Giardia

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Giardia lamblia (Giardia intestinalis) Compiled by PB Wadigamangawa

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Introduction Background Giardia lamblia was originally identified by von Leeuwenhoek in the 1600s and was first recognized in human stool by Vilem Dusan Lambl (1824-1895) in 1859 and by Alfred Giard (1846-1908) after whom it is named. Although it was the first protozoan parasite described, its role as a pathogenic organism was not recognized until the 1970s, after community outbreaks and after the appearance of the disease in travelers returning from endemic regions. Prior to that time, the organism was thought to be a harmless commensal organism of the intestine. Giardia lamblia

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Giardiasis is the most prevalent protozoal infection of the human intestine. G lamblia is one of the most common causative agents of epidemic and endemic diarrheal illness throughout the world. This organism has a worldwide distribution and is a major cause of epidemic childhood diarrhea in developing countries. Prevalence rates vary from 4-42%.

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Pathophysiology Giardia has one of the simplest life cycles of all human parasites. The life cycle is composed of 2 stages, as follows: (1) the trophozoite which exists freely in the human small intestine, and (2) the cyst, which is passed into the environment. No intermediate hosts are required. Upon ingestion of the cyst contained in contaminated water or food, excystation occurs in the stomach and the duodenum in the presence of acid and pancreatic enzymes. The trophozoites pass into the small bowel where they multiply rapidly, with a doubling time of 9-12 hours. As trophozoites pass into the large bowel, encystation occurs in the presence of neutral pH and secondary bile salts. Cysts are passed into the environment, and the cycle is repeated.

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The trophozoite form of G lamblia is teardrop-shaped and measures 9-21 micrometers long by 5-15 micrometers wide. The trophozoite has a convex dorsal surface and a flat ventral surface that contains the ventral disk or the sucker, a rigid cytoskeleton axostyle, composed of microtubules and microribbons. The trophozoite also contains 4 pairs of flagella, directed posteriorly, that aid the parasite in moving. Two symmetric nuclei with prominent karyosomes produce the characteristic facelike image that appears on stained preparations.

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The cyst form of the protozoan is smooth-walled and oval in shape, measuring 8-12 micrometers long by 7-10 micrometers wide. As the cyst matures, nuclear division occurs and readies the cyst to release 2 trophozoites upon excystation. Once the host is infected, trophozoites may appear in the duodenum within minutes.7 Excystation occurs within 5 minutes of exposure of the cysts to an environment with a pH between 1.3 and 2.7.

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After infection, the trophozoites attach to the enterocytes via the ventral adhesive disk. This may occur through the presence of lectin on the surface of the trophozoite or through other mechanical means. Encystation is a continuous process during infection. As the trophozoites encounter neutral pH and/or secondary bile salts, encystation-specific secretory vesicles (ESVs) appear. After 15 hours, cyst wall proteins are visible. Within 24 hours after the appearance of ESVs, the trophozoite is covered with these cyst wall proteins, the form of the cyst has emerged, and new antigenic differences are present.

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Mode of transmission The infectious load required to produce disease may be as low as 10 cysts. Subjects become infected through ingestion of infectious G lamblia cysts by the following ways Contaminated water supplies: Giardia is one of the most common causes of water-borne diarrhea outbreaks. Sources of contaminated water include public facilities that improperly filter and treat water, water in developing countries, or rivers and lakes used by hikers. Travelers and hikers are at a high risk for infection. Contaminated food: Food that may have been washed in contaminated water, exposed to manure, or prepared by an infected person can transmit the disease Person-to-person contact: Infection may be caused by poor hygiene and most commonly occurs in daycare centers, nursing homes, and in sexually active homosexual males. Up to 50% of children infected with Giardia in daycare centers, and up to 20% of infected sexually active homosexual males, pass cysts in their stool. Family members, daycare workers, and others in contact with infected stool may then themselves become infected. The protozoan is known to have multiple strains with varying abilities to cause disease, and several different strains may be found in one host during infection.

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Mortality/Morbidity Most infected subjects are asymptomatic, and most infections are self-limited. However, chronic infections, marked by chronic diarrhea/steatorrhea and malabsorption, can occur and can last from weeks to months. Death is rare and usually occurs in malnourished children. G lamblia has been implicated as the chief cause of growth retardation in infected children, even after control of other agents that cause diarrhea.

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Approximately 15% of cases of giardiasis are asymptomatic, with cyst passage only. Approximately 50% of patients infected with Giardia may present with a variety of symptoms, including acute watery diarrhoea, chronic diarrhoea with malabsorption and weight loss, and abdominal cramping. Acute diarrhoea is the most common symptom of Giardia infection, occurring in 90% of symptomatic subjects. Steatorrhoea, vitamin B-12, vitamin A, protein, and D-xylose malabsorption all have been documented in patients with chronic infection.

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Lab diagnosis Examination of stool The most common way to diagnose giardiasis is by the visualization of the Giardia cysts by experienced professionals. The cysts are detected 50-70% of the time in the first stool specimen examined. Over 90% of the time the cysts are detected after 3 stool specimens have been examined. So more than 1 specimen may be required. Another method of diagnosis that is commonly used as a screening tool in outbreaks or in daycare centers is antigen assay of stool. This method detects a certain protein found in the wall of Giardia. A stool sample is mixed with a solution that detects the cysts in the stool. String test The string test involves swallowing a fuzzy string enclosed in a gelatin capsule. The free end of the string is taped to the person's cheek. Once swallowed, the string collects secretions and mucus from the small bowel. Four hours later the string is pulled back out and examined for organisms. Aspiration and biopsy This is the most invasive method of diagnosis. After passage of a small telescope through the person's mouth and stomach into the small bowel, the doctor removes a small amount of tissue for examination. This method is reserved for difficult cases in which the cause of diarrhea cannot be determined using other methods. It allows for visualization of all abnormalities of the small intestine, which may cause diarrhoea including giardiasis.

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