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 1GastrointestinalMedium
A 19-year-old college student presents with 3 days of profuse watery diarrhea after returning from a camping trip where he drank from a stream. Stool ova and parasite exam shows pear-shaped flagellated trophozoites. He has no fever. What is the most likely diagnosis?
AGiardia lamblia
BCyclospora cayetanensis
CBalantidium coli
DEntamoeba histolytica
ECryptosporidium parvum
Reveal answer & full explanation
Correct answer: A — Giardia lamblia
AGiardia lamblia✓
BCyclospora cayetanensis
CBalantidium coli
DEntamoeba histolytica
ECryptosporidium parvum
Why Giardia lamblia is correct
Giardia lamblia is the most common intestinal parasite in the US
Transmission is fecal-oral via contaminated water (camping, travel)
Classic features: watery to greasy/foul-smelling diarrhea, bloating, flatulence, no fever, no bloody stools
Stool ova and parasite (O&P) exam shows pear-shaped binucleate trophozoites or cysts; stool antigen is more sensitive
Treatment: metronidazole 250 mg three times daily (TID) x 5-7 days, or tinidazole 2 g single dose (preferred)
Why the others are wrong
Cryptosporidium parvum — causes watery diarrhea primarily in immunocompromised patients and is treated with nitazoxanide
Entamoeba histolytica — causes bloody dysentery and liver abscess, not the nonbloody watery diarrhea seen here
Cyclospora cayetanensis — comes from contaminated produce and shows acid-fast oocysts, not flagellated trophozoites
Balantidium coli — like Giardia it is diagnosed by finding trophozoites on stool ova and parasite exam and can spread through fecally contaminated water, but its trophozoites are large, oval, and covered with cilia rather than pear-shaped and flagellated; infection is classically linked to pig exposure and can cause bloody dysentery
Question 2Infectious 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?
ACryptosporidium infection
BGiardia lamblia disease
CCampylobacter enteritis
DNorovirus gastroenteritis
EETEC traveler's diarrhea
Reveal answer & full explanation
Correct answer: B — Giardia lamblia disease
ACryptosporidium infection
BGiardia lamblia disease✓
CCampylobacter enteritis
DNorovirus gastroenteritis
EETEC traveler's diarrhea
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, and metronidazole and nitazoxanide are the other accepted first-line options.
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 — 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 — 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.
ETEC traveler's diarrhea — ETEC is the leading cause of traveler's diarrhea and spreads through contaminated food and water, but it causes an acute, watery, self-limited diarrhea that usually resolves within a few days; it does not cause 2 weeks of greasy, foul-smelling stools with bloating and weight loss.
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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.
Educational use only. This outline is a study aid for PA students and is not medical advice or a substitute for clinical judgment. FirstPassPA is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCCPA or PAEA. PANCE® and PANRE® are registered trademarks of the National Commission on Certification of Physician Assistants; End of Rotation™ is a program of the Physician Assistant Education Association.