Question

How many of the following statement(s) is/are true with respect to the life cycle of the Plasmodium parasite?

I. The merozoites are initially formed in the liver cells and attack the RBCs upon their release.
II. Rupturing of RBCs releases hemozoin into the blood.
III. The Plasmodium parasite has a single host.
IV. When a female Anopheles mosquito bites a malaria patient, it picks up the sporozoites from the human blood.

Answer 1

Malaria is a protozoan disease caused by the Plasmodium sp. and transmitted through the bite of the female Anopheles mosquito. This disease is characterised by recurring fever, chills, headache, muscle pain, vomiting, etc.

The Plasmodium parasite completes its life cycle in two different hosts, i.e, humans and the female Anopheles mosquito. So statement III is incorrect.

• Life cycle of Plasmodium in the mosquito starts when the mosquito sucks blood of an infected malaria patient and picks up the immature gametocytes from the patient’s bloodstream (step 1). These further develop into mature gametes within the gut of the mosquito (step 2).
• Fertilisation of the male and female gametes results in the formation of zygotes which develop into motile ookinetes (step 3). These dig through the midgut wall of the mosquito and form oocysts on the exterior surface (step 4). Oocysts eventually burst to release sporozoites (step 5) that travel up and reside in the mosquito’s salivary glands.
• When the female Anopheles mosquito sucks blood of a human, it injects the sporozoite containing saliva into the bloodstream (step 6). These sporozoites attack the liver cells (step 7) within which they undergo asexual reproduction (exoerythrocytic cycle) to produce multiple smaller parasitic structures called merozoites (step 8).
• The liver cells rupture and release the merozoites which start attacking red blood cells (step 9) and multiply asexually (erythrocytic cycle) within them to produce more merozoites that continue the cycle by attacking more RBCs. So statement I is correct.
• Within the RBCs, some of the merozoites grow into feeding stages called trophozoites which further develop and undergo nuclear divisions to form schizonts (step 10). Cytoplasmic divisions in the schizonts lead to formation of progeny merozoites (Step 11)
• Within the infected RBCs, the excretory products of the parasite along with the iron part of the haemoglobin forms a toxic substance called hemozoin. Rupturing of RBCs releases hemozoin which immediately triggers the immune response of the body resulting in fevers and chills. The RBCs rupture periodically as the merozoites complete their life cycle within them. This results in recurring fever and chills. So statement II is correct.
• In some of the infected blood cells the trophozoites develop into sexual forms of the parasite, called gametocytes, that circulate in the bloodstream (step 12).
• When a female Anopheles mosquito sucks blood from such an infected person, it picks up the gametocytes and the life cycle within the mosquito ensues. Hence statement IV is incorrect.

Thus only 2 statements, i.e, statements I and II are true.