Diagram depicting the
life cycle of human malaria
(Asexual phase in human body and sexual phase in the mosquito)
Asexual
phase in the human host
Tissue schizogony (Pre- erythrocytic
schizogony):
This phase starts with the inoculation of
the parasite into the human blood by the bite of a female anopheles mosquito. Within half
an hour, the sporozoites reach the liver and invade the liver cells. The mechanism
of targeting and invading the hepatocytes with this rapidity is not yet clear.
Within the liver cells, the trophozoites
start their intracellular asexual division. At the completion of this phase,
thousands of extra erythrocytic merozoites are released from each liver cell. The time
taken for the completion of the tissue phase is variable, depending on the infecting
species; (8 - 25 days for P. falciparum, 8 - 27 days for P. vivax, 9 - 17 days for P.
ovale, 15 - 30 days for P. malariae) and this interval is called as pre-patent
period.
In case of P. vivax and
P. ovale, some
sporozoites may go into hibernation - the cryptobiotic phase- in which they are
called as hypnozoites. They can lie dormant for months or years and on reactivation
they cause clinical relapse.
Erythrocytic schizogony:
The merozoites released from the liver
cells attach onto the red blood cell membrane and by a process of invagination, enter the
red cell. Within the red blood cell, the asexual division starts and the parasites develop
through the stages of rings, trophozoites, early schizonts and mature schizonts; each
mature schizont consisting of thousands of erythrocytic merozoites. These merozoites
are released by the lysis of the red blood cell and they immediately invade uninfected red
cells. This repetitive cycle of invasion - multiplication - release - invasion
continues. The intra erythrocytic cycle takes about 48 hours in P. vivax,
P. ovale
and P. falciparum infections and 72 hours in case of P. malariae infection. It occurs
synchronously and the merozoites are released at approximately the same time of the day.
The contents of the infected cell that are released with the lysis of the RBC stimulate
Tumor Necrosis Factor and other cytokines, which results in the characteristic clinical
manifestations of the disease.
A small proportion of the merozoites in
the red blood cells undergo transformation into gametocytes - male and female. Mature
gametocytes appear in the peripheral blood after a variable period and enter the mosquito
when it bites an infected individual. (Appear on the 5th day of primary
attack in P. vivax and P. ovale, and thereafter become more numerous; appear at about 5 -
23 days after primary attack in P. malariae; appear after 8 - 11 days of the primary
attack in P. falciparum, rising in number until 3 weeks and falling thereafter, but
may circulate for several weeks).
For
details See Pathophysiology
Sexual phase in
the mosquito
Sporogony:
The gametocytes continue their
development in the mosquito. The male and female gametes fuse and form into a zygote. This
transforms into an ookinete which penetrates the gut wall and becomes an oocyst. The
oocyst divides asexually into numerous sporozoites which reach the salivary gland of the
mosquito. On biting a man, these sporozoites are inoculated into human blood stream. The
sporogony in the mosquito takes about 10 - 20 days and thereafter the mosquito remains
infective for 1 - 2 months.
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