Page 446 - Review of Medical Microbiology and Immunology ( PDFDrive )
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CHAPTER 52 Blood & Tissue Protozoa
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FIGURE 52–1 Plasmodium species. Life cycle. Right side of figure describes the stages within the human (blue arrows). Cycle A (top right) is
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the exoerythrocyte stage that occurs in the liver. Note that only P. vivax and P. ovale produce hypnozoites (a latent form) in liver cells (not shown).
Cycle B (bottom right) is the erythrocyte stage that occurs in the red blood cell. Note that at step 6 in the cycle, merozoites released from the ruptured
schizonts then infect other red blood cells. The synchronized release of merozoites causes the periodic fever and chills characteristic of malaria. Left side
of figure describes the stages within the mosquito (red arrows). Humans are infected at step 1 when mosquito injects sporozoites. Mosquito is infected
at step 8 when mosquito ingests gametocytes in human blood. (Source: Dr. Alexander J. da Silva and Melanie Moser, Centers for Disease Control and Prevention.)
(heterozygotes) are protected against malaria because their
red cells have too little ATPase activity and cannot produce
areas where malaria is endemic. Both male and female
carriers of the mutated gene are protected against
sufficient energy to support the growth of the parasite. Peo-
ple with homozygous sickle cell anemia are also protected
Malaria is transmitted primarily by mosquito bites, but
but rarely live long enough to obtain much benefit.
transmission across the placenta, in blood transfusions,
The receptor for P. vivax is the Duffy blood group anti-
and by intravenous drug use also occurs.
gen. People who are homozygous recessive for the genes malaria.
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that encode this protein are resistant to infection by P. vivax.
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Partial immunity based on humoral antibodies that
block merozoites from invading the red cells occurs in
More than 90% of black West Africans and many of their
American descendants do not produce the Duffy antigen
infected individuals. A low level of parasitemia and low-
and are thereby resistant to vivax malaria.
nition. In contrast, a nonimmune person, such as a
People with glucose-6-phosphate dehydrogenase (G6PD)
first-time traveler to an area where falciparum malaria is
deficiency are also protected against the severe effects of
falciparum malaria. G6PD deficiency is an X-linked
endemic, is at risk of severe, life-threatening disease.
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