|Basic Malaria Microscopy (part I and II) (WHO - OMS, 1991, 72 p.)|
By the end of this Unit you should be able to:
· describe the life cycle of the malaria parasite in humans
· describe that part of the life cycle which is spent in the female Anopheles mosquito (the vector of malaria)
· recall the various stages of the malaria parasite found in human blood.
The liver phase
When an infected female Anopheles mosquito bites a human being, malaria parasites are introduced into that person.
The parasites move quickly to the liver, where each invades a liver cell. Over a period of 7-21 days the parasite grows and reproduces. Finally the liver cell bursts, releasing parasites into the bloodstream, where each attaches to and invades a red blood cell.
This description is invariably true of Plasmodium falciparum and P. malariae infections. In P. vivax and P. ovale infections, however, some of the parasites remain in the liver and do not reproduce immediately. It is these dormant parasites that are responsible for the relapses that occur in patients with P. vivax and P. ovale infections.
The blood phase
From the work you have done in earlier Learning Units you will already be familiar with the different stages of the blood phase, which are shown diagrammatically in Fig. 5.
In the mosquito
The sexual phase of the malaria life cycle takes place in the stomach of the mosquito. Soon after the female Anopheles mosquito has ingested blood from an infected person, the male gametocytes (microgametocytes) each produce 4-8 flagella. These flagella enter, and fertilize, the female gametocytes (macrogametocytes). The mobile products of this fertilization burrow through the stomach wall and develop into cysts in the lining of the abdomen. When the cysts rupture, they release sporozoites which eventually enter the salivary glands. After a period of time which varies according to the species of mosquito and the ambient temperature but which is usually between 7 and 14 days, the anopheline mosquito is able to transmit malaria.