Question
Describe the life cycle of Plasmodium (malaria parasite), explaining the stages in the human host and the mosquito host. Which stage is responsible for the recurring fever pattern?
(NCERT Class 12, high-frequency NEET question)
Solution — Step by Step
The female Anopheles mosquito injects sporozoites (the infective form) into the human bloodstream during a blood meal. The sporozoites travel to the liver within minutes.
Only the female Anopheles mosquito transmits malaria — she needs a blood meal for egg development. Males feed on nectar.
In the liver, sporozoites invade hepatocytes (liver cells) and undergo asexual reproduction (schizogony). Each sporozoite multiplies to produce thousands of merozoites inside the liver cell.
The liver cell ruptures, releasing merozoites into the bloodstream. This stage is asymptomatic — the patient does not feel sick yet.
Merozoites invade red blood cells (RBCs) and go through a cycle:
- Ring stage (trophozoite): Merozoite enters the RBC, feeds on haemoglobin.
- Schizont stage: The parasite divides asexually within the RBC, producing 8-24 new merozoites.
- RBC rupture: The infected RBC bursts, releasing merozoites and toxic waste (haemozoin) into the blood.
The released merozoites infect new RBCs, and the cycle repeats every 48 hours (for P. vivax and P. falciparum) or 72 hours (for P. malariae).
The synchronous rupture of RBCs causes the characteristic recurring fever — chills and high fever coincide with merozoite release and haemozoin toxicity.
Some merozoites develop into gametocytes (male and female sexual forms) inside RBCs instead of continuing asexual reproduction.
When a female Anopheles mosquito bites an infected person, she ingests these gametocytes. Inside the mosquito’s gut:
- Gametocytes mature into gametes (microgamete and macrogamete).
- Fertilization occurs, forming a zygote.
- The zygote becomes a motile ookinete, which penetrates the gut wall and forms an oocyst.
- The oocyst divides to produce thousands of sporozoites.
- Sporozoites migrate to the mosquito’s salivary glands, ready to infect the next human.
The mosquito is the definitive host (sexual reproduction occurs here), and the human is the intermediate host (asexual reproduction occurs here).
Why This Works
Plasmodium has evolved a complex two-host life cycle that maximises its transmission. The liver stage allows massive multiplication before the immune system detects the infection. The blood stage causes disease but also produces gametocytes needed for transmission. The mosquito vector provides the link between human hosts.
The recurring fever pattern is directly tied to the synchronised 48/72-hour erythrocytic cycle — each burst of RBC lysis triggers an immune response that causes fever and chills.
NEET commonly tests: “Which is the infective stage to humans?” Answer: sporozoites (injected by mosquito). “Which host is definitive?” Answer: mosquito (sexual reproduction). “Why does fever recur every 48 hours?” Answer: synchronous rupture of RBCs releasing merozoites and haemozoin.
Common Mistake
The most frequent error: calling the human the definitive host. By biological convention, the definitive host is where sexual reproduction occurs — that is the mosquito. The human is the intermediate host (asexual reproduction). This is counter-intuitive because humans suffer the disease, but the terminology is based on the parasite’s reproductive cycle, not on who gets sick.
Another mistake: confusing sporozoites and merozoites. Sporozoites are injected by the mosquito and invade the liver. Merozoites are released from the liver (and later from RBCs) and invade red blood cells. They are different stages of the same parasite.