Parasites, in general, may be single-celled or
Single celled parasites are known as Protozoa.
Multi cellular parasites are known as Metazoa or
Helminths or more commonly as worms.
Protozoans can be further sub divided into
Sarcodina, includes the amebas like Entamoeba
histolytica (amebiasis), Entamoeba coli, Entamoeba
dispar; Acanthameba and Naeglaria spp.
Sporozoa, includes sporozoans like Cryptosporidium
spp.(Cryptosporidiosis), Plasmodium spp. (malaria),
Toxoplasma spp.(Toxoplasmosis), Cyclospora spp.,
Isospora spp. Microsporidia(diarrhoea), Babesia spp.
(influenza like symptoms.
Mastigophora, includes flagellates such as Giardia
lamblia (Giardiasis, watery foul-smelling diarrhea),
Trichomonas vaginalis (Trichomoniasis),
Trypanosoma spp. Such as cruzi (Chagas
disease), gambiense (Sleeping sickness) ,
Leishmania spp. donovani ( Kala-azar), tropica,
mexicana and brasiliensis ( Leishmaniasis.
Ciliata, includes ciliates like Balantidium
Metazoans are sub-divided into Platyhelminthes,
commonly known as Flat-worms and
Nemathelminthes commonly known as
roundworms or nematodes.
Platyhelminthes have two important classes that
are clinically important that are:
Cestoda or tapeworms, such as Taenis spp.
Trematoda or flukes, such as Schistosoma spp.
(schistosomiasis); Clonorchis spp.(clonorchiasis);
Diseases caused by Intestinal protozoa:
Entamoeba histolytica causes amebic dysentry
and liver abscesses.
The life cycle includes two significant stages of
Trophozoites (motile, feeding, reproducing form
surrounded by a cell membrane) and
Cysts ( nonmotile, nonreproducing surrounded by
a thick wall).
Pathogenesis and Clinical findings:
The parasite is acquired by the ingestion of the
cysts through the fecal-oral route (contaminated
food and water).
The ingested cysts differentiate into trophozoites
in the ileum.
These trophozoites then colonize the cecum and
The trophozoites have the capability to secrete necrotic
enzymes and cause localized necrosis in the colonic
Further invasion into the muscularis layer results in flask
shaped ulceration that damages the intestinal epithelium
They can also form a granulomatous lesion called an
ameboma in the cecum or recto-sigmoid areas of the
colon, that may resemble as and must be distinguished
from a adenocarcinoma of the colon.
Any further invasion results in the entry of the trophozoites
into the portal circulation, now having the potential to cause
a systemic disease involving the liver causing abscess that
can penetrate the diaphragm and cause lung disease.
Acute intestinal amebiasis presents as a bloody,
mucus containing diarrhea accompanied by lower
abdominal discomfort, and flatulence.
Chronic amebiasis presents with milder symptoms
such as occasional diarrhea, weight loss and
Amebic abscess of the liver is characterized by
more severe symptoms with a right upperquadrant pain, weight loss, fever and a tender
Aspiration of the liver abscess yields a brownish-yellow
Examination of diarrheal stools reveal trophozoites
characteristically containing ingested red blood cells.
Non-diarrheal stools often reveal the presence of cysts.
Cysts however are passed intermittently and hence at least
three specimens should be examined.
The cysts typically contain four nuclei and are an important
Serological testing can also be useful as antibodies
although non-protective are produced against the
trophozoite antigens especially in invasive amebiasis.
The indirect hemagglutination test is usually positive and
diagnostic in patients with an invasive disease.
It is especially important to distinguish E.histolytica from
other spp. of Entamoeba.
Their trophozoite nucleus, typically possess a central
nucleolusand fine chromatin granules along the nuclear
membrane, unlike other Entamoebas.
The E.histolytica cysts are smaller and contain four nuclei
unlike others that may contain eight nuclei.
PCR based assays to detect E.histolytica antigens
and nucleic acids are however highly specific.
A wet mount in saline or iodine may also be useful
to distinguish between amebic and bacillary
dysentry which may contain many inflammatory
cells such as polymorphonuclear leucocytes.
Treatment of choice for symptomatic intestinal
amebiasis and liver abscesses is Metonidazole.
Asymtomatic carriers treated with iodoquinol or
Prevention to avoid fecal contamination of
cultivated and consumed food.
Boiling water is effective as cysts not very
resistant and get killed.
Chlorination, however may not be effective.
Giardia lamblia causes Giardiasis.
Giardiasis is characterized by watery but nonbloody, foul smelling diarrhea accompanied by
persistent nausea, flatulence and abdominal
cramps but no fever.
Their life cycle also include the trophozoite and a
The trophozoite is pear-shaped with two nuclei,
four pairs of flagella and a suction disk that helps
attach to the intestinal walls.
The cysts are oval, thick walled and several
characteristic internal fibres. On encystation in the
intestinal tract, each cyst gives rise to two
Ingested cysts through contaminated food and
water (fecal-oral route) results in encystation in
The trophozoite thus formed now attaches to the
intestinal walls. They do not invade the mucosa
and do not enter the blood stream.
This however leads to inflammation of the
duodenal mucosa affecting the absorption of
proteins and fats.
Examination of diarrheal stools may show the
presence of both trophozoites and cysts.
Non-diarrheal stools may contain cysts alone.
ELISA test to detect the presence of cyst wall
antigen is a important diagnostic criteria.
Treatment and prevention:
Treatment of choice is Metronidazole or
Prevention includes boiled, filtered or iodine
Cryptosporidium parvum or Cryptosporidium
hominis causes Cryptosporidiosis.
Cryptosporidiosis manifests itself as a watery,
non-bloody diarrhea causing large fluid loss,
especially in the more severe forms in
The life cycle of Cryptosporidium parvum,
belonging to sporozoa sub-group, involves both
the sexual and asexual cycles involving oocytes,
that result in schizonts, then merozoites , followed
by micro(male) and macro(female) gametes , that
ultimately unites to form a zygote and
differentiates finally into an oocyst.
Involves the transmission or ingestion of oocytes
through the fecal-oral route.
The oocysts excyst in the jejunum of the small
intestines, where the trophozoites attach to the gut
Invasion does not occur.
The pathogenesis of diarrhea is as yet uncertain.
Laboratory diagnosis involves the finding of oocysts in
fecal smears using a modified acid-fast staining
Treatment may not be required in immunocompetent
patients as the disease is self-limiting. Nitazoxanide is
the drug of choice, if necessary.
For immuno-compromised patients however,
paromomycin may just help in reducing diarrhea.
Prevention using purified water, either boiled or filtered
. Chlorination is not effective.
Among the clinically significant urino-genital
protozoa is Trichomonas vaginalis.
They cause Trichomoniasis, one of the most
common infections world-wide, that presents a
watery, foul-smelling, greenish vaginal discharge
Infected males are generally asymptomatic.
Trichomonas vaginalis is a pear shaped flagellate,
possessing four anterior flagella and a central
It has an undulating membrane.
It exists only as a trophozoite.
Transmission is through sexual contact, hence
does not need a cyst form.
It locates itself in the vagina (female) and the
Laboratory diagnosis generally involves a wet
mount of vaginal/prostate secretions to find the
pear shaped trophozoites with a typical, jerky
motion, along with neutrophils.
The drug of choice is Metronidazole for both
partners to prevent reinfection.
Maintaining a low pH of the vagina is helpful.