Biomedical Engineering Reference
In-Depth Information
diatoms, and a colorless group containing organisms that have morphologic
features similar to fungi.
Class Heterokontophyta has undergone extensive taxonomic revision in
the recent past. The golden and brown algae (currently in Class
Heterokontophyta) were previously classified under Class Plantae. Adding to
the confusion, green and red algae are currently placed in Class
Archaeplastida (Chapter 24). Various species of the colorless group (cur-
rently in Class Heterokontophyta) had been incorrectly placed in Class
Fungi. Oomycota, a “colorless” class of heterokonts, contains the organisms
that produce late blight of potato (Phytophthora infestans), and sudden oak
death (Phytophthora ramorum). Oomycota, despite its suffix (mycota, an
alternative name for fungus), is not a member of Class Fungi.
Given the taxonomic turmoil within Class Heterokontophya, students
should be grateful that there is only one heterokont genus that is infectious
in humans: Blastocystis.
Heterokontophyta
Blastocystae
Blastocystida
Blastocystidae
*Blastocystis
Blastocystis infects a variety of animals. The general rule for naming species
of Blastocystis is to apply the genus to the species in which the organisms are
recovered. Hence, multiple species of Blastocystis recovered from human
feces were named Blastocystis hominis. Species of the organism recovered
from rat feces were all named Blastocystis ratti. Obviously, this taxonomic
disaster cannot persist. Efforts to identify individual species by species-spe-
cific genotyping (rather than bundling different species under the host species
name) have begun [80].
Infection seems to be through acquisition of the cyst form of the organ-
ism,
through a fecal
oral
route. Human
human, animal
human, and
human
animal transmission all may occur, but the relative frequencies of
these different transmissions are unknown. Incidence is highest where
humans are exposed to animal feces, implying that animal
human transmis-
sion is common. In the USA the prevalence rate of infection of Blastocystis
hominis is 23%, with the highest rates found in the western states [81].
Four different morphologic forms of Blastocystis have been observed: cyst,
vacuolar, amoeboid, and granular. After the cyst is ingested, the other forms
emerge in the intestine. Replication of the organism occurs in the vacuolar form.
The length of infection varies from weeks to years. Many Blastocystis infections
do not manifest clinically, and an asymptomatic carrier state is common. Disease,
when it occurs, can closely mimic irritable bowel syndrome. When species of
Blastocystis hominis are identified by type-specific laboratory testing, we will be
in a better position to determine whether specific variants of the organism are
Search WWH ::




Custom Search