Published by Australian Academy of Science
Activity 2 | Malaria – a growing threat

Aspects of the life cycle of the malarial parasite

Malaria is characterised by recurring bouts of fever every few days, with no significant symptoms in between.

• Explain this phenomenon in terms of the life cycle of the malarial parasite.

Teachers notes

Malarial parasites initially travel to the host's liver. In the host's liver cells the parasite multiplies repeatedly. After about 5 days there are in the order of 40,000 new parasites, called merozoites. At this stage, the liver cell bursts and the merozoites attack red blood cells. In the red blood cells, the parasite divides again, forming about 16 new merozoites which are released to invade new red cells.

The merozoites burst out of the blood cells at regular intervals. The release of the parasites and associated toxins causes a fever. The length of the growth cycle is specific for the species of Plasmodium. There are four types of malarial parasite which infect humans: Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale and Plasmodium malarial. The first three have a 48-hour blood cycle (causing fever every 2 days); Plasmodium malarial has a 72-hour blood cycle (causing fever every 3 days).

The first attack of fever usually occurs about 2 weeks after a person has been infected. During this time the parasites are multiplying in the liver, and then in the blood, and no fever is experienced by the infected person.

Often people infected with Plasmodium vivax or Plasmodium ovale experience no symptoms for a number of years after their initial illness. During this time the parasite remains dormant in the liver, but can become active again, causing a relapse.