Plasmodium vivax, a prevalent protozoal parasite and human pathogen, is responsible for causing malaria in humans. While P. vivax is generally less virulent in comparison to Plasmodium falciparum, it can still lead to severe illness and even fatalities due to the enlargement of the spleen. These parasites are transmitted via female Anopheles mosquitoes.
Table of Contents
- Classification of Plasmodium vivax
- Host of Plasmodium vivax
- Habitat of Plasmodium vivax
- Plasmodium vivax Genome
- Plasmodium vivax Characteristics
- Plasmodium vivax Structure
- Life cycle of Plasmodium vivax
- (i) Life-Cycle of P. vivax in the Human body (Asexual Cycle)
- (ii) Life-Cycle of P. vivax in Mosquito (Sexual Cycle)
Classification of Plasmodium vivax
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Domain: |
Eukaryota |
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(unranked): |
Diaphoretickes |
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Clade: |
TSAR |
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Clade: |
SAR |
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Infrakingdom: |
Alveolata |
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Phylum: |
Apicomplexa |
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Class: |
Aconoidasida |
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Order: |
Haemospororida |
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Family: |
Plasmodiidae |
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Genus: |
Plasmodium |
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Species: |
P. vivax |
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Domain: |
Eukaryota |
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Host of Plasmodium vivax
- Plasmodium vivax requires two distinct hosts to complete its life cycle: the human body serves as the primary host, while the female Anopheles mosquito plays the role of the secondary or intermediate host.
- Various mosquito species, including maculatus, stephensi, fluvialitis, and culicifacies, are involved in the transmission of malaria, facilitating the spread of the disease.
Habitat of Plasmodium vivax
- Plasmodium Vivax is most prevalent in Asia, Latin America, and certain regions of Africa.
- A recent research investigation revealed its primary origin in wild chimpanzees and gorillas in central Africa.
- This protozoal parasite accounts for approximately 65% of malaria instances in Asia and South America.
- Plasmodium vivax is capable of undergoing sporogonic development at lower temperatures, similar to Plasmodium falciparum.
- Approximately 71 mosquito species are known to transmit Plasmodium vivax.
Plasmodium vivax Genome
- These organisms possess a total of 12-14 linear chromosomes, each containing 12-14 linear chromosomes themselves.
- The genome size is estimated to fall within the range of 35-40 megabases (Mb).
- GC-rich isochores are associated with protein-coding genes, whereas AT-rich isochores found at the telomeres pertain to chromosome ends.
- The estimated number of genes is approximately 5000.
Plasmodium vivax Characteristics
- Anaerobic respiration is their metabolic process.
- They engage in both sexual and asexual reproduction.
- Their nutritional method is saprozoic, accomplished through osmotrophy.
- Organs related to locomotion, like the contractile vacuole, are not present.
- They rely on a host as an essential component of their life cycle.
Plasmodium vivax Structure
The electron microscope has been utilized to observe the detailed structure of Plasmodium vivax.
In a thin-film blood smear photomicrograph stained with Giemsa, a mature Plasmodium vivax trophozoite was observed, displaying numerous Schüffner's dots.
- Within the red blood cell, Plasmodium vivax is encased by a double membrane, with the plasmalemma closely adjacent to the cytoplasm.
- The cytoplasm houses ribonucleoproteins with small dense particles.
- Plasmodium vivax's endoplasmic reticulum appears as vesicles of varying shapes and is not highly developed. These vesicles can be either smooth or rough and are scattered throughout the cytoplasm.
- The double-membraned mitochondria feature peripheral cristae and a central region devoid of distinct structure.
- The quantity of mitochondria in Plasmodium vivax varies with the life stage. For instance, merozoites possess one mitochondrion, while trophozoites contain several.
- The Golgi apparatus is composed of numerous small vesicles arranged in rows.
- The plasmalemma is linked to a double-layered concentric body, which appears to originate from the plasmalemma.
- In 1965, Rudzinska hypothesized that the concentric bodies perform the essential functions of mitochondria.
- The cytoplasm also contains 1-2 double-membrane vacuoles with a structureless matrix. The functions of these double-membrane vacuoles remain unknown.
- Plasmodium vivax has a relatively large nucleus, with granular and fine fibrillar material making up the nucleoplasm.
- The nucleus features a double membrane to which RNA particles are attached.
- A centrally located nucleolus is found within the nucleus.
- The cytoplasm contains Pinocytosis vacuoles, which serve as food vacuoles.
- Occasionally, these food vacuoles may contain hemozoin, the presence of which depends on the Plasmodium species.
Life cycle of Plasmodium vivax
As previously mentioned, Plasmodium vivax undergoes its life cycle within two distinct hosts. This life cycle incorporates both asexual and sexual processes. The asexual phase takes place within the human host, while the sexual phase occurs within the female Anopheles mosquito, involving gametogony, syngamy, and sporogony.
(i) Life-Cycle of P. vivax in the Human body (Asexual Cycle)
Within the human body, the life cycle of P. vivax progresses through the following stages:
(a) Inoculation
- When an infected female Anopheles mosquito feeds on blood, it delivers Plasmodium sporozoites, the parasites, along with its saliva, into the human bloodstream.
- These sporozoites exhibit dimensions of 11 to 12 microns in length and 0.5 to 1 micron in width, taking on a sickle-shaped appearance with an oval nucleus.
- They are capable of gliding, enabling their movement within the body.
- Subsequently, these sporozoites infiltrate the liver's parenchymatous cells, departing from the bloodstream. Within the liver, they undergo a minimum of two schizogonic cycles.
(b) Schizogony in Liver Cells
- Within the liver cell, the sporozoite matures into a large, spherical schizont.
- Subsequently, the schizont undergoes multiple divisions, giving rise to thousands of small, spindle-shaped cells known as merozoites. This process is referred to as schizogony.
- Afterward, the merozoites are released from the ruptured schizont into the liver's sinusoids or venous passages.
- These merozoites, resulting from this asexual multiplication, are termed cryptozoites or cryptomerozoites. This phase is known as pre-erythrocytic schizogony, and these cryptozoites exhibit resistance to medications and the host's immune response.
- Another round of asexual multiplication occurs in liver cells, known as exo-erythrocytic schizogony, where the cryptozoites develop into schizonts upon entering new liver cells.
- The second-generation merozoites produced are referred to as metacryptozoites or phanerozoites.
- These merozoites subsequently re-enter the bloodstream and infect other red blood cells.
- It is believed that there are two types of metacryptozoites, namely micro-metacryptozoites and macro-metacryptozoites. Micro-metacryptozoites infect red blood cells and continue the erythrocytic schizogony, while macro-metacryptozoites enter fresh liver cells to continue the exo-erythrocytic schizogony.
(c) Pre-patent and Incubation Periods
- The pre-patent period indicates the duration from the initial sporozoite infection to the first detection of parasites in the blood. For P. vivax, this period typically averages around 8 days.
- The incubation period signifies the interval between human infection by sporozoites and the onset of the first malaria symptoms. In the case of P. vivax, this incubation period typically ranges from 10 to 17 days.
(d) Schizogony in Erythrocytes
- A third phase of schizogony, called erythrocytic schizogony, occurs within the red blood cells.
- When the micro-metacryptozoite feeds on erythrocytes, it transforms into a ring-shaped trophozoite, forming a vacuole within itself, and relocating the nucleus to one side. This stage is referred to as the signet ring stage.
- The size of the ring-shaped trophozoite is approximately one-third to one-half of the erythrocyte.
- These trophozoites continue to grow and eventually develop into rounded, amoeboid cells known as schizonts.
- The schizonts then undergo schizogony, a process through which they give rise to 12 to 24 oval-shaped merozoites.
- Subsequently, the infected erythrocyte ruptures, releasing the merozoites into the bloodstream, where they enter new erythrocytes and repeat this cycle approximately every 48 hours.
(e) Formation of Gametocytes
- Merozoites undergo further development, giving rise to two distinct types of gametocytes: macrogametocytes and microgametocytes.
- The exact trigger for gametocyte formation remains unknown.
- Female macrogametocytes are characterized by cytoplasm rich in nutrients and a small, eccentric nucleus.
- In contrast, male microgametocytes have clear cytoplasm and a large, centrally located nucleus.
- Both microgametocytes and macrogametocytes contain hemozoin and cause an enlargement of erythrocytes.
- These gametocytes enter the body of an Anopheles mosquito when it bites a human.
(ii) Life-Cycle of P. vivax in Mosquito (Sexual Cycle)
The life cycle of P. Vivax within the Mosquito is accomplished by the following stages;
(a) Gametogony
- Upon entering the stomach of a female Anopheles mosquito, microgametocytes undergo a process known as ex-flagellation.
- This process yields 6-8 haploid daughter nuclei derived from the microgametocyte's nucleus.
- Subsequently, these nuclei migrate towards the periphery of the microgametocyte, while the cytoplasm extends outward, forming flagellum-like structures, each containing one daughter nucleus. These structures, termed microgametes, measure 20-25 microns in length, resulting in the formation of 6-8 microgametes.
- These microgametes then move towards the mosquito's stomach.
- On the other hand, macrogametocytes mature and develop into female gametes or macrogametes, which are non-motile and form a cytoplasmic receptive cone.
(b) Fertilisation
- The nucleus and cytoplasm of both gametes merge, forming a rounded zygote. The merging of male and female gametes is referred to as syngamy.
- While several microgametes approach the macrogamete, only one microgamete successfully accomplishes fertilization.
- When two gametes with differing characteristics fuse (anisogametes), this process is termed anisogamy.
(c) Ookinete and Encystment
- Approximately 24 hours later, the previously immobile and spherical zygotes undergo elongation, transforming into a worm-like structure with pointed ends and gaining motility. This stage is now referred to as ookinetes or vermicules.
- An ookinete measures 15 to 22 microns in length and 3 microns in width.
- Subsequently, the ookinete attaches itself to and penetrates the stomach wall of the mosquito, positioning itself beneath the outer epithelial layer.
- Following this, the ookinete forms a thin, elastic, membranous cyst, becoming encysted. In this encysted state, it is known as an oocyst.
- The oocyst proceeds to grow and develop into a sporont.
(d) Sporogony
- The oocyst develops slender spindle-shaped haploid cells called sporozoites, with the division of the nucleus through the process of meiosis and mitosis.
- A single oocyst can develop ten thousand sporozoites through the process of sporogony.
- After that, the oocyst lysed the sporozoites are released into the hemolymph of the mosquito. After that, they move towards the salivary glands and enter the duct of the hypopharynx.
- When the infected mosquito bites a human it will inject the sporozoites within the bloodstream and the life cycle will repeat again.
