Model Answer
0 min readIntroduction
Malaria, a mosquito-borne infectious disease, remains a significant global health concern, particularly in tropical and subtropical regions. *Plasmodium vivax* is one of the most widespread malaria parasites affecting humans, responsible for a substantial proportion of malaria cases outside of Africa. Unlike *P. falciparum*, *P. vivax* is characterized by its ability to form dormant liver stages called hypnozoites, leading to relapses weeks or months after the initial infection. Understanding its complex life cycle is crucial for developing effective control and elimination strategies. This answer will provide a detailed, illustrated account of the *Plasmodium vivax* life cycle, encompassing both the mosquito and human phases.
The Life Cycle of *Plasmodium vivax*
The life cycle of *Plasmodium vivax* is complex, involving two hosts: the female *Anopheles* mosquito and the human. It can be broadly divided into two stages: the sexual reproduction stage within the mosquito and the asexual reproduction stage within the human.
I. Mosquito Stage (Sexual Reproduction)
This stage begins when a female *Anopheles* mosquito takes a blood meal from an infected human.
- Gametocyte Ingestion: The mosquito ingests male (microgametocytes) and female (macrogametocytes) sexual forms of the parasite present in the human blood.
- Gametogenesis: Within the mosquito’s midgut, the gametocytes undergo maturation. The microgametocyte undergoes exflagellation, producing 8 motile microgametes.
- Fertilization: A microgamete fertilizes a macrogamete, forming a zygote.
- Ookinete Formation: The zygote transforms into a motile, elongated ookinete, which penetrates the mosquito’s midgut wall.
- Oocyst Development: The ookinete develops into an oocyst on the outer surface of the midgut. Inside the oocyst, numerous sporozoites are formed through multiple rounds of asexual reproduction (sporogony).
- Sporozoite Release: The oocyst ruptures, releasing sporozoites which migrate to the mosquito’s salivary glands.
II. Human Stage (Asexual Reproduction)
This stage begins when an infected mosquito injects sporozoites into the human bloodstream during a blood meal.
- Liver Stage (Pre-erythrocytic Stage): Sporozoites travel to the liver and invade liver cells (hepatocytes).
- Schizogony: Inside the liver cells, sporozoites undergo asexual multiplication, forming schizonts containing numerous merozoites.
- Liver Cell Rupture & Merozoite Release: The schizonts rupture, releasing merozoites into the bloodstream. *P. vivax* is unique in forming hypnozoites – dormant forms that remain in the liver and can reactivate months or years later, causing relapses.
- Erythrocytic Stage (Blood Stage): Merozoites invade red blood cells (erythrocytes).
- Trophozoite Formation: Inside the red blood cells, merozoites develop into trophozoites, feeding on hemoglobin.
- Schizogony (in RBCs): Trophozoites undergo further asexual multiplication, forming schizonts containing more merozoites.
- RBC Rupture & Merozoite Release: The schizonts rupture, releasing merozoites to infect more red blood cells, leading to cyclical fever and other malaria symptoms.
- Gametocyte Formation: Some merozoites differentiate into gametocytes (male and female) within the red blood cells, completing the cycle and making the infected individual infectious to mosquitoes.
The erythrocytic cycle of *P. vivax* takes approximately 48 hours, resulting in a paroxysmal fever every other day (tertian fever). *P. vivax* typically infects reticulocytes (young red blood cells), leading to a lower parasite density compared to *P. falciparum*.
| Stage | Location | Key Features |
|---|---|---|
| Sporozoite | Mosquito Salivary Glands | Infective stage for humans |
| Oocyst | Mosquito Midgut | Site of sporozoite development |
| Merozoite | Human Bloodstream/RBCs | Infective stage for RBCs |
| Hypnozoite | Human Liver | Dormant stage causing relapses (unique to *P. vivax*) |
| Gametocyte | Human RBCs | Sexual stage ingested by mosquito |
Conclusion
The life cycle of *Plasmodium vivax* is a complex interplay between the parasite, the mosquito vector, and the human host. The unique ability of *P. vivax* to form hypnozoites in the liver presents a significant challenge for malaria eradication, as these dormant forms can reactivate and cause relapses. Effective control strategies must therefore target both the mosquito vector and the human host, including the use of primaquine to eliminate hypnozoites and prevent relapses. Continued research into the parasite’s biology and the development of new interventions are crucial for achieving a malaria-free world.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.