Giardia and Giardiasis Flashcards
Non-kinetoplastid flagellates
Retortamonadida
Diplomonadida
No mitochondrion or golgi apparatus.
Recurrent flagellum in a cytosomal groove and occupy anoxic environments.
Retortamonadida
Harmless commensal species.
Ex. Chiloastic misnill, Retotamonas intestinalis
Diplomonadida
Giardia duodenalis, human parasite.
Giardia duodenalis
Diplomonadida, hexamitidae
The most common flagellate of the human digestive tract.
AKA: G. lamblia and G. intestinalis.
Discovered by Antony Leeuwenhoek.
A cosmopolitan species, but most common in warm climates.
Children are more susceptible.
Morphology: Giardia duodenalis
12-15 um long; dorsoventrally flattened, rounded at anterior end and pointed at posterior end.
Concave bilobed adhesive disc on ventral surface; used along with a ventral flagella to adhere on intestinal wall.
The pair of ventral flagella, is found in a ventral groove. These flagella and three other flagella arise form kinetosomes (basal bodies) located between anterior portions of the two nuclei.
A pair of dark staining transverse median bodies lie behind the adhesive disc. These bodies are unique to Giardia, but with unknown function
Life cycle: Giardia duodenalis
- Infection occurs when a person swallows Giardia cysts from contaminated water, food, hands, surfaces, or objects.
- When Giardia cysts are swallowed, they pass through the mouth, esophagus, and stomach into the small intestine where each cyst releases two trophozoites through a process called excystation. The Giardia trophozoites then feed off and absorb nutrients from the infected person. The adhesive disc fits on the epithelia cells. In severe infection, the free surface of each cell of the intestine will be covered by a parasite.
- Giardia trophozoites multiply by splitting in two in a process called longitudinal binary fission, remaining in the small intestine where they can be free or attached to the inside lining of the small intestine. Three cell divisions are required before the parasite is mature. New Cysts have two nuclei. Older Cysts have 4 nuclei.
- The Giardia trophozoites then move toward the colon and transform back into cyst form through a process called encystation. Watery diarrhea releases trophozoites only, while more solid stools release the cysts. This process is induced by cholesterol depletion that triggers a Golgi-like complex to produce vesicles that contain the cyst wall materials.
- Both Giardia cysts and trophozoites can be found in the stool of someone who has giardiasis and may be observed microscopically to diagnose giardiasis. Giardia cysts are immediately infectious when passed in the stool or shortly afterward, and the cysts can survive several months in cold water or soil.
Metabolism: Giardia duodenalis
Aerotolerant anaerobe: Doesn’t use oxygen, but tolerates it.
No mitochondrion, but consumes oxygen if present.
The ends of metabolism are ethanol, acetate, and CO2.
All energy is produced by substrate level phosphorylation.
Pathogenesis: Giardia duodenalis
Evidence of protective immunity.
Dense coating of the parasite on the intestinal wall damages microvilli and interferes with absorption of fats and other nutrients.
The gall bladder can be infected, resulting in jaundice.
The disease is controlled by humoral immunity.
As in the case of trypanosomes, there is marked antigenic variation
Diagnosis: Giardia duodenalis
Recognition of trophozoites or cysts in stained fecal smears.
In some cases, when parasites are not released in stool, duodenal
aspiration is performed.
Serological tests may be used although they don’t distinguish past
and current infection.
Epidemiology: Giardia duodenalis
Giardiasis is highly contagious. When a member of a family becomes infected, others usually will be infected.
Prevalent throughout the world including the US.
Higher incidence in warm developing countries
Transmission depends on the swallowing of mature cysts.
Evidence of zoonosis also exists. Beavers, dogs, cats and sheep serve as reservoirs. Farm livestock, especially calves are commonly infected (up to 100%).
Of all animals, beavers have highest epidemiological significance in human giardiasis.
Zoonosis of Giardiasis has been linked to specific genotypes of the parasite.
Prevention and Control: Giardia duodenalis
Prevention is therefore through sanitation
Treatment: Giardia duodenalis
Treatment is with quinacrine or metronidazole.
It is recommended that all household occupants should be treated
simultaneously to avoid reinfection.
Symptoms: Giardia duodenalis
Some individuals notice increase in mucous production, diarrhea, dehydration, intestinal pain, flatulence and weight loss.
Stool is fatty but never contains blood.
The disease is not fatal but can be quite uncomfortable.
