Intestinal protozoa
Amoeba: Entamoeba histolytica
Flagellates: Giardia lamblia
Coccidians: Toxoplasma gondii, Cryptosporidium parvum
1. Protozoa colonize and infect the oro-pharynx, duodenum and colon
2. The organisms are transmitted by the fecal-oral route (food/water)
3. Outbreaks of diarrhea and dysentery are especially problematic in
daycare centers
4. The cyst forms of protozoa are resistant to chlorine and ozone and
can become important when the municipal water supply is
overburdened with these organisms—esp. farming communities
Read Vela Chapter 7
Case study
E. histolytica
Entamoeba histolytica
Primitive unicellular micro-organisms
Life cycle divided into two stages
trophozoite—actively motile feeding stage—food and human
blood cells
when environmental conditions are favorable
cyst—dormant, highly resistant, infectious stage
when temperature or moisture levels drop
Replication involves simple binary fission of trophozoite or division
to produce numerous infectious trophozoites in a mature cyst.
Motility—extension of a pseudopod (false-foot) and then drawing
up the rest of the cell to meet the pseudopod in a “snail like”
movement.
Entamoeba histolytica
E. histolytica life cycle
Encounter—fecal-oral route
patients with diarrhea excrete the trophozoite form which is
killed by drying in the environment or by the acidity of the
stomach
asymptomatic patients excrete infectious cysts that are resistant
to drying and acid
1. Ingestion of cysts
2. Passage of cysts through the stomach where gastric acid stimulates the
release of the infectious trophozoites from the cysts
3. Trophozoites move to the duodenum where they divide
4. Trophozoites travels to the colon where they attach to colonic epithelial
cells
5. After attachment they produce a cytotoxin that kills epithelial cells so they
can gain access to deeper tissues
6. Continue to divide in colon where amoeba/cysts are excreted in stool OR
7. Trophozoites invade the deeper mucousa and enter the peritoneal cavity
8. Trophozoites are carried in the circulation to the liver but can also be
carried to the lungs, brain and heart
Epidemiology of E. histolytica
Worldwide distribution—especially prevalent in warmer climates
but also endemic cases found in cold areas (ie. Alaska, Canada)
Many infected individuals can be asymptomatic and serve as reservoirs
Ffor disease
Carrier passes cysts that contaminate water supply and food—esp
children in daycare centers
Flies, ants and cockroaches can also serve as vectors for the spread
of cysts
Sewage containing cysts can contaminate municipal water supply
wells and springs.
Use of human feces as fertilizer can contribute to the spread
Prevalence of infection in U.S. is 2-5% in warmer countries 15-50%
Clinical diseases of E. histolytica
Amoebic dysentery!!! Related to the destruction of the colonic
epithelial cells by the organism.
Flask shaped ulcerations of the intestinal mucousa with inflammation
Secondary bacterial infection
symptoms: abdominal pain, cramping passage of numerous
watery and bloody stools
If untreated patients can die of dehydration
Amoeba can invade deeper tissues and enter the blood circulatory
system where they especially infect the liver as trophozoites are removed from blood as they enter the liver.
abscess formation in the liver is common
pain in the liver and elevation of the diaphragm
Treatment and prevention of E. histolytica
Metronidazole—penetrates deeper tissues and destroys amoeba present
in liver, brain, lungs etc.
the organism’s metabolism converts the drug into its lethal form
A second drug is used to eradicate the amoeba present in the intestinal
lumen (paromomycin)
Prevention: When traveling to areas where E. histolytica is epidemic
or endemic
AVOID drinking water ALSO ice cubes
filter and boil water
thoroughly wash unpeeled fruits and raw vegetables
Giardia lamblia trophozoite
Giardia attached to intestinal microvilli by sucking disks
Upon detaching clear impressions from the
Sucking disks are left on the surface of the microvilli
G. lamblia is a flagellate and moves by lashing its flagella that moves
organism through fluid environments.
G. lamblia attaches to the intestinal villi of duodenum via an adhesive
disk
Cysts are resistant to the amounts of chlorine put in municipal water
systems (2 parts per million) therefore water systems should ALSO
filtrate water
G. lamblia case study
G. lamblia life cycle
G. lamblia life cycle
1. Infection initiated by the ingestion of infectious cysts (only 10 are
required for infection
2. Acid in the stomach stimulates the release of trophozoites from the
cyst
3. Trophozoites are released in the duodenum and jejunum (upper part
of small intestines) where they multiply by binary fission
4. Trophozoites attach to the intestinal villi by means of a sucking disk
5. Trophozoites can develop into cysts for survival outside of the host
6. Trophozoites cause an explosive diarrhea such that cysts are released
into the environment
7. Trophozoites remain in the G-I tract and almost never found
elsewhere in the body.
Epidemiology of G. lamblia
G. lamblia found everywhere in the world
Often found in streams, lakes mountain resorts—reservoir animals
such as beavers and muskrats perpetuate the infectious cycle
Approximately 50% of infected humans are asymptomatic and are
important carriers of disease
Giardiasis is acquired through
the consumption of inadequately treated water
ingestion of uncooked vegetables and fruits
person-person spread (esp. daycare centers, families with
infected children)
Giardia can be maintained in the municipal water supply, unless water
treatment plant uses filtration AND chemicals to eradicate the
protozoa
Clinical diseases of G. lamblia
Symptomatic disease ranges from mild diarrhea to severe dysentery
The incubation period before symptomatic disease is approx. 10 days
The onset of disease is sudden and consists of
foul-smelling watery diarrhea (seldom bloody)
abdominal cramping
flatulence
Spontaneous recovery occurs in 2 weeks HOWEVER
Chronic disease with several relapses may occur.
Clinical diagnosis of G. lamblia
With the onset of diarrhea the patient’s stool are examined for
trophozoites and cysts.
Giardia may appear in stool on a given day and not be present on the
following day
one stool sample over a period of three days should be
examined before making a negative diagnosis.
Samples can be collected through duodenal aspiration or via biopsy
of upper small intestines.
Treatment and prevention of G.
lamblia
Eradicate Giardia from BOTH asymptomatic carriers and diseased
patients.
Campers/travelers should boil AND filter water taken from
lakes and streams AND from municipal water in areas where disease
is endemic
Municipal water supplies should maintain functioning filtration
Systems since the cysts are resistant to chlorine and ozone treatment
Coccidia—Crytosporidium and
Toxoplasma
Reproduce by sexual and asexual reproduction
Most coccidiae have multiple hosts
C. parvum found in farm animals/ reptiles and fish where
they reproduce sexually they reproduce asexually in humans.
T. gondii found in herbivores, birds and carnivores (sexual)
and humans (asexual) cats are especially important carriers of
disease
Hard to eradicate because these protozoa are zoonotic
Coccidia life cycle Cryptosporidium
and Toxoplasma
T. gondii cysts in brain tissue
Toxoplasma gondii
Toxoplasma gondii