Parasites

Morphology

Cytology:

• Trophozoites 10-20 µm, pear- or kite-shaped
• Bilaterally symmetric - two nuclei with central karyosomes giving a characteristic "face-like" / "eyeball" appearance 
• The anterior portion is broad and rounded, whereas the posterior extremity gradually tapers to a pointed termination
• Ventral adhesive disk, paired axonemes and median bodies, eight flagella
• Characteristic falling-leaf motility in fresh wet mounts 
• Cysts 8-12 µm (up to 19 µm), oval to ellipsoid
  • mature forms with 4 nuclei and visible fibrils
  • thin wall, refractile
  • best visualized in fresh wet prep, iodine wet mount, trichrome, or iron hematoxylin stains

Histology: 

• Trophozoites adhere to the mucosal surface and between villi in the duodenum and proximal jejunum, without tissue invasion 
• Duodenal histology is often normal or shows minimal changes; when present, findings may include mild to moderate villous blunting, increased intraepithelial lymphocytes, crypt hyperplasia, and eosinophilic infiltration of lamina propria 
• Severe cases can mimic celiac disease

Differential Diagnosis:

• Balantidium coli
• Amebiasis
• Cryptosporidiosis
• Celiac disease

Clinical notes

Acute or chronic watery or greasy, foul-smelling diarrhea, bloating, flatulence, weight loss, malabsorption, steatorrhea; post-infectious lactose intolerance common; asymptomatic carriage occurs.
Rarely, Giardia can spread to the biliary and pancreatic ducts, leading to gallbladder, liver, and pancreatic complications (1).

Ancillary Testing

Stool antigen tests (EIA or DFA), multiplex PCR panels, stool ova and parasites (O&P) test on 2–3 specimens on different days, trichrome or iron hematoxylin stains, immunocyto-/-histochemistry: CD117 positive

Additional Information

Transmission is fecal-oral, primarily through contaminated water; direct person-to-person and foodborne spread also occur.
Cysts are infectious immediately upon excretion and environmentally hardy, remaining viable for at least 3 months in cold water. As few as 10–100 cysts can cause infection. Cysts are moderately chlorine-tolerant; boiling (≥1 minute) and appropriate filtration (≤3–5 µm pore size) are more reliable for inactivation than routine chlorination alone (2, 3).