Page 444 - Review of Medical Microbiology and Immunology ( PDFDrive )

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Blood & Tissue Protozoa 52
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CHAPTER C ONTENT S

Introduction
Self-Assessment Questions
Plasmodium
Toxoplasma
Summaries of Organisms
Pneumocystis
Practice Questions: USMLE & Course Examinations
Trypanosoma
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mebooksfree.com mebooksfree.com mebooksfree.com There are two phases in the life cycle: the sexual cycle, which mebooksfree.com
INTRODUCTION
occurs primarily in mosquitoes, and the asexual cycle,
The medically important organisms in this category of pro-
1
which occurs in humans, the intermediate hosts.
tozoa consist of the sporozoans Plasmodium and Toxoplasma
The sexual cycle is called sporogony because sporo-
and the flagellates Trypanosoma and Leishmania. Pneumocystis
zoites are produced (sporogonic cycle is labeled C in
is discussed in this book as a protozoan because it is con-
Figure 52–1), and the asexual cycle is called schizogony
sidered as such from a medical point of view. However,
because schizonts are made.
molecular data indicate that it is related to yeasts such as
The life cycle in humans begins with the introduction of
Saccharomyces cerevisiae. Table 51–2 summarizes several
important features of these blood and tissue protozoa.
mosquito. The sporozoites are taken up by hepatocytes
The medically important stages in the life cycle of the sporozoites into the blood from the saliva of the biting
within 30 minutes. This “exoerythrocytic” phase (labeled A
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blood and tissue protozoa are described in Table 52–1.
in Figure 52–1) consists of cell multiplication and differen-
tiation into merozoites. Plasmodium vivax and P. ovale
PLASMODIUM
is the cause of relapses seen with vivax and ovale malaria.
Disease
Merozoites are released from the liver cells and infect
red blood cells. During the erythrocytic phase (labeled B in
Malaria is caused primarily by four plasmodia: Plasmodium
Figure 52–1), the organism differentiates into a ring-
vivax, Plasmodium ovale, Plasmodium malariae, and
shaped trophozoite (Figures 52–2A and B and 52–3). The
Plasmodium falciparum. Plasmodium vivax and P. falciparum
ring form grows into an ameboid form and then differenti-
are more common causes of malaria than are P. ovale and
P. malariae. Plasmodium vivax is most widely distributed
After release, the merozoites infect other erythrocytes (step
and P. falciparum causes the most serious disease. A fifth
6 in Figure 52–1). This cycle in the red blood cell repeats at
species, Plasmodium knowlesi, is found in Southeast Asia. ates into a schizont filled with merozoites (Figure 52–2C).
regular intervals typical for each species. The periodic
Worldwide, malaria is one of the most common infec-
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mebooksfree.com mebooksfree.com mebooksfree.com 1 cytes within red blood cells (gametogony) and completed in mosquitoes 433 mebooksfree.com
release of merozoites causes the typical recurrent symp-
tious diseases and one of the leading causes of death.
toms of chills, fever, and sweats seen in malaria patients.
Important Properties
The sexual cycle is initiated in humans with the formation of gameto-
The life cycle of Plasmodium species is shown in Figure 52–1.
The vector and definitive host for plasmodia is the female
with the fusion of the male and female gametes, oocyst formation, and
Anopheles mosquito (only the female takes a blood meal).
production of many sporozoites (sporogony).
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