Parasite load as a marker of pathogenicity in Dientamoeba fragilis infections

Dientamoeba fragilis is a globally widespread intestinal parasite and one of the most frequent in humans, often leading to primary care consultations. The pathogenic potential of this parasite remains unclear due to limited evidence and contradictory study results. This study investigated the pathogenicity of D fragilis, focusing on the critical knowledge gap regarding the relationship between parasite load and associated symptomatology. A prospective case-control study matched by household unit was conducted, considering individuals with gastrointestinal symptoms and D fragilis in stool as cases, and their asymptomatic household members with D fragilis in stools as controls. D fragilis detection was performed in parallel using light microscopy and real time-polymerase chain reaction. The study was carried out in 7 primary care centers over 12 months. Parasite load was measured as the number of trophozoites per field at a magnification of 40x by microscopy and by cycle threshold values in RT-PCR. A total of 218 individuals were recruited: 74 symptomatic cases and 144 household members, of whom 57 (39.6%) were D fragilis-positive asymptomatic controls. The proportion of individuals with D fragilis and a parasite load less than 1 trophozoite per field was higher in asymptomatic individuals (controls) than in symptomatic cases (47.7% vs 3.1%, respectively) (P < .001). Parasite load is associated with the presence of gastrointestinal symptoms, supporting the pathogenicity of D fragilis. Any diagnostic approach for D fragilis should incorporate or be complemented by quantitative information to accurately estimate parasite load and enhance treatment decision-making.