Biomedical Engineering Reference
In-Depth Information
Cryptosporidium, Cyclospora, and Cystoisospora, the three enteric cocci-
dians, are genera under Class Eimeriorina. Two of the coccidians are genera
under Class Sarcocystidae: Sarcocystis and Toxoplasma. These genera pro-
duce a specialized structure, containing dormant, infectious organisms, filling
the husk of a host cell. These structures have specialized names, but most
healthcare workers use the short but inaccurate term, “cysts.”
Sarcocystis suihominis and Sarcocystis bovihominis can cause disease in
humans. As with other coccidians, oocysts are released into the gut lumen
and pass into the environment via feces. The intermediate host ingests con-
taminated water, and infection leads to the development of cysts in host
cells, usually muscle; hence the name sarcocyst (from the Greek, “sarx,”
flesh). When the intermediate host is eaten by a primary host, the organisms
within the sarcocyst reproduce, and their oocytes are eventually released into
the gut, where they mix with fecal material and leave the host. The sarco-
cyst's primary host has a purely enteric infection. The intermediate host, in
which ingested organisms invade muscle throughout the body, and develop
sarcocysts within muscle cells, has an enteric and a systemic disease.
As their names would suggest, infections occur in swine (hence, suihominis),
or cows (hence, bovihominis). On rare occasions, humans become infected
and may serve as either primary or secondary hosts. Though sarcosporidiosis
is considered a rare disease in humans, the animal reservoir for human infec-
tions is large. Various species of Genus Sarcocystis commonly infect a wide
range of animals worldwide. In Southeast Asia, post-mortem biopsies of
human tongues revealed sarcocysts in 21% of the sampled population [79].
This would indicate that many human infections are asymptomatic.
The sister genus to Sarcocystis is Toxoplasma. About one third of the
human population has been infected (i.e., about 2.3 billion people) by the
only species that produces human toxoplasmosis: Toxoplasma gondii.This
number is somewhat higher than the number of tuberculosis infections
worldwide (about 2 billion). In the United States of America, the prevalence
rate of Toxoplasma infection is about 11%.
Toxoplasma has a life cycle very similar to Sarcocystis. The most com-
mon primary host of Toxoplasma is the cat. Humans are the intermediate
host. For most infected individuals, infection does not cause overt disease.
Serious disease occurs most often in immune-compromised hosts, particu-
larly individuals with AIDS or individuals who are pregnant. Transplacental
infections may occur. Disease occurs in the locations where the cysts
develop, often in the brain (causing encephalitis), or eyes (causing chorioreti-
nitis), or through which the organisms migrate (e.g., lymph nodes). Latent
cysts can produced a reactivation of toxoplasmosis, and this occurs most
often in individuals who were infected early in life and later became
immune-compromised due to disease or immunosuppressive therapy.
