a. Four Species of Plasmodium. There are four species of Plasmodium that
are known to cause malaria in man: Plasmodium vivax, P. falciparum, P. malariae, and
P. ovale. The diseases produced are known as vivax, falciparum, quartan, and ovale
(1) Sexual cycle of the parasite. Females of certain species of Anopheles
mosquitoes (Figure 5-3) serve as the arthropod vectors.
When the mosquito bites, and feeds on an infected person, she
ingests sexual forms of Plasmodium along with the blood meal (Figure 5-2).
Fusion of a male and female Plasmodium pair produces a cell
thatdevelops into a motile form that penetrates the stomach wall of the mosquito and
encysts on the outer wall of the stomach.
Spore forms produced in large numbers in the cyst break out and
begin to migrate to the mosquito salivary gland about 10 to 14 days after the infected
blood was first ingested.
This process is referred to as the sexual cycle of the parasite.
(2) Infection development.
The spore forms are transmitted to man when the infected mosquito
Within humans, the spore forms enter liver cells, where they enlarge
and develop large numbers of a form that then may enter the red blood cells of the
-- This phase, plus the early portion of the red blood cell phase,
corresponds to the incubation period of the clinical disease.
Inside the red blood cells, the parasites develop into small, ring-
-- These grow rapidly and almost fill the cells.
-- Some blood forms may develop into large and small sexual forms
that will infect a feeding female anopheline mosquito.