Protozoan small bowel infection (Giardia duodenalis) causing prolonged greasy diarrhea, bloating, and malabsorption after exposure to contaminated water.
Also known as: giardiasis, Giardia lamblia, Giardia intestinalis, Giardia duodenalis, beaver fever
Overview
Diarrheal illness caused by Giardia duodenalis (also called G. lamblia or G. intestinalis), a flagellated binucleate protozoan that colonizes the proximal small intestine without invading tissue. Most commonly reported intestinal parasite in the US.
Epidemiology
Roughly 1.2 million US cases annually (CDC estimates). Peaks in summer/fall. Outbreaks linked to recreational water exposure (lakes, pools, hot tubs) and untreated drinking water; daycare and MSM populations also at risk.
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Question 1Infectious DiseaseEasy
A 28-year-old man presents with 2 weeks of greasy, foul-smelling, non-bloody diarrhea, abdominal bloating, excessive flatulence, and a 4-lb weight loss. He returned from a backpacking trip 3 weeks ago, during which he drank from mountain streams without filtering the water. He denies fever and reports no blood in his stool. On examination he is afebrile with mild diffuse abdominal tenderness and no peritoneal signs. Which of the following is the most likely diagnosis?
ANorovirus gastroenteritis
BGiardia lamblia disease
CCampylobacter enteritis
DCryptosporidium infection
Reveal answer & full explanation
Correct answer: B — Giardia lamblia disease
ANorovirus gastroenteritis
BGiardia lamblia disease✓
CCampylobacter enteritis
DCryptosporidium infection
Why Giardia lamblia disease is correct
The triad of subacute (1-2 week) onset, greasy/foul-smelling/steatorrheic non-bloody diarrhea, and bloating with flatulence and weight loss is the classic giardiasis syndrome.
Untreated surface water during backpacking is the hallmark exposure; Giardia duodenalis cysts are ingested, excyst in the duodenum, and trophozoites attach to small bowel mucosa causing villous blunting and malabsorption (no tissue invasion, so no fever or bloody stool).
Preferred test is a stool antigen EIA (sensitivity 85-100%); first-line treatment is single-dose tinidazole 2 g PO, with metronidazole or nitazoxanide as alternatives.
Why the others are wrong
Cryptosporidium infection also causes diarrhea after recreational or untreated-water exposure, but it produces watery (not classically greasy/steatorrheic) diarrhea, is most severe and protracted in immunocompromised hosts, and is diagnosed by acid-fast oocysts in stool.
Norovirus gastroenteritis is a viral illness with prominent vomiting and a short, self-limited course of under 72 hours; it does not produce 2 weeks of steatorrhea and weight loss.
Campylobacter enteritis is a bacterial gastroenteritis that typically presents acutely with fever and often bloody/inflammatory diarrhea diagnosed by stool culture; the afebrile, non-bloody, malabsorptive picture here argues against it.
Question 2Infectious DiseaseMedium
A 29-year-old man presents with 2 weeks of foul-smelling, greasy diarrhea, bloating, excessive flatulence, and a 4-lb weight loss. He returned 3 weeks ago from a backpacking trip during which he drank from mountain streams. He reports no fever and no blood in the stool. Vital signs are normal. The abdomen is mildly diffusely tender without guarding or rebound. Which of the following is the most appropriate next diagnostic test?
AStool bacterial pathogen culture
BStool Giardia antigen immunoassay
CStool fecal leukocyte (lactoferrin)
DColonoscopy with random biopsies
Reveal answer & full explanation
Correct answer: B — Stool Giardia antigen immunoassay
AStool bacterial pathogen culture
BStool Giardia antigen immunoassay✓
CStool fecal leukocyte (lactoferrin)
DColonoscopy with random biopsies
Why Stool Giardia antigen immunoassay is correct
The vignette is classic giardiasis: a backpacker who drank untreated surface water now has subacute (about 2 weeks of) foul-smelling, greasy (steatorrheic), non-bloody diarrhea with bloating, flatulence, and weight loss, and no fever, a malabsorptive, non-inflammatory picture.
Stool antigen detection (EIA or rapid immunoassay) is the preferred first-line test, with sensitivity of roughly 85-100%, far better than a single ova-and-parasite exam because Giardia is shed intermittently.
IDSA and CDC guidance favor stool antigen or stool nucleic acid amplification (multiplex GI PCR panels) as the initial diagnostic approach when giardiasis is suspected.
Why the others are wrong
Stool bacterial pathogen culture is appropriate for acute, febrile, or bloody (inflammatory) diarrhea suggesting Campylobacter, Salmonella, or Shigella; this patient has a non-bloody, afebrile, malabsorptive course, and culture will not detect a protozoan.
Stool fecal leukocyte (lactoferrin) testing is a marker of inflammatory or invasive colitis; Giardia is non-invasive and does not cause fecal leukocytes, so the result would be negative and non-discriminating.
Colonoscopy with random biopsies is low-yield because Giardia infects the proximal small bowel, not the colon, and a noninvasive stool test should come first.
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Drinking untreated surface water (camping, backpacking, hiking)
International travel to endemic regions
Daycare attendance, household contact with infected child
Men who have sex with men (oral-anal contact)
Immunocompromise (HIV, CVID, IgA deficiency) — chronic infection
Inadequate water sanitation
Pathophysiology
Cysts are ingested via contaminated water/food and excyst in the duodenum to release trophozoites. Trophozoites attach to small bowel mucosa via a ventral adhesive disc, causing villous blunting, brush border enzyme reduction, and malabsorption. Some trophozoites encyst and pass in stool as the infectious form. No invasive disease.
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