Question

Which stage of Plasmodium vivax is infectious to humans?

• A. Trophozoite
• B. Sporozoite
• C. Merozoite
• D. Gametocyte

Answer 1

Answer:

B. Sporozoite

Understanding Plasmodium vivax and Malaria Transmission

• Malaria is a life-threatening disease caused by parasites of the genus Plasmodium, which are transmitted to humans through the bites of infected female Anopheles mosquitoes.
• Among the five species of Plasmodium that cause malaria in humans, Plasmodium vivax is one of the most widespread and causes 'benign tertian malaria'.

• The Life Cycle of Plasmodium vivax: Key Stages

  • The life cycle of Plasmodium vivax involves two hosts: a human (intermediate host) and an Anopheles mosquito (definitive host).
  • The stage of Plasmodium vivax that is infectious to humans is the Sporozoite.
  • When an infected female Anopheles mosquito bites a human, it injects these sporozoites, which are spindle-shaped, motile forms, into the human bloodstream.
  • These injected sporozoites quickly travel to the liver cells (hepatocytes), where they undergo asexual reproduction to form schizonts and then merozoites. This is the exo-erythrocytic (pre-erythrocytic) stage.
  • A unique feature of Plasmodium vivax (and P. ovale) is the formation of dormant liver stages called hypnozoites. These hypnozoites can remain quiescent in liver cells for weeks or even years, causing relapses of malaria long after initial infection and treatment.
  • Once mature, the liver schizonts rupture, releasing thousands of merozoites into the bloodstream.
  • These merozoites then invade red blood cells (erythrocytes) and multiply asexually. This is the erythrocytic stage, responsible for the clinical symptoms of malaria.
  • Within the red blood cells, merozoites develop into ring forms, trophozoites, and then asexual schizonts, which rupture to release more merozoites, leading to cyclical fevers.
  • Some merozoites develop into male and female gametocytes, which are the sexual stages. These gametocytes circulate in the human bloodstream.
  • When an uninfected female Anopheles mosquito bites an infected human, it ingests these gametocytes.
  • Sexual reproduction (sporogony) occurs within the mosquito gut, eventually leading to the formation of oocysts containing sporozoites, which migrate to the mosquito's salivary glands, ready to infect another human.

• Competitive Exam Facts:

  • Plasmodium vivax preferentially invades reticulocytes (young red blood cells).
  • Infection with Plasmodium vivax is often less severe than P. falciparum, hence 'benign' tertian malaria, but it can still be debilitating.
  • Individuals with Duffy blood group negative (FyFy) are largely resistant to Plasmodium vivax infection, as the Duffy antigen acts as a receptor for the merozoites to enter red blood cells. This explains the lower prevalence of P. vivax in parts of West Africa where the Duffy negative phenotype is common.
  • Chloroquine used to be the primary drug for P. vivax, but resistance is growing. Primaquine is essential to target hypnozoites and prevent relapses.