Biomedical Engineering Reference
In-Depth Information
intestine). From this location, oocysts are carried within feces and eventually
contaminate the water where feces are deposited. One method of coccidian
infection is through oocysts that contaminate food and water. A second strat-
egy for long-term survival is achieved by some coccidians that encyst within
the tissues of the host organism. This coccidian cyst is formed from the husk
of a host cell that has been filled by multiplying small forms of the organ-
ism. These so-called cysts, which are really modified host cells filled with
infective organisms, are stable and durable. When the host tissues are eaten
by another animal, the cysts release an infective form of the organism, and
the coccidian life cycle repeats, beginning in the intestines.
The disease produced by any of the organisms belonging to Class
Coccidia is known collectively as coccidiosis. This term is applied most
often to coccidian infections in animals. Coccidiosis must not be confused
with the similar-sounding coccidioidomycosis (Ascomycota, Chapter 36).
Cryptosporidiosis is caused by Cryptosporidium parvum. It is typically
transmitted by drinking water that has been contaminated with relatively
stable oocysts carried by the feces of an infected animal. Infection is con-
fined to the mucosa of the small intestine, typically producing a watery diar-
rhea that is self-limited in immune-competent individuals. Cryptosporidiosis
infection can produce a severe gastroenteritis in immune-deficient indivi-
duals. In some cases of Cryptosporidiosis occurring in immune-deficient per-
sons, the organism is never completely cleared from the small intestine.
Cryptosporidiosis is a common disease that occurs worldwide.
Like all members of Class Apicomplexa, Genus Cryptosporidium is an
obligate intracellular organism. Casual histologic examination of infected
mucosa shows that the organisms perch atop the apical surface of the epithe-
lial cells lining the small intestinal villi (i.e., extracellular growth). Careful
examination demonstrates that the cryptosporidia attached to the mucosal
surface are actually wrapped by host cell membranes (i.e., intracellular
growth). Students should avoid confusing cryptosporidiosis with the similar-
sounding cryptococcosis (Class Basidiomycota, Chapter 35).
One species of Genus Cyclospora produces disease in humans:
Cyclospora cayetanensis. As in cryptosporidiosis, stable oocysts carried in
water contaminated by infected animals, pass into the gastrointestinal tract
and produce sporozoites that grow in the small intestine. The infection pro-
duces diarrhea. On histologic stains, Cyclospora cayetanensis can be mis-
taken for Cryptosporidium parvum, but Cyclospora cayetanensis is the larger
of the two organisms.
Cystoisospora (synonymous with Isospora) belli has a life cycle, mode of
transmission, and clinical presentation (enteritis), similar to that of
Cryptosporidium parvum and Cyclospora cayetanensisis. Isosporiasis is an
uncommon disease, but when it occurs, it tends to arise in immune-deficient
individuals. Diagnosis can usually be made after careful stool examination.
The oocysts are ellipsoidal and large.
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