. Gametocytes, not taken up by the insect host, do not live for more than a week in the human blood (Fig. 5.7).
B. Mosquito Cycle or Sexual Cycle of P. vivax:
When the female Anopheles sucks blood from a malaria-victim, the gametocyte continue further development in her stomach while the schizonts are destroyed and digested. The gametocytes however survive. These are liberated and become lodged in the cavity of the gut and they become ready for further development.
(a) Development of Microgametes:
After reaching the stomach of female Anopheles mosquito microgametocytes show maximum activities. 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 macrogamete.
(b) Development of Macrogametes:
The female gametocyte matures into a macrogamete within the stomach of the mosquito. A macrogamete is non-motile and spherical in shape. This larger cell persists as un-fertilised egg. The nucleus now migrates towards the periphery and a cytoplasmic swelling occurs at one side.
The microgamete now approaches towards the egg. One of the microgametes now fuses with a mature macrogamete— the process is known as fertilization and the product of fertilization is a zygote. The zygote is the only diploid condition of P. vivax.
(d) Further Development of the Zygote to form Ookinete and Oocyst:
The zygote remains rounded and motionless for some-time, but soon it puts out a pseudopodium and is converted into ookinete. It is elongated, vermiform and motile. It measures 15-22 μm in length and 3 μm in width. At the centre of the body there is an irregular shaped nucleus. It consists of dense cytoplasm with pigment granules, many mitochondria and ribosomes. Now it starts penetrating the gut-wall of the host.
Ultimately the ookinete protrudes into the body cavity of the mosquito and here it becomes spherical and begins to encyst. Now the ookinete is known as oocyst. It gradually expands and its diameter becomes 6.5 μm to 60 μm. The resulting oocyst reproduces asexually by sporogony.
(e) Sporogony and Migration of Sporozoites:
During sporogony the nucleus of the oocyst breaks up into a number of fragments by multiple fission, the first division being meiotic. Eventually a large number of spindle shaped sporozoites are formed within the expending 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 into the liver where they start the pre-erythrocytic schizogony and the entire life cycle is repeated in the new host.