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Pathogenic flagellates: Giardia lamblia, causative agent for Giardiasis



Flagellates are unicellular microorganisms. Their locomotion is by lashing a tail-like appendage called a flagellum or flagella and reproduction is by simple binary fission.

There are three groups of flagellates:

• Luminal flagellates: Giardia lamblia, Dientmoeabfragilis
• Hemoflagellates:  Trypanosoma species, Leishmania species.
• Genital flagellates:  Trichomonas vaginalis

Luminal flagellates: Giardia lamblia

The life cycle consists of two stages, the trophozoite and cyst. The trophozoite is 9-12 μm long and 5-15μm wide anteriorly. It is bilaterally symmetrical, pear-shaped with two nuclei (large central karyosome), four pairs of flagella, two axonemes, and a suction disc with which it attaches to the intestinal wall. The oval cyst is 8-12μm long and7-10μm wide, thick-walled with four nucleus and several internal fibera. Each cyst gives rise to two trophozoites during excystation in the intestinal tract.

Transmission is by ingestion of the ineffective cyst

The life cycle consists of two stages, the trophozoite and cyst. The trophozoite is 9-12 μm long and 5-15μm wide anteriorly

Pathogenesis

Infection with G.lamblia is initiated by ingestion of cysts. Gastric acid stimulates excystation, with the release of trophozoites in duodenum and jejunum. The trophozoites can attach to the intestinal villi by the ventral sucking discs without penetration of the mucosa lining, but they only feed on the mucous secretions. In symptomatic patients, however, mucosa-lining irritation may cause increased mucous secretion and dehydration. Metastatic spread of disease beyond the GIT is very rare.



Epidemiology

Giardia lamblia has a worldwide distribution, particularly common in the tropics and subtropics. It is acquired through the consumption of inadequately treated contaminated water, ingestion of contaminated uncooked vegetables or fruits, or person-to-person spread by the faecal-oral route. The cyst stage is resistant to chlorine in concentrations used in most water treatment facilities. Infection exists in 50% of symptomatic carriage, and reserves the infection in endemic form.

Clinical disease: Giardiasis

Symptomatic giardiasis ranges from mild diarrhea to severe malabsorption syndrome. Usually, the onset of the disease is sudden and consists of foul smelling, watery diarrhea, abdominal cramps, flatulence, and streatorrhoea. Blood and pus are rarely present in stool specimens, a feature consistent with the absence of tissue destruction.

Immunity

The humoral immune response and the cellular immune mechanism are involved ingiardiasis. Giardia – specific IgA is particularly important in both defense against andclearance of parasite.

Laboratory diagnosis

Examination of diarrhoeal stool- trophozoite or cyst, or both may be recovered in wet preparation. In examinations of formed stool (e.g. in asymptomatic carriers) only cysts are seen. Giardia species may occur in “showers”, i.e. many organisms may be present in the stool on a given day and few or none may be detected the next day. Therefore one stool specimen per day for 3 days is important.

If microscopic examination of the stool is negative in a patient in whom giardiasis is highly suspected duodenal aspiration, string test (entero-test), or biopsy of the upper small intestine can be examined. In addition to conventional microscopy, several immunologic tests can be implemented for the detection of parasitic antigens.

Treatment

For asymptomatic carriers and diseased patients the drug of choice is quinacrinehydrochloride or metronidazole.



Prevention

- Asymptomatic reservoirs of infection should be identified and treated.
- Avoidance of contaminated food and water.
- Drinking water from lakesand streams should be boiled, filtered and/or iodinetreated.
- Proper waste disposal and use of latrine.
Pathogenic flagellates: Giardia lamblia, causative agent for Giardiasis Pathogenic flagellates: Giardia lamblia, causative agent for Giardiasis Reviewed by gafacom on November 03, 2019 Rating: 5

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