Microbiology · Year 2 · from Microbiology
Case 2: Giardiasis
Presentation
A 28-year-old man presents with 3 weeks of bloating, foul-smelling diarrhea, and excessive flatulence. He recently returned from a backpacking trip in Colorado where he drank water from mountain streams. His stools are pale, greasy, and float. He has lost 8 pounds. He denies fever, blood in stool, or tenesmus. Physical examination is notable for diffuse abdominal distension and hyperactive bowel sounds. Stool ova and parasite examination reveals oval cysts with multiple nuclei.
Clinical Image
Microscopic appearance of Giardia lamblia: (A) Trophozoite showing characteristic "face-like" appearance with two nuclei and flagella, (B) Cyst form with multiple nuclei seen on stool O&P examination.
Image Source: Lecture image - Giardia morphology
Questions
- What is the diagnosis, and how was this infection acquired?
- Why does this organism cause steatorrhea and malabsorption?
- What diagnostic tests are available for this infection?
- What is the appropriate treatment?
Answers
- Diagnosis and transmission: This is giardiasis caused by Giardia lamblia (also called G. intestinalis or G. duodenalis). The infection was acquired through consumption of fecally contaminated water from mountain streams - the classic "hiker's diarrhea" or "beaver fever." Giardia cysts are hardy and survive in cold mountain water; beavers and other animals serve as reservoirs. Giardia is also a common cause of waterborne outbreaks and daycare-associated diarrhea.
- Mechanism of steatorrhea and malabsorption: Giardia causes malabsorption through multiple mechanisms:
- Mechanical blockade: Trophozoites attach to the duodenal and jejunal brush border via a ventral sucking disk, physically covering the absorptive surface
- Microvillus damage: Attachment causes blunting and shortening of microvilli
- Brush border enzyme deficiency: Disruption of lactase and other enzymes causes secondary disaccharidase deficiencies
- Fat malabsorption: Results in steatorrhea (fatty, foul-smelling, floating stools)
- Notably, there is no mucosal invasion, hence no blood or leukocytes in stool
- Diagnostic tests:
- Stool antigen testing (EIA): Most commonly used; sensitive and specific
- Stool O&P examination: Identifies cysts (and occasionally trophozoites); may require multiple samples due to intermittent shedding
- Stool PCR: Highly sensitive
- String test (Enterotest) or duodenal aspirate: Rarely needed, can identify trophozoites
- Stool antigen testing has largely replaced microscopy as first-line
- Treatment:
- First-line: Metronidazole 250 mg three times daily for 5-7 days (or tinidazole single dose)
- Alternative: Nitazoxanide (especially for children)
- Pregnancy: Paromomycin (non-absorbable aminoglycoside)
- Treatment failure may occur; retreatment with same or different agent
- Lactose intolerance may persist for weeks after treatment; temporary dairy avoidance may help
- Household contacts with symptoms should be tested and treated