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INTRODUCTION

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A man can be riddled with malaria for years on end, with its chills and its fevers and its nightmares, but if one day he sees that the water from his kidneys is black, he knows he will not leave that place again, wherever he is, or wherever he hoped to be.

­—Beryl Markham: West with the Night (1942)

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APICOMPLEXA

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When the paroxysms fall on even days, the crises will be on even days; and when the paroxysms fall on odd days, the crises will be on odd days. Furthermore, it is necessary that one know that if crises fall on days other than those mentioned above, there will be a relapse, and this may be deadly. But it is essential to pay attention and know at which times the crises will lead to death and in which to recovery, or during which is there tendency to fair better or worse.

—Hippocrates (Translated from the ancient Greek in his work—Epidemics)

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GROUP CHARACTERISTICS

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The Apicomplexa are obligate intracellular protozoan parasites. The name of this group of parasites derives from the complex of organelles located at the apical end of parasite life cycle stages that are involved in penetrating cells. These organelles include the rhoptries, micronemes, and associated microtubular complexes located in this region of the parasite. The Apicomplexa have alternating cycles of sexual and asexual reproduction. Asexual multiplication within the host occurs by a process of multiple fission termed schizogony. The nucleus of a trophozoite divides into several parts, forming a multinucleated schizont. The cytoplasm then condenses around each nuclear portion to form new daughter cells, or merozoites, which burst from their intracellular location to invade new host cells. After the completion of one or more of these asexual cycles, some merozoites differentiate into male and female gametocytes, initiating the sexual phase of the life cycle. In the case of malaria, the gametocytes reach maturity in the mosquito host and effect fertilization, forming a zygote, or motile ookinete. In other Apicomplexa, this process may occur in intestinal cells. The zygote then becomes an oocyst for these parasites. Sporozoites are formed within the oocyst by an asexual process of sporogony and when released, penetrate host tissue cells, and begin another asexual cycle as trophozoites. The only phase of this life cycle that is diploid is when the zygote is formed. All other stages in the life cycle are haploid. The general apicomplexan cell plan is illustrated in Figure 51–1.

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Intracellular protozoa with alternating sexual and asexual cycles

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FIGURE 51–1.

The apicomplexan cell. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein’s Microbiology, 7th edition. McGraw-Hill, 2008.)

Graphic Jump Location Graphic Jump Location
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Two sporozoan infections, malaria and toxoplasmosis, are common ...

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