Plasmodium spp uses a vertebrate host in which asexual reproduction takes
place and in an invertebrate host (mosquito) where sexual reproduction
takes place. Since the definitive host is defined as the host in which
sexual reproduction takes place, the mosquito technically is the definitive
host. The life cycle as depicted here, begins with the female mosquito (Anopheles
spp), biting the human host and injecting sporozoites with its saliva. The
sporozoites travel to the liver where they penetrate hepatic cells and undergo a
rapid division called schizogeny often referred to as the exoerythocytic
phase or simply liver phase. The resulting schizonts are known as
cryptozoites. The resulting thousands of merozoites emerge from the
liver cell and enter the bloodstream, penetrating erythrocytes and beginning the
erythrocytic phase of the life cycle. Some remain in the liver and become
hypnozoites, resulting in later relapses in the case of Plasmodium vivax
and P. ovale in humans. Once in RBCs, the merozoites are now referred to
as trophozoites which ingests RBC cytoplasm and forms a vacuole which gives the
trophozoite the appearance of a ring (ring stage). The trophozoite almost
immediately begins to divide producing another schizogonic phase. As the
schizont doles out cytoplasm to the dividing nuclei, it is known as a segmenter,
and when the divisions are complete, individual cells are known as merozoites.
Merozoites emerge in a periodic fashion depending upon the species, and cause
the classic chills and fever associated with malaria. Plasmodium vivax
and P. ovale demonstrate a 48 hour cycle, while P. malariae
infections reveal a 72 hour cycle. P. falciparum periodicity is
also a 48 hour cycle but the regularity of the cycle is not as rigid as in P.
vivax or P. ovale. Emerging merozoites will infect other
RBCs and repeat the cycle but some develop into macro- and microgametocytes
leading to the sexual cycle in the mosquito. If a susceptible mosquito
ingests gametocytes, the gametocytes will develop into macro-and microgametes (microgametes
exflagellate as seen in the diagram), which will in turn fuse in the act of
sexual union in the midgut of the mosquito. The resulting zygote becomes
motile and penetrates the lining of the mosquito gut, ending up encysted on the
outside wall, bathed in the hemolymph of the hemocoel. At this point this
encysted form is known as an oocyst. Meiosis occurs and the resulting
sporoblasts repeatedly divide forming 1000s of haploid sporozoites. The
sporozoites migrate through the hemocoel, entering the salivary glands where
they remain throughout the life of the mosquito, infecting new hosts with each
blood meal. Contaminated blood passed during transfusions, needle-sharing
amongst drug addicts, accidental needle sticks in hospital and laboratory
workers, and rarely, transplacental passages of blood phases also can lead to
infections.
