Question
Describe the life cycle of Plasmodium, the causative agent of malaria. Trace all stages in the human host and the mosquito vector.
Solution — Step by Step
Plasmodium requires two hosts to complete its life cycle:
- Human (intermediate/secondary host): Asexual reproduction (schizogony) occurs here
- Female Anopheles mosquito (definitive/primary host): Sexual reproduction (sporogony) occurs here
The parasite alternates between sexual and asexual phases, making it uniquely difficult to eliminate. Four species infect humans: P. vivax, P. falciparum, P. malariae, P. ovale.
When an infected female Anopheles bites a human:
- Sporozoites (elongated, infective stage) enter the bloodstream with mosquito saliva
- Sporozoites travel to liver cells (hepatocytes) within 30 minutes
- Inside hepatocytes, each sporozoite undergoes pre-erythrocytic schizogony (asexual division)
- Produces thousands of merozoites per liver cell
- Infected liver cells burst, releasing merozoites into blood
Note: In P. vivax and P. ovale, some sporozoites become dormant hypnozoites in the liver, causing relapses months/years later.
Merozoites invade red blood cells (RBCs):
- Merozoite enters RBC → becomes ring-form trophozoite (visible in blood smear)
- Trophozoite feeds on haemoglobin → grows into schizont
- Schizont divides → 8–24 new merozoites per RBC
- RBC bursts → merozoites released → infect new RBCs
This erythrocytic cycle repeats every 48 hours (P. vivax, P. falciparum) or 72 hours (P. malariae). The synchronised RBC bursting releases haemozoin (malarial pigment) and parasite products → triggers fever, chills, and headache in characteristic periodic cycles.
Incubation period: 10–14 days from mosquito bite to first symptoms.
After several erythrocytic cycles, some merozoites differentiate into:
- Macrogametocytes (female gametocytes): Large, rounded
- Microgametocytes (male gametocytes): Smaller, irregular shape
Gametocytes circulate in blood without dividing. They are taken up when an Anopheles mosquito feeds on an infected person.
Inside the mosquito’s stomach (midgut):
- Gametocytes emerge as gametes: microgamete (male) and macrogamete (female)
- Fertilisation → zygote (only diploid stage in the cycle)
- Zygote → ookinete (motile, elongated)
- Ookinete penetrates gut wall → becomes oocyst on outer gut wall
- Oocyst undergoes asexual division → thousands of sporozoites
- Oocyst bursts → sporozoites migrate to salivary glands
- Ready to infect next human when mosquito bites
Duration in mosquito: 10–14 days (extrinsic incubation period; temperature-dependent).
Diagram Summary
Mosquito bites human
↓
Sporozoites → Liver (Pre-erythrocytic schizogony)
↓
Merozoites → RBCs (Erythrocytic schizogony — fever)
↓
Some merozoites → Gametocytes (in blood)
↓
Mosquito bites infected human — takes up gametocytes
↓
Fertilisation in mosquito gut → Zygote → Ookinete → Oocyst
↓
Sporozoites → Mosquito salivary glands (cycle complete)Why This Works
The dual-host cycle gives Plasmodium major evolutionary advantages: rapid asexual multiplication in humans (one sporozoite → millions of merozoites), genetic recombination via sexual reproduction in the mosquito (generating diversity to evade immune responses), and efficient spread through the mosquito vector.
For NEET, three key facts tested repeatedly: (1) Female Anopheles transmits malaria (not male — males don’t bite); (2) sexual reproduction in mosquito (definitive host); (3) asexual reproduction in human (intermediate host). The word “definitive” refers to sexual reproduction, not the “more important” host.
Common Mistake
Students often say “the mosquito is the intermediate host because it carries the parasite.” Wrong — the mosquito is the definitive host because sexual reproduction occurs in it. The human is the intermediate host (asexual reproduction). Also: malaria is transmitted ONLY by female Anopheles mosquitoes. Male mosquitoes feed on plant nectar, not blood. Saying “Anopheles mosquito” without specifying female is technically incomplete in NEET answers.