Giardia lamblia/intestinalis/duodenalis is a flagellated protozoa occurring in temperate and tropical climates worldwide. It is the most common gastrointestinal parasitic disease in the United States, and although infections may be asymptomatic, it can cause chronic diarrhea, weight loss, anemia, and malabsorption in many if not treated (with metronidazole). The cyst, which is the infective form, is chlorine-resistant and may survive in water for months. It is transmitted via ingestion of parasite-contaminated food, water, or fecal-oral route. Although generally inhabiting the small bowel, stomach and colon sites have been reported as well.

Endoscopic appearance and histology of duodenal mucosa are usually unremarkable. Rarely, there may be mild to moderate villous blunting and increased lamina propria inflammatory cells (neutrophils, plasma cells, lymphocytes). The organisms are located on the luminal surface of the intestinal epithelium without tissue invasion and are pear-shaped and binucleated, distributed in a “falling leaves” pattern. They stain positively for CD117. Marked decrease/absence of lamina propria plasma cells in the context of giardiasis should raise concern for immunodeficiency (e.g. CVID).