FINAL DIAGNOSIS GIARDIASIS
This case illustrates how giardiasis can be present in patients with no suspicion of the disease. Giardiasis is an infection of the small intestine caused by the flagellated protozoan Giardia lamblia. Giardia lamblia has the distinction of being the first microscopic organism to be recognized as a cause of infection, when Van Leeuwenhoek had diarrhea, examined his own stool with a primitive microscope and described the organisms in 1681. Giardiasis is present in all climates, but is especially common in developing countries with tropical or subtropical climates. Giardiasis is very common. There are estimated to be 2.5 million cases of giardiasis per year in the United States and Giardia lamblia can be identified in approximately 4% of stool specimens submitted to laboratories in the United States.
Giardiasis has three routes of transmission: water-borne (particularly in contaminated community water supplies), direct fecal-oral and food-borne. The cyst form of the organism is resistant to chlorine disinfection and generally must be mechanically filtered out of water supplies. As few as 10 cysts are capable of causing infection. Direct fecal-oral transmission can occur in day care centers for infants and small children. Food-borne outbreaks can occur in restaurants and picnics where salads and other uncooked foods are served. Giardiasis occurs year-round, but has a peak incidence in late summer and early fall.
The life cycle of Giardia lamblia consists of two stages: trophozoite and cyst. Infection occurs after oral ingestion of cysts. Each cyst gives rise to two trophozoites during excystation in the intestinal tract. After excystation, trophozoites colonize and multiply in the upper small intestine. The trophozoite is 9 to 21 microns in length, 5 to 15 microns in width, motile, flagellated and pear-shaped, with two nuclei, two claw-shaped median bodies crossing the middle, 4 pairs of flagella, and a large suction disk with which it attaches to the intestinal wall (Image 4). The oval cyst is thick-walled, measuring 8 to 12 microns in length and 7 to 10 microns in width, with four nuclei and several internal fibers (Image 6). Host immunity is humoral and cellular, including lymphocyte- and macrophage-mediated mechanisms.
Histologically, the small intestinal mucosa is usually intact, but there may be blunting of villi and an increased number of inflammatory cells in the lamina propria. The trophozoites can be seen along the epithelial surface. The organism is approximately the same size as the enterocyte nucleus; it has a characteristic pear shape, with a tapered posterior region. The trophozoite has prominent paired nuclei, paired median bodies and four pairs of flagellae. Electron microscopic examination of epithelial cells beneath adherent trophozoites shows deformation and blunting of the individual microvilli.
Giardia infection is asymptomatic in 60% of patients. The most common symptoms of giardiasis are diarrhea (present in 90% of symptomatic patients), malaise (86%), flatulence (75%), foul-smelling greasy stools (75%), abdominal cramps (71%), bloating (71%), nausea (69%), anorexia (66%) and weight loss (66%). Vomiting, fever and grossly blood stool are usually absent. Patients with giardiasis who develop chronic diarrhea may have profound malaise, weight loss and malabsorption with greasy and foul-smelling or frothy yellowish stools. Giardiasis may have been the cause of B12 and iron deficiency in this case.
Stool examination for trophozoites or cysts is a common method of diagnosis. Because of intermittent shedding of the organisms in stool, at least three stool specimens, collected every other day, should be examined. Stool can be examined fresh or after preservation. A saline wet mount of fresh liquid stool in the acute stages of disease may show trophozoites, which have a characteristic motility resembling a falling leaf. Radiologic study using barium can show increased transit time and irregular thickening of small bowel mucosal folds, but these findings are non-specific and the barium can interfere with microscopic examination of the stool (Image 5). In semi-formed stool, trophozoites are usually not found, and the stool should be examined fresh for cysts.
If no organisms are seen in the stool, a string test sampling of duodenal contents may yield the diagnosis. One commercially available form of the string for this test has a gelatin capsule at the end, which the patient swallows. The free end of the string is secured at the mouth. After incubation from four hours to overnight, the string is retracted and the bile-stained mucus on the distal end squeezed onto a slide for microscopic examination and diagnosis.
Duodenal biopsy may yield the diagnosis. An advantage of biopsy, particularly in patients with human immunodeficiency virus infection or malabsorption, is the ability to diagnose alternative conditions in the differential diagnosis, such as Whipple's disease.
Antigen detection assays using species-specific antibody reagents are now available. These commercial assays use either ELISA or direct fluorescence antibody techniques and are over 90% sensitive and nearly 100% specific. These tests are more sensitive than the standard "O&P" (ova and parasite) examination of stool and have become the test of choice.
Contributed by Xiaoyan Wang, MD, PhD, Wen-Wei Chung, MD, PhD, Larry Nichols, MD, and Jeannette Dunn, MD