Prevalence of Giardia duodenalis in Stray Dogs in Istanbul and Diagnostic Comparison of Microscopy versus qPCR

Giardia duodenalis is a common intestinal protozoan parasite affecting dogs and posing One Health concerns. We conducted a cross-sectional study of 2,485 fecal samples collected from stray dogs in Ümraniye, Istanbul, to determine Giardia prevalence and compare the performance of conventional microscopy (direct smear and zinc sulfate flotation) with a real-time PCR (qPCR) assay targeting the Giardia 5.8S rRNA gene. DNA was extracted using magnetic beads and amplified in a closed-tube qPCR assay. Using qPCR as the reference standard, we calculated sensitivity, specificity, and predictive values for each microscopy method and measured inter-test agreement (Cohen’s κ). Real-time PCR detected Giardia DNA in 1,097 of 2,485 samples (44.1%). In contrast, microscopy identified only 298 positives (12.0%) by direct smear and 401 (16.1%) by ZnSO₄ flotation. Direct smear showed sensitivity 25.5%, specificity 98.7%, positive predictive value (PPV) 94.0%, and negative predictive value (NPV) 62.6%. ZnSO₄ flotation had sensitivity 34.6%, specificity 98.5%, PPV 94.8%, and NPV 65.6%. Combining both microscopy methods (positive by either) increased sensitivity slightly to 35.6%. Cohen’s κ indicated moderate agreement between each microscopy method and PCR. These results confirm that PCR greatly increases detection of Giardia in canine feces, whereas conventional microscopy though highly specific misses most infections when only a single sample is examined. Our findings highlight the utility of molecular diagnostics for surveillance and control of canine giardiasis in urban settings.