Infectious Disease · Gastrointestinal Infections
The facts most likely to be tested
Giardiasis is caused by the protozoan Giardia duodenalis (also known as *G. lamblia* or *G. intestinalis*) which is transmitted via the fecal-oral route through contaminated mountain stream water or well water.
Patients typically present with foul-smelling, greasy, floating stools known as steatorrhea due to malabsorption of fats.
The classic clinical presentation includes bloating, flatulence, abdominal cramps, and watery diarrhea without blood or mucus.
The gold standard for diagnosis is the identification of trophozoites (pear-shaped with two nuclei) or cysts in the stool ova and parasite (O&P) exam.
Stool antigen testing (via ELISA or DFA) is highly sensitive and specific and is often the preferred initial diagnostic test in clinical practice.
The pathophysiology involves the parasite adhering to the duodenal and jejunal mucosa, causing villous atrophy and subsequent malabsorption.
The first-line treatment for symptomatic giardiasis is tinidazole or metronidazole.
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A 28-year-old hiker presents to the clinic complaining of 2 weeks of bloating, flatulence, and foul-smelling, greasy stools that float in the toilet. He reports drinking unfiltered water from a mountain stream during a backpacking trip in the Rockies 10 days ago. He denies fever, blood in his stool, or recent travel abroad. Physical examination reveals mild diffuse abdominal tenderness without guarding or rebound.
What is the most appropriate initial diagnostic test to confirm the suspected diagnosis?
Stool antigen testing
The patient's history of drinking stream water and symptoms of steatorrhea are classic for Giardiasis; stool antigen testing is the most sensitive and commonly used diagnostic modality.
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Etiology / Epidemiology
Caused by Giardia duodenalis; transmitted via fecal-oral route from contaminated mountain stream water or daycare centers.
Clinical Manifestations
Presents with greasy, foul-smelling stools (steatorrhea) and flatulence; classic malabsorption syndrome.
Diagnosis
Stool antigen assay is the gold standard; microscopic exam shows trophozoites or cysts.
Treatment
Tinidazole is the first-line treatment; avoid alcohol during therapy.
Prognosis
Self-limiting but can cause chronic malabsorption and weight loss if untreated.
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Epidemiology & Etiology
Commonly known as beaver fever, this protozoan infection is endemic in wilderness water sources. It is highly prevalent in daycare centers due to poor hygiene. Infection occurs via ingestion of cysts in contaminated water or food.
Pertinent Anatomy
The parasite colonizes the proximal small intestine (duodenum and jejunum). It adheres to the intestinal villi, causing blunting and mechanical obstruction of nutrient absorption.
Pathophysiology
Ingested cysts excyst in the stomach acid to release trophozoites. These attach to the brush border, leading to malabsorption of fats and carbohydrates. The resulting osmotic diarrhea and steatorrhea are hallmark features.
Clinical Manifestations
Patients report bloating, flatulence, and greasy, foul-smelling stools that float. Symptoms typically appear 1-3 weeks post-exposure. Red flags include significant weight loss and failure to thrive in pediatric populations.
Diagnosis
The stool antigen assay (ELISA) is the most sensitive and specific diagnostic tool. Microscopic examination of stool for trophozoites or cysts may require multiple samples. Entero-test (string test) is rarely used but historically significant.
Treatment
Tinidazole is the preferred first-line agent due to a single-dose regimen. Metronidazole is an alternative, though it requires a longer course. Disulfiram-like reaction occurs if combined with alcohol; teratogenic potential in the first trimester.
Prognosis
Most cases resolve spontaneously, but chronic infection leads to vitamin deficiencies and growth retardation. Patients should be monitored for resolution of symptoms; repeat testing is rarely required unless symptoms persist.
Differential Diagnosis
Cryptosporidiosis: usually associated with immunocompromised states
Lactose intolerance: lacks the infectious history and foul-smelling stool
Celiac disease: chronic condition with villous atrophy, not acute infection
Traveler's diarrhea: typically bacterial (ETEC) with shorter incubation
Whipple disease: systemic symptoms like joint pain and lymphadenopathy