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Daniel J. Carucci, Malcolm J. Gardner, Herve Tettelin, Leda M. Cummings, Hamilton O. Smith, Mark D. Adams, Stephen L. Hoffman and J. Craig Venter
 |
Figure 1. The life cycle of Plasmodium
falciparum malaria parasite.
Malaria is caused by infection with an obligate, intracellular protozoan parasite
of the genus Plasmodium. Of the four species that infect humans (Plasmodium
falciparum, Plasmodium vivax, Plasmodium ovale and Plasmodium
malariae), it is P. falciparum that is responsible for virtually
all deaths. The life cycle of Plasmodium spp. is complex and somewhat
specific to the parasite species.
(a) P. falciparum infection in humans begins when an
infected Anopheles sp. mosquito takes a blood meal and injects infective
sporozoites into the peripheral circulation.
(b) Within minutes, these sporozoites invade hepatocytes
in the liver and, over approximately one week, undergo asexual multiplication,
producing tens of thousands of merozoite forms of the
parasite.
(c) When the infected hepatocyte ruptures, merozoites are
released into the peripheral circulation.
(d) The merozoites invade red blood
cells (rbcs) and
(e) complete another round of multiplication within 48–72
h, with the production of 16–20 additional merozoites per rbc, which devour
the rbc haemoglobin in the process.
(f) The released merozoites invade additional rbcs and carry
on the cycle. It is the synchronous release of merozoites that is thought to
be responsible for the periodic fevers associated with malaria.
(g) Some invading merozoites do not divide, but differentiate
into male (microgametocyte) and female (macrogametocyte)
sexual forms.
(h) These sexual forms are taken from the bloodstream by a
feeding Anopheles sp. mosquito and
(i) fertilise in the mosquito midgut to
form zygotes. These zygotes further differentiate into motile forms, called
ookinetes, migrate through the mosquito gut wall and divide within oocysts on
the external gut wall to form thousands of sporozoites.
(j) The infective sporozoites are released into the mosquito
haemocoele and move to the salivary gland, where they await injection into another
human host, thus completing the life cycle (fig001dcn).
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