Question

Explain life cycle of sporozoite

Answer 1

The life cycle of Plasmodium, the malarial parasite, involves two hosts: humans and the female Anopheles mosquito. The sporozoite is the infective stage for humans.

Here's the life cycle starting from the sporozoite:

1. Human Infection (Sporozoite Entry): An infected female Anopheles mosquito bites a human and injects sporozoites (the infective stage) into the bloodstream.

2. Liver Stage (Exo-erythrocytic Schizogony):

   • Sporozoites travel through the blood to the liver cells (hepatocytes).

   • Inside liver cells, they multiply asexually (schizogony) to form thousands of merozoites.

   • The infected liver cells rupture, releasing merozoites into the bloodstream. (In P. vivax and P. ovale, some sporozoites can remain dormant as hypnozoites in the liver, causing relapses).

3. Blood Stage (Erythrocytic Schizogony):

   • Merozoites invade Red Blood Cells (RBCs).

   • Inside RBCs, they grow into trophozoites, then multiply asexually (schizogony) to produce more merozoites.

   • The infected RBCs rupture, releasing merozoites and toxins (like hemozoin) into the blood, causing symptoms such as fever and chills.

   • The released merozoites infect new RBCs, continuing the asexual cycle in the blood.

4. Gametocyte Formation:

   • Some merozoites, instead of continuing the asexual cycle, develop into sexual stages called gametocytes (male microgametocytes and female macrogametocytes) within RBCs.

5. Mosquito Infection (Gametocyte Ingestion):

   • When an uninfected female Anopheles mosquito bites an infected human, it ingests blood containing gametocytes. Gametocytes are the only stage that can develop further in the mosquito.

6. Mosquito Stage (Sporogony):

   • In the mosquito's midgut, gametocytes mature into male and female gametes.

   • Fertilization occurs, forming a zygote.

   • The zygote develops into a motile ookinete, which penetrates the midgut wall and forms a cyst-like structure called an oocyst on the outer surface of the midgut.

   • Inside the oocyst, asexual multiplication (sporogony) occurs.

7. New Sporozoite Formation:

   • The mature oocyst ruptures, releasing thousands of new sporozoites.

   • These sporozoites migrate to the mosquito's salivary glands, making the mosquito infective. The cycle is now complete, and the mosquito is ready to transmit the infection to another human.

Answer 2

The explanation above details the life cycle of the sporozoite.