In this article we will discuss about Plasmodium Vivax:- 1. Habit, Habitat and Distribution of Plasmodium Vivax 2. Hosts 3. Structure 4. Life Cycle.
Habit, Habitat and Distribution of Plasmodium Vivax:
Plasmodium vivax lives as an intracellular parasite in the red blood corpuscles (R.B.Cs) of man in the form of its mature adult condition, called trophozoite. The species of Plasmodium are reported from reptiles, birds and various mammals.
However, Plasmodium is widely distributed in tropical and temperate countries of the world but they are no longer a problem in the colder countries of the world. Countries like India, Sri Lanka, Bangladesh, Nepal, Pakistan, etc., are worst affected. In our country, states like Bihar and Uttar Pradesh suffer a greater setback by the infection of this parasite. In fact, the infection of Plasmodium is a global problem.
Hosts of of Plasmodium Vivax:
Plasmodium vivax has two hosts; man and female Anopheles mosquito. Man is considered to be the primary host and female Anopheles mosquito, the secondary or intermediate host. The common species of Anopheles, which transmit malaria parasite in India, are A. maculatus, A. stephensi, A. fluvialitis and A. culicifacies.
Structure of Plasmodium Vivax:
As referred to, the parasite, in its mature adult condition, is called trophozoite. The trophozoite is amoeboid, uninucleated having vacuolated and granular cytoplasm.
An ultra structure of the trophozoite is described below:
Ultra structure of trophozoite:
The ultra structure of Plasmodium Vivax (Fig. 19.1) has been revealed by the electron microscope. According to electron microscopic studies, the Plasmodium in a red blood corpuscle possesses a double membrane, the plasma lemma closely applied to the cytoplasm. The cytoplasm of Plasmodium Vivax contains small dense particles probably containing ribonucleoproteins.
The endoplasmic reticulum is not well developed and appears as vesicles of variable shapes. The vesicles are either smooth surfaced or rough surfaced and are loosely scattered in the cytoplasm. The mitochondria possess double membrane and show peripheral cristae and a structure less central region.
The number of mitochondria varies with the age, the merozoite has only one mitochondrion, while the trophozoite has several mitochondria. The Golgi apparatus is composed of small vesicles arranged in rows. A double layered concentric body is also found in the cytoplasm attached with the plasma lemma of Plasmodium Vivax.
It appears that the concentric body originates from plasma lemma. Rudzinska (1965) suspect that the concentric bodies serve the function of mitochondria. One or two double membrane vacuoles with structure less matrix, also occur in the cytoplasm.
The function of these vacuoles is unknown. The nucleus is large and its nucleoplasm is composed of granular and fine fibrillar material. The nuclear membrane is double, to which RNA particles are attached. The nucleolus lies centrally in the nucleus. Pinocytosis vacuoles are common in the cytoplasm and serve as food vacuoles. The food vaculoes may also contain hemozoin depending upon the species of Plasmodium.
The mode of nutrition is saprozoic, occurs by osmotrophy. Organ of locomotion, contractile vacuole, etc., are not found. Respiration takes place anaerobically. Reproduction occurs both by sexual and asexual methods.
Life Cycle and Plasmodium Vivax:
The life cycle of Plasmodium vivax is digenetic involving two hosts as mentioned earlier. Its life cycle is completed both by asexual and sexual phases.
Asexual phase of its life cycle is completed in man by schizogony (differentiated into exoerythrocytic schizogony involving pre- and post-erythrocytic schizogonic cycles, and erythrocytic schizogony) and sexual phase of its life cycle is completed in female Anopheles mosquito by gametogony, syngamy and sporogony.
(a) Part of Life-Cycle of P. Vivax in Man (Asexual Cycle):
It is completed in the following way:
Inoculation:
When an infected female Anopheles bites a man to suck his blood, then along with its saliva it injects the sporozoite stage of Plasmodium into the human blood. The parasite remains always in the body of one of the two hosts, hence, the sporozoites do not possess any protective covering.
The sporozoite, infective stage, is minute measuring about 11 to 12 microns in length and 0.5 to 1 micron in width, sickle-shaped cell with an oval nucleus; mosquito inoculates sporozoites in thousands.
The sporozoites are capable of slight gliding movement. In about half an hour the sporozoites disappear from the blood stream, and they enter the parenchymatous cells of the liver where they undergo at least two schizogonic cycles.
Ultra Structure of Sporozoite:
The sickle-shaped body of the sporozoite is covered externally by an elastic, firm pellicle having longitudinally arranged contractile microtubules. These microtubules help in the gliding movements shown by the sporozoite. Its anterior end bears an apical cup being made of three or more concentric rings.
A pair of elongated reservoir like secretory organelles, comparable to roptries of the sporozoite of Monocystis, open into the apical cup.
These organelles are supposed to secrete some secretion which facilitates its penetration into the liver cells. Nucleus is single and vesicular having a nucleolus in its centre. There is a single mitochondrion and a large number of convoluted tubules of unknown function. However, the micropyle represents the cytostome of other protozoans.
Schizogony in Liver Cells:
In the liver cells, the sporozoite grows to form a large, round schizont. The schizont divides by multiple fission to form about one thousand to several thousand small spindle-shaped cells called merozoites; this multiple fission is called schizogony. The schizont ruptures and merozoites are liberated into the sinusoids or venous passages of the liver.
This phase of asexual multiplication is pre-erythrocytic schizogony and the merozoites produced by it are also called cryptozoites or cyptomerozoites; these cryptozoites are immune to medicines and the resistance of the host.
A second phase of asexual multiplication known as an exo-erythrocytic schizogony occurs in the liver cells in which the cryptozoites enter into new liver cells and grow into schizonts, the schizont divides to form merozoites; the merozoites of the second generation are termed metacryptozoites or phanerozoites.
The exo-erythrocytic schizogony may continue in more liver cells to form a reservoir of merozoites, or some merozoites after at least two cycles of schizogony may re-enter the blood stream when they invade erythrocytes.
It is supposed that the merozoites of second generation, i.e., metacryptozoites are of two types; the more numerous and smaller are micro-metacryptozoites, while larger and less in number are macro-metacryptozoites.
In fact, the micro-metacryptozoites invade the R.B.Cs and start erythrocytic schizogony, while the macro-metacryptozoites enter fresh liver cells to continue the exo-erythrocytic schizogony. The merozoites attack only the young and immature corpuscles, (the merozoites of P. malariae attack only old corpuscles, while those of P. falciparum attack all kinds of corpuscles indiscriminately).
Pre-patent and Incubation Periods:
The pre-patent period is the duration between the initial sporozoite infection and the first appearance of parasite in the blood. In case of P. vivax, it is about 8 days on an average. The incubation period is the time taken from the infection of man by sporozoites till the appearance of first malarial symptom.
In case of P. vivax, it is about 14 days on an average ranging from 10 to 17 days. Of course, during the incubation period the host shows no symptoms of malaria.
Schizogony in Erythrocytes:
In the erythrocytes, a third multiplication phase of schizogony occurs which is known as erythrocytic schizogony. The micro-metacryptozoite feeds on erythrocytes, a vacuole appears in it, the nucleus is pushed to one side, and the micro-metacryptozoite is changed into what is called as the ring-shaped trophozoite, the signet ring stage, which is 1/3 to 1/2 the size of the erythrocyte.
The signet ring stage is not found in P. falciparum. The trophozoite grows to become rounded and amoeboid, this is the full grown trophozoite and is known as a schizont. The large schizont makes the erythrocyte to become very large. The schizont shows yellowish-brown pigment granules of haemozoin derived from the iron of haemoglobin of erythrocyte; the enlarged erythrocyte acquires granules called Schuffner’s dots.
The schizont now undergoes multiple fission to form 12 to 24 oval-shaped merozoites; this phase of asexual multiplication is erythrocytic schizogony. The much weakened erythrocyte bursts and the merozoites are liberated into the plasma from where they enter new erythrocytes, then they repeat the erythrocytic schizogony once every 48 hours.
However, the merozoites may again go from the blood to the liver cells and invade them to undergo another phase of asexual multiplication which is called post-erythrocytic schizogony.
Formation of Gametocytes:
After many generations of schizogony in the blood, some of the merozoites slowly grow large producing much haemozoin, these are inside erythrocytes and do not change in schizonts but they grow and are transformed into two types of gametocytes called macro gametocytes and microgametocytes.
The condition which brings about the formation of gametocytes is not known. Gametocytes appear in the peripheral blood at various intervals after the onset of fever, they remain inactive while in the human blood. The macro gametocytes are female, they are round with the food laden cytoplasm and a small eccentric nucleus.
The microgametocytes are male, they have a clear cytoplasm and a large central nucleus. Both gametocytes contain large amounts of haemozoin; they enlarge the erythrocytes. Gametocytes remain in the human blood for several weeks, but are unable to develop any further, it is necessary for them to be taken into the body of an Anopheles’, if this does not happen they degenerate and die.
(b) Part of Life-Cycle of P. Vivax in Mosquito (Sexual Cycle):
Many species of Anopheles, but not all species, act as intermediate hosts. If the gametocytes are sucked up along with human blood by a female Anopheles then they reach the stomach where corpuscles are dissolved and the gametocytes are set free.
Gametogony:
The microgametocytes, after release in the stomach of mosquito, undergo the process of ex-flagellation. The cold-bloodedness of the mosquito is said to stimulate this process. However, the nucleus of microgametocytes divides into 6-8 haploid daughter nuclei.
These nuclei migrate towards the periphery of microgametocyte. The cytoplasm pushes out forming long flagellum like structures having one daughter nuclei in each. Thus, 6-8 flagellum like male gametes or microgametes measuring from 20-25 microns in length are formed. Soon these gametes separate and start moving actively in the stomach of mosquito.
On the other hand, the macro gametocytes undergo maturation process, thereby two polar bodies are pushed out and a female gamete or macrogamete is formed. The female gamete is non-motile and develops a cytoplasmic or receptive cone.
Fertilisation:
If microgamete happens to reach the macrogamete, then it enters into the female gamete at the point of cytoplasmic cone and finally complete fusion of nucleus and cytoplasm of the two gametes occurs. This results in the formation of rounded zygote.
Several microgametes may approach a macrogamete but only one of them enters the macrogamete and others shed off. The fusion of male and female gametes is called syngamy. Here, the gametes are dissimilar (anisogametes), hence, their fusion is called anisogamy.
Ookinete and Encystment:
The zygotes, thus, formed remain rounded and motionless for 24 hours but soon they elongate to become worm-like having pointed ends and motile. The zygotes are now called ookinetes or vermicules. An ookinete measures about 15 to 22 microns in length and 3 microns in width.
The ookinete moves and bores through the wall of the stomach of mosquito and comes to lie beneath the outer epithelial layer. (The ultrastructure of ookinete shows the presence of a central, irregular nucleus, dense cytoplasm, brown pigment granules, many mitochondria and ribosomes in it. It also shows the presence of contractile fibrils, the microtubules).
However, here they become spherical and secrete a thin elastic membranous cyst. The cyst is also partly secreted by the surrounding tissues of the stomach. Thus, the ookinetes become encysted and in this condition it is referred to as the oocyst. The oocyst grows in size and sometimes called sporont.
As many as 50 such oocysts can be seen on the stomach of the host mosquito. Howard (1906) has observed that the ookinetes which do not succeed in boring the stomach wall pass out from mosquito’s body with faecal matter.
Sporogony:
The nucleus of oocyst first divides by meiosis and then by mitosis several times (Bano, 1959) and its cytoplasm develops vacuoles forming faintly-outlined cells called sporoblasts. Particles of chromatin arrange themselves around the periphery of each sporoblast. Then the cytoplasm forms slender spindle-shaped haploid cells known as sporozoites.
Each oocyst may have ten thousand sporozoites, and group of sporozoites gets arranged around the vacuoles. This phase of asexual multiplication in which sporozoites are formed is called sporogony which is completed in 10-20 days from the time the gametocytes are taken in by the mosquito, the time depending on the temperature.
The oocyst bursts and sporozoites are liberated into the haemolymph of the mosquito, from where they reach its salivary glands and enter the duct of the hypopharynx. The sporozoites will infect a human host when the mosquito bites and the life cycle is repeated again.