PSV-22 Characterization of the gut microbiota of horses based on two different housing conditions and parasitic load.

Abstract Strongyles are parasitic nematodes affecting the large intestine and cecum of equines, leading to gastrointestinal disturbances, including colic and impaired nutrient absorption due to mucosal damage and inflammation. Effective parasite management is increasingly challenging due to widespread resistance to anthelmintic treatments, necessitating alternative strategies. Recent research suggests a complex interplay between gut microbiota and parasitic infections, influenced by factors such as diet, immune response, and housing conditions. Gut microbiome modulation has emerged as a potential alternative strategy for controlling strongyles, as specific microbial communities may enhance parasite resistance and mitigate infection severity. The objective of the study was to investigate the impact of housing conditions (pasture vs. semi-intensive) on gut microbiota composition and diversity in horses with varying strongyle fecal egg counts (FEC) and to explore potential links between microbial communities and parasite resistance. Twenty-five horses were divided into two housing conditions: pasture (n=13) and semi-intensive (SI; n=12). Within each group, horses were classified as low or high shedders based on strongyle egg counts (< 300 eggs/g = low, >300 eggs/g = high). A factorial design was used. Fecal samples were collected in the fall for flotation analysis and gut microbiota assessment. Sequence processing was performed using Mothur 1.47.0, with downstream analysis in R 3.2.7. Significant differences (P < 0.05) in beta diversity metrics (Bray-Curtis, Jaccard Index, and Theta YC) were observed between pastured and SI-housed horses, indicating distinct microbial community structures influenced by housing conditions. Heatmaps and log2 fold-change analysis revealed a higher abundance (P < 0.05) of Firmicutes and Elusimicrobiota at the phylum level in SI-housed horses with low FEC. At the order level, Bryobacterales, Endomicrobials, Pedosphaerales, and Streptomycetales showed significant differences (P < 0.05) between low and high FEC horses in SI housing. Elusimicrobiota, commonly found in herbivore and insect guts, is known for its role in cellulose degradation, potentially aiding fiber digestion in horses. Endomicrobials, associated with ruminant microbiomes, may enhance fiber fermentation and nutrient absorption. Bryobacterales and Pedosphaerales, typically found in soil, may enter the gut through grazing and contribute to nutrient cycling and organic matter degradation. Streptomycetales, known for their antibiotic-producing capabilities, could help regulate microbial balance and inhibit pathogenic bacteria, potentially contributing to parasite resistance. However, no significant differences (P > 0.05) were found in alpha diversity, richness, or overall microbial diversity. These findings highlight the impact of housing on the equine gut microbiome and its potential role in parasite management. Given the increasing resistance of strongyles to traditional anthelmintics, targeted microbiome modulation could offer an alternative approach for parasite control by leveraging the interplay between microbiome-associated metabolites and hormonal responses. Future research should further investigate these interactions to better understand their collective impact on microbial composition and function across different housing conditions.