Biomedical Engineering Reference
In-Depth Information
individuals infected with Trichomonas vaginalis are asymptomatic, the
organism can cause urethritis in a significant percentage of infected men
[71]. Like all members of Class Trichomonadida, it exists only in the
trophozoite form, and transmission is typically caused by the exchange of
infected secretions during sexual intercourse. The trophozoites can be easily
visualized in cervical Pap smears, by light microscopic examination.
Dientamoeba fragilis causes diarrhea and other gastrointestinal symptoms.
It causes disease worldwide. Its reservoir seems to be restricted to humans
and other primates. Like the other members of Class Trichomonadida, it is
found only in the trophozoite form. The organism lives in the large intestine.
Though it can be found in stools, it degenerates quickly (hence the “fragilis” in
Dientamoeba fragilis), and the organism can be difficult to demonstrate. A 2005
study employed a sensitive and specific PCR (polymerase chain reaction) test
on stool specimens from 6750 patients with gastrointestinal symptoms and on
a control set of 900 asymptomatic individuals. The study found that about 1%
of the symptomatic patients tested positive for Dientamoeba fragilis. None of
the asymptomatic patients demonstrated infection [72]. This would indicate
that the organism, when present, causes symptoms; and that Dientamoeba is a
significant cause (on the order of 1%) of gastrointestinal complaints. Because
the trophozoite is passed in stools, and because the organism has no cyst stage,
the presumed method of transmission is through fecal
oral exchange.
Metamonada
Fornicata
Diplomonadida
Hexamitidae
*Giardia
Members of Class Diplomonadida have a so-called mirror morphology, with
two sets of nuclei, flagella, and cytoplasm, symmetrically arranged about a
central axis [73]. There is only one human pathogen in Class Diplomonada:
Giardia lamblia (alternately known as Giardia intestinalis or Giardia duode-
nalis, and formerly known as Lamblia intestinalis). Giardia lamblia can be
found worldwide and infects a wide variety of animals, including cats, dogs,
and birds. Its infection in beavers has inspired the rhyming couplet, beaver
fever. Unlike the pathogens in Class Trichomonadida, Giardia grows in two
forms: trophozoite and cyst. The organism lives in the small intestine, in con-
trast with Entamoeba histolytica (Class Amoebozoa, Chapter 22), which lives
in the large intestine. Entamoeba histolytica and Giardia lamblia both have
cyst forms, but the two organisms can be distinguished by the morphology of
their trophozoites: Giardia trophozoites have flagella; Entamoeba trophozoites
do not. The cyst form supports the long-term survival of the organism in
contaminated water supplies. Transmission can occur from the fecal
oral
route or from ingesting contaminated water. Giardiasis is characterized by
diarrhea and associated gastrointestinal complaints.
