Giardia duodenalis is a flagellated protozoan parasite that causes giardiasis. It has both a trophozoite and cyst stage. The trophozoite lives in the small intestine where it attaches to epithelial cells and feeds on mucus, interfering with absorption. It can cause diarrhea and malabsorption. The cyst forms when trophozoites pass through the large intestine and are excreted in feces. Cysts are hardy and infect new hosts when ingested. Giardiasis is common worldwide and transmitted through contaminated water. Treatment involves metronidazole antibiotics.

1. GIARDIA
Shilpa.k
Microbiology Tutor
AIMSRC

2. Giardia duodenalis (G. lamblia; G.
intestinalis)
– Giardiasis.
– Most distinctive of
the flagellates.
– Has both a
trophozoite and
cyst stage.

3. Giardia duodenalis Trophozoite
Trophozoites are
binucleated (looks like a
face). 12-15 μm.
Ventral surface bears
adhesive disk to adhere to
surface of intestinal cell.
8 flagella (2 anterior, 2
posterior, 2 ventral, and 2
caudal) - all arise from
kinetosome.
Median bodies occur
behind adhesive disk -
function is unknown.

4. Giardia duodenalis Trophozoite
Light microscope photos of trophozoites

5. Giardia duodenalis
• Lives in the upper part of the small
intestine (duodenum, jejunum, and
upper ileum).
• Here the trophozoites attach to the
epithelial cells.

6. Giardia duodenalis Trophozoite
Scanning EM view of trophozoite surface showing the
adhesive disk.
ventral dorsal

7. • Feeds on mucous that forms in response to irritation.
• Also absorbs vitamins and amino acids.
• Interferes with absorption in host especially lipids.
• Giardia can also interfere with vitamin/nutrient
absorption.
– Vitamin A vision
– Vitamin D rickets: Both of these are due to long
standing infections.

8. Cyst of Giardia duodenalis
The cyst forms as trophozoites become dehydrated
when they pass through the large intestine.
Morphology:
• ovoid in shape; 8-12 µm long x 7-10
µm wide
• thin cyst wall.
• Four nuclei present, often
concentrated at on end.
• Flagella shorten and are retracted
within cyst.
• Axonemes provide internal support.

9. Cyst of Giardia duodenalis
Cyst may remain viable in the external
environment (usually water) for many
months.
-14 billion cysts can be passed in
1 stool sample
-Moderate infections: 300
million cysts.

10. Cyst of Giardia duodenalis

11. Symptoms
• Range from none abdominal
discomfort causing acute or chronic
diarrhea and other GI signs.
• Gray, greasy, voluminous malodorous
diarrhea!
• Flatulence.

12. Giardia duodenalis
• Giardia trophozoites are attracted to bile
salts: so sometimes you can get infections
in bile ducts and gall bladder, causing
jaundice and colic.
• This is irritating but not life threatening
infection like E. histolytica.

13. Pathogenesis and Pathology
• Nutrient malabsorption and physical
blockage and damage to microvilli.
• Trophozoites attach to small intestine
cause damage (mechanical and toxins).

14. Pathogenesis and Pathology
1) Fat/CHO digestion decreases and causes maldigestion.
2) Absorption decreases due to villus blunting causing
malabsorption.
3) Malabsorption and maldigestion causes
diarrhea.
4) Physical damage: clubbing of villi; decreases
villus-to-crypt ratio; brush borders of cells are
irregular.

19. Giardia trophozoite
Trophozoite attaches to surface of epithelial cells with its
adhesive disk.

20. Epidemiology
• Get infected by ingesting cysts through
contaminated water.
• Most common intestinal flagellate of
people.
• World wide distribution; prevalence
ranges from 2.4-67.5%.
• Reservoir hosts can play a significant
role.

21. Reservoir Hosts
Transmission from animals to humans is
controversial; dependent on strain or type involved.

22. Human Infections
• There are hot spots: Vacations and Travels
Camping.
• Colorado ski resorts are notorious for
outbreaks drinking from Mountain Springs,
washing utensils/drinking water that is not
treated.
• Day care centers.

23. Diagnosis
• Trophozoites in diarrheic feces; cysts in
formed feces.
• At least 3 exams (one every other day)
before judge negative.
• ELISA tests: detect soluble antigen.

24. Treatment and Prognosis
• Drug of choice is Flagyl, Metronidazole:
15 mg/kg/day in 3 divided doses for 5–7
days
Giardia thrives in people not
necessarily hard to treat, but keeping
those who were infected from becoming
reinfected.

25. REFERENCES
• Centers for Disease Control and Prevention
1600 Clifton Rd. Atlanta, GA 30333, USA
• Garcia textbook of Medical parasitology