A Comprehensive Review of the Equine Gut Microbiome in Health and Disease

Molecular microbiology has revolutionized our understanding of the complex host-associated microbiomes required for normative development and physiology. Horses and other members of the family Equidae are particularly reliant on the early maturation and lifelong maintenance of an unusually rich hindgut microbiome for optimal digestion and overall health and performance. Research on the equine gut microbiome has accelerated in the past several years, necessitating a renewed appraisal of the field. The present work is a comprehensive and critical review of the literature regarding the bacterial gastrointestinal microbiome of horses. First, the developmental trajectory of the foal gut microbiome is discussed, followed by descriptions of the taxonomic membership of the core equine gut microbiome, its primary functions and effects on host physiology, and intrinsic and extrinsic factors that shape the equine microbiome during health, with a focus on diet and supplements. Next, evidence supporting adverse effects on the equine gut microbiome of gastrointestinal conditions including colic and colitis, extraintestinal conditions including obesity and laminitis, and pharmacological interventions including antibiotics and non-steroidal anti-inflammatory drugs is summarized. Lastly, clinical and experimental research investigating the effects of treatments targeting the gut microbiome of horses, including probiotics, prebiotics, and fecal microbiome transfer, is critically examined. Conclusions summarize the connection between natural (i.e., wild) equine behavior and the health of the equine gut microbiome and the impacts of human management.