Giardia duodenalis Colonization Slightly Affects Gut Microbiota and Hematological Parameters in Clinically Healthy Dogs

**Background:** Giardia duodenalis is a common gastrointestinal parasite in dogs, but its role in asymptomatic carriers remains debated. The host–parasite interaction may involve gut microbiota changes, though confounders such as diet, environment, co-infections, and clinical status have limited prior studies. This study aimed to investigate the impact of Giardia on gut microbiota and hematological/biochemical parameters in a homogeneous population of asymptomatic dogs.

**Methods:** Thirty-one German shepherd dogs from a single breeding and training facility were enrolled. All were clinically healthy, had no pharmacological therapy in the prior 2 months, and tested negative for other intestinal parasites by sedimentation-floatation. Giardia and Cryptosporidium were detected via immunofluorescence (Merifluor). Gut microbiota was analyzed by 16S rRNA gene amplicon sequencing (V3–V4 regions) on an Illumina MiSeq platform. Hematological parameters were measured with a Sysmex XN-1000V analyzer; serum biochemistry (including cobalamin, folate, cCRP) was performed on a Cobas c501/c601; fecal calprotectin was measured by ELISA. Statistical analyses included alpha diversity (Chao1, Shannon), beta diversity (UniFrac, PERMANOVA), volcano plots, and PLS-DA. Comparisons were made between Giardia-positive (GP, n=13) and Giardia-negative (GN, n=18) dogs, and stratified by sex (GPF, GPM, GNF, GNM).

**Key Results:** Giardia prevalence was 42% (8/19 females, 5/12 males). Positive dogs were significantly younger than negatives (median age 15–24 months vs. older; p=0.0003). No significant differences were found in alpha diversity (Chao1, Shannon) between GP and GN groups overall. Beta diversity was significantly different between GP and GN (p=0.0025) and between GNF and GPF (padj=0.02), but not between GNM and GPM. In males, Giardia-positive dogs showed reduced Gammaproteobacteria and increased Fusobacteria compared to negatives. PLS-DA identified differential abundance of several taxa: Erysipelotrichales and Lactobacillales were higher in Giardia-positive dogs; Clostridiales were higher in Giardia-negative males; Burkholderiales were higher in Giardia-negative females. All hematological and biochemical parameters were within normal ranges. Among females, Giardia-positive dogs had significantly lower triglycerides than negatives (p=0.0051). Among males, Giardia-positive dogs had higher cCRP than negatives (p=0.0725, at significance limit). Within positive dogs, females had significantly higher lipase (p=0.0429) and lower cCRP (p=0.0204) than males. No significant differences were found for cobalamin, folate, calprotectin, or alkaline phosphatase.

**Clinical Implications:** The findings suggest that in clinically healthy dogs, Giardia colonization is associated with subtle shifts in gut microbiota—particularly enrichment of taxa linked to anti-inflammatory effects (Lactobacillales) and lipid metabolism (Erysipelotrichales)—without causing substantial dysbiosis or overt disease. The mild elevation of cCRP in positive males may indicate a low-grade pro-inflammatory state, but all values remained within reference ranges. The authors argue that routine antiparasitic treatment of asymptomatic Giardia-positive dogs may be unnecessary and could disrupt a delicate host–microbiome–parasite equilibrium. Limitations include the small sample size and cross-sectional design, which may have prevented detection of additional differences.