Giardia lamblia (Giardia intestinalis, Giardia duodenalis) : Morphology, Life Cycle, Cyst, Diseases, Symptoms, & Treatment
- Giardia lamblia (Giardia intestinalis, Giardia duodenalis) is a flagellate protozoan.
- It is the most common intestinal protozoan pathogen.
- Giardia lamblia lives in the duodenum and upper jejunum and is the only protozoan parasite found in the lumen of the human small intestine.
History
- This flagellate was observed by Leeuwenhoek (1681) in his own stools.
- It is named Giardia after Professor Giard of Paris and lamblia after Professor Lambl of Prague who gave a detailed description of the parasite. Infection may be asymptomatic
or cause diarrhoea.
Geographical Distribution
- Worldwide in distribution.
Epidemiology
- The infection is worldwide, especially in children.
- Endemicity is very high in some areas.
- Enhanced susceptibility to giardiasis is associated with blood group A, achlorhydria, use of cannabis, chronic pancreatitis, malnutrition and immune defects such as 19A deficiency and hypogammaglobulinaemia.
- HIV infection has not apparently been associated with increased risk of giardiasis.
- Cats, dogs, cattle, sheep and many wild animals have been found naturally infected.
Morphology
Giardia lamblia occurs in the vegetative and cystic forms.
1. Trophozoite (vegetative form)
- Configuration : Rounded anteriorly and pointed posteriorly,
- Size : About 15 μm long, 9 μm wide and 4 μm thick.
- Shape : Variously such as pyriform, heart-shaped or racket-shaped.
- Dorsally it is convex and ventrally it has a concave sucking disc which occupies almost the entire anterior half of the body.
- Bilaterally symmetrical and possesses 2 nuclei, one on either side of the midline, two axostyles running along the midline, 4 pairs of flagella and 2 sausage shaped parabasal or median bodies lying transversely posterior to the sucking disc.
- Motility : Motile, with a slow oscillation about its long axis, which has been likened to the motion of a ‘falling leaf.’
- Division : Longitudinal binary fission.
- It lives in the duodenum and upper part of the jejunum attached by means of the sucking disc to the epithelial cells of the villi and crypts feeding by pinocytosis.
2. Cyst (cystic form)
- Shape : Ovoid
- Size : About 12 μm by 8 μm
- Surrounded by a tough hyaline cyst wall.
- The young cyst contains two and the mature cyst four nuclei situated at one end.
Life Cycle
Cysts → Passed in stools → Ingestion of cysts in contaminated food and water → Cyst hatches out into two trophozoites → Trophozoites divides successively by binary fission → Trophozoites as they pass down the colon develop into cysts.
- Cysts remain viable in soil and water for several weeks.
- There may be up to 2,00,000 cysts present per gram of faeces.
- In diarrhoeic stools trophozoites also may be present, but they die outside and are not infectious.
Encystation occurs in the colon. The trophozoite retracts its flagella into the axonemes which remain as curved bristles in the cyst.
Mode of Infection
- Infection is acquired by the ingestion of cysts in contaminated food and water. Infectivity is high, as few as 10 cysts being capable of initiating infection.
- Direct person-to-person transmission may also occur in children, male homosexuals and the mentally-ill.
Pathogenesis
- Giardia lamblia is seen typically within the crypts in the duodenum. It does not invade tissues, but remains tightly attached by means of the sucking disc to the epithelial surface in the duodenum and jejunum. This may cause abnormalities of villous architecture.
- It has been suggested that enormous numbers of the parasite adhering to the mucosal surface of the small intestine may interfere with absorption. Increased bacterial colonisation of the small intestine has been observed in subjects with giardiasis and steatorrhoea.
- Occasionally giardia may colonise the gallbladder, causing biliary colic and jaundice.
Clinical Features
The incubation period is variable, but is usually about 2 weeks.
- Often no clinical illness results, but in some abnormalities of villous architecture may lead to mucus diarrhoea, dull epigastric pain and flatulence.
- The diarrhoea in some cases may be steatorrhoeic with excess mucus and fat, but no blood.
- Children may develop chronic diarrhoea, malabsorption, weight loss and a sprue-like syndrome.
Diagnosis
- The cysts and trophozoites can be found in diarrhoeal stools.
- Only the cysts are seen in asymptomatic carriers.
- When the cysts are sparse, concentration by formalin ethyl acetate or zinc sulphate centrifugal floatation is useful.
- Entero test
- ELISA and immunochromatographic strip tests : Routinly not used.
- Antibody demonstration is not useful in diagnosis.
Prophylaxis
- Like other faecal-oral infections, prevention is by better personal hygiene and prevention of food and water contamination. Iodine is effective in disinfecting drinking water.
Treatment
Only symptomatic cases need treatment.
- Drugs of choice : Metronidazole, Tinidazole
- Furazolidone is slower in action, but is preferred in children as it has fewer adverse effects.
