Results of a Clinical Trial Showing Changes to the Faecal Microbiome in Racing Thoroughbreds after Feeding a Nutritional Supplement

SIMPLE SUMMARY: Bacteria within the gastrointestinal tract communicate with the immune system and the brain, have a role in energy metabolism and are responsible for gastrointestinal health and gut wall integrity. Veterinary research has reported links between gut bacteria and health. The so-called good bacteria can maintain health while proliferation of pathogenic bacteria can occur in certain conditions, causing colitis, inflammation, colic and dysbiosis. Genetic sequencing of gut bacteria can be used to provide a picture of the composition of the gut microbiome. This information can be used to manage the diet to provide an environment where the good gut bacteria can thrive. This study showed that feeding a prebiotic nutritional supplement resulted in mean and median percent decreases in Bacteroidetes, increases in Firmicutes and the Firmicutes:Bacteroidetes ratio that were significantly greater than zero for the treated horses only. Supplemented horses (8/10) were more likely than control horses (2/10) to show an increase in Firmicutes of a ≥9% with ≥24% increase in Clostridia, ≥5% decrease in Bacteroidetes, ≥16% increase in the F:B ratio and ≥2% increase in Actinobacteria (p = 0.01). This provides useful information for further investigations on long-term effects on the microbiome and on health and racing-related outcomes. ABSTRACT: Next-generation sequencing (NGS) has been used to evaluate the effect of various interventions on the equine microbiome. The aim of this randomised blinded clinical trial was to determine if a prebiotic nutritional supplement would result in a change from baseline in the faecal microbiome composition of racing Thoroughbred horses in training being fed a high concentrate/grain-based diet to be more similar to that found in forage fed/pasture grazed horses. Thirty-two horses on one training yard were randomised to either receive the supplement or not. Faecal samples were collected at baseline, 6 and 12 weeks for NGS of the 16S ribosomal subunit gene. Twenty-two horses completed the trial, met the inclusion criteria and were included in the intention to treat analysis; 20 horses were included in the per protocol analysis. The mean and median percent decreases in Bacteroidetes, increases in Firmicutes and the Firmicutes:Bacteroidetes ratio were significantly greater than zero for the treated horses only. Supplemented horses (8/10) were more likely than control horses (2/10) to show an increase in Firmicutes of a ≥9% with ≥24% increase in Clostridia, ≥5% decrease in Bacteroidetes, ≥16% increase in the F:B ratio and ≥2% increase in Actinobacteria (RR = 4, 95% CI: 1.1–14.4, p = 0.01). This provides useful information for further investigations on long-term effects on the microbiome and on health and racing-related outcomes.