CBSE · Class 12 · Biology · Human Health and DiseasesDescribe the complete life cycle of the malarial parasite Plasmodium with neat points. Explain the stages that occur in both the human body and the female Anopheles mosquito.

Introduction

Plasmodium is a digenetic endoparasite that causes malaria in humans. It requires two hosts to complete its life cycle: a primary host (human) and a secondary/vector host (female Anopheles mosquito).

A. Stages in the Human Body (Asexual Cycle / Schizogony)

1. Infection:

   • When an infected female Anopheles mosquito bites a human, it injects the infectious stage of the parasite called sporozoites along with its saliva into the bloodstream.

2. Hepatic Phase (Liver Stage):

   • The sporozoites reach the liver cells (hepatocytes) through blood circulation within 30 minutes.
   • Inside liver cells, the parasite reproduces asexually (schizogony) and eventually ruptures the liver cells, releasing merozoites into the blood.

3. Erythrocytic Phase (RBC Stage):

   • Merozoites attack Red Blood Cells (RBCs) and develop through ring stage and trophozoite stage.
   • The parasite feeds on hemoglobin, breaking it down into a toxic substance called haemozoin.
   • Rupture of RBCs releases haemozoin and new merozoites, causing periodic high chills and recurring fever every 3–4 days.

4. Gametogony:

   • Some merozoites inside RBCs differentiate into sexual stages called male (microgametocytes) and female (macrogametocytes) gametocytes, which remain in peripheral blood.

B. Stages in Female Anopheles Mosquito (Sexual Cycle / Sporogony)

1. Ingestion:

   • When a female Anopheles mosquito bites an infected human, it sucks up blood containing gametocytes.

2. Fertilization and Development:

   • Fertilization and maturation of gametes occur in the mosquito's gut/stomach.
   • The zygote transforms into an elongated motile form called ookinete, which penetrates the stomach wall and forms an oocyst.

3. Sporogony:

   • The oocyst undergoes asexual division (sporogony) to form thousands of sporozoites.
   • The oocyst ruptures, releasing sporozoites which migrate to the salivary glands of the mosquito, making it capable of infecting another human.