Objective: Gastrointestinal parasitosis is a significant cause of morbidity and mortality. Definitive diagnosis is usually made by stool tests and/or serology but may require tissue evaluation. Although pathologists are usually familiar with common parasites, it is not well established whether the diagnosis could be suspected without seeing the “parasite” itself.

Material and Method: Resection or biopsy specimens of 32 cases with Giardia intestinalis (n=20), Enterobius vermicularis (n=5), Entamoeba histolytica (n=4), Fasciola hepatica (n=1), Strongyloides spp. (n=1) and Taenia saginata (n=1) infections were retrospectively re-evaluated for accompanying mucosal changes, and compared with nonparametric tests.

Results: The most common changes were congestion (65.6%) and eosinophilic infiltration (50%). Chronic active mucosal inflammation accompanied 37.5% of the cases. More than 10 eosinophils/HPF were present in 43.8%. Only one case of G. intestinalis, E. vermicularis, E. histolytica, and F. hepatica showed more than 50 eosinophils/HPF. Mucosal architectural abnormalities were present in 34.4%. Granulomas, giant cells and Charcot-Leyden crystals were only seen accompanying F. hepatica. No statistically significant difference was found between parasite subspecies regarding presence of inflammation, lymphoid aggregates, architectural distortion, congestion, ulceration and increase of eosinophils.

Conclusion: Parasites induce nonspecific inflammation, slight mucosal architectural changes, mild eosinophilic infiltrate or granuloma formation. They may cause ulceration, bowel obstruction or perforation. Parasitosis should also be considered when evaluating cases mimicking inflammatory bowel disease, celiac disease or those that do not fulfill diagnostic criteria.