Page 441 - Review of Medical Microbiology and Immunology ( PDFDrive )

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PART VI Parasitology
430
with acquired immunodeficiency syndrome [AIDS]).
Cryptosporidium parvum is the former name that is no
oocysts excyst in the small intestine, where the trophozo-
ites (and other forms) attach to the gut wall. Invasion does
longer used.
not occur. The jejunum is the site most heavily infested.
The pathogenesis of the diarrhea is uncertain; no toxin has
Important Properties
been identified.
The life cycle of C. hominis is shown in Figure 51–8. Some
Cryptosporidia cause diarrhea worldwide. Large out-
aspects of the life cycle remain uncertain, but the following
stages have been identified. Oocysts release sporozoites,
in the United States are attributed to inadequate purifica-
which form trophozoites. Several stages ensue, involving breaks of diarrhea caused by cryptosporidia in several cities
tion of drinking water. Other outbreaks are related to
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the formation of schizonts and merozoites. Eventually
swimming in fecally contaminated pools and lakes. The
microgametes and macrogametes form; these unite to pro-
cysts are highly resistant to chlorination but are killed by
duce a zygote, which differentiates into an oocyst. This
cycle has several features in common with other sporozoa
(e.g., Isospora). Taxonomically, Cryptosporidium is in the
subclass Coccidia.
Clinical Findings
The disease in immunocompromised patients presents
Pathogenesis & Epidemiology
primarily as a watery, nonbloody diarrhea causing large
The organism is acquired by fecal–oral transmission of
fluid loss. Symptoms persist for long periods in immuno-
compromised patients, whereas they are self-limited in
oocysts from either human sources (primarily) or from
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Cryptosporidiosis
(Cryptosporidium hominis)

Thick-walled oocyst
ingested by host

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water

Contamination of water

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Thick-walled oocyst
exits host

FIGURE 51–8
Cryptosporidium hominis. Life cycle. Top blue arrow shows cysts being ingested. Within the intestine, the oocyst produces
trophozoites that cause diarrhea. Bottom blue arrow shows cysts being passed in the stool and entering the environment. Red arrow indicates
survival of cysts in the environment. (Source: Dr. Alexander J. da Silva and Melanie Moser, Centers for Disease Control and Prevention.)

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