Behaviors in horses that indicate welfare issues, such as stereotypies and aggressiveness, have been linked to the make-up of their gut bacteria by researchers.
The findings add further weight to the possible existence of a so-called gut-brain axis in horses, with mental wellbeing and gut health closely inter-related.
Núria Mach and her colleagues, writing in the journal Scientific Reports, identified a range of factors that affect the make-up of a horse’s microbial gut community, with one of them being their chosen discipline.
For their study, the researchers from France and Spain took blood and fecal samples from 185 healthy sport horses cared for in similar conditions eight months apart.
They also assessed 41 host-related factors, which ranged from diet, bedding, housing, and sporting discipline to age, sex, parasite infection status, behavior, fecal pH, and their loading of fungi and protozoa.
Although kept under similar conditions — they were individually housed — the horses were trained for different disciplines and enrolled in different specialities.
Forty-six percent were trained for dressage while 25% were showjumpers and another 25% took part in eventing. Overall, 30% of the horses were enrolled in specialties with higher mental and physical demands, taking part in Gala and Cadre Noir displays.
In all, 78% were body-scored as ideal, while 14% were classed as overweight and 8% were categorized as thin.
While core bacteria were identified across the horses, there was a high degree of variability seen between horses, especially so among rarer species of gut microbes.
Considerable variability was found when the fecal microbiota of individual horses was resampled eight months later. Indeed, the changes were such that individual horses could not have been recognised by their gut’s “unique microbial fingerprint”.
“Therefore, the stability of fecal microbiota communities should not be assumed even in healthy individuals,” the authors said.
A heatwave was occurring when the second samples were taken, and rainfall had been scarce. However, no dehydration occurred in the horses, nor were there changes in food availability or quality when compared to the first sampling point.
Their work identified that equitation factors were highly associated with the variability in gut microbiota, pointing to a relationship between gut microbiota and high levels of physical and mental stressors.
Discipline was estimated to be responsible for 5.79% of the variation seen in the study.
Horses that had a specialty, taking part in Gala and Cadre Noir displays, accounted for another 6.5% of the variation seen, and were clearly grouped together by their microbial communities, the researchers said.
Bedding was also responsible for variation, with two clear clusters identifiable — those on straw bedding and those housed on wood shavings or wood pellets.
Hypervigilant behavior was calculated as being a factor in 4.97% of the variation seen, while oral stereotypies tended to influence the fecal microbiota composition less, at around 2.5%.
The behavior-related findings reinforce the notion for the existence of the microbiota-gut-brain axis, the authors said.
As more elite equine athletes suffer from stress, targeting the microbiota offers a new opportunity to investigate the bidirectional interactions within the brain-gut microbiota axis, they said.
The authors said there were many factors that could have been potentially associated with the gut microbiota divergences observed eight months apart.
But the evidence indicated that these changes in healthy sport horses kept in the same riding school and fed the same diet were primarily related to equitation factors, such as specialty and discipline.
Physical and mental stressors were likely the main variables involved, they said.
“In our study, the prevalence of stress was believed to be higher in horses that were trained for dressage and jumping, as most of them were elite athletes that traveled to international competitions, trained many hours per day, five days a week, for several weeks without taking time off from intense training.”
They said that behaviors related to mental distress, including hypervigilance and to a lesser extent, oral stereotypies, were also related to the composition of the fecal microbiota in the horses.
Interestingly, specific kinds of bacteria were significantly linked to behaviors that indicate welfare deterioration.
The strongest effect was observed in animals experiencing oral stereotypies. Levels of Roseburia, a butyrate-producing genus were among the core microbiota, positively correlated to the prevalence of oral stereotypies.
Mach, N., Ruet, A., Clark, A. et al. Priming for welfare: gut microbiota is associated with equitation conditions and behavior in horse athletes. Sci Rep 10, 8311 (2020). https://doi.org/10.1038/s41598-020-65444-9