Interactions between fecal microbiota and athletic horses

Context and Objectives

The intestinal microbiota is recognized as a central player in regulating the physical, metabolic, and mental health of mammals.
In horses, although several studies have examined the effects of diet or exercise on gut microbial composition, few have simultaneously integrated environmental, physiological, and behavioral variables within a longitudinal framework.
This study aims to fill this gap by analyzing the interactions between fecal microbiota, riding conditions, hematological parameters, and behavioral indicators of well-being in 185 athletic horses monitored over an eight-month period.

Methodology

The horses, housed in the same national riding school in France, underwent two sampling campaigns (T1 and T2, eight months apart).
Fecal samples were analyzed using high-throughput sequencing of the V3–V4 region of the 16S rRNA gene.
In parallel, 41 environmental and individual variables were measured, including:

• Equestrian discipline and specialty

• Type of bedding

• Dietary data

• Stereotypic behaviors (cribbing, weaving, aggressiveness, hypervigilance)

• Hematological parameters (MCHC, RWR, etc.)

• Fecal pH and microbial loads (bacteria, protozoa, fungi)

Main Results

1. Structure and Dynamics of the Microbiota

• High interindividual variability: Microbial composition differed more between individuals than within the same individual over time, especially for rare bacterial taxa.

• Stable microbial core: A set of 29 bacterial genera, mostly from the Firmicutes phylum, constituted the core microbiota, present in 99% of samples.

• Temporal dynamics: A significant drift in microbial composition was observed between T1 and T2, with an increase in alpha diversity (Chao1, Fisher) and an overrepresentation of certain rare genera (e.g., Anaeroplasma, Enterococcus, Erysipelotrichaceae).

2. Factors Influencing Microbial Composition

Multivariate analysis revealed that six variables together explained 32% of the beta diversity variance:

• Equestrian specialty (6.5%)

• Discipline (5.8%)

• Type of bedding (5.2%)

• Hypervigilance (4.9%)

• MCHC (6%)

• Red-to-white cell ratio (RWR) (5.7%)

Horses engaged in high-intensity physical and mental disciplines (e.g., Gala, Cadre Noir) exhibited more homogeneous microbiota profiles, suggesting a structuring effect of chronic stress.

3. Behavior and Microbiota

Significant correlations were established between certain behavioral indicators of discomfort and microbial composition:

• Oral stereotypies: associated with an overabundance of Roseburia, Acinetobacter, Desulfurispora, Helicobacter.

• Aggressiveness: correlated positively with Streptococcus and Butyrivibrio, and negatively with Anaeroplasma.

• Hypervigilance: tended to be associated with Denitrobacterium and Dehalobacterium.

• Environmental withdrawal: linked to Diplorickettsia, Anaerorhabdus, Novosphingobium.

These associations strengthen the hypothesis of a functional gut–microbiota–brain axis in horses.

4. Microbiability of Traits

The concept of microbiability (m²), analogous to genetic heritability, was used to estimate the proportion of phenotypic variance explained by the microbiota:

Behavioral traits:

• Oral stereotypies: 24.2%

• Locomotor stereotypies: 16.2%

• Aggressiveness: 13%

• Hypervigilance: 9%

Hematological parameters:

• Eosinophils: 32.1%

• RWR: 25.7%

• Hemoglobin: 21.9%

Discussion

The results demonstrate that even under homogeneous management conditions, the gut microbiota of athletic horses is influenced by factors related to training intensity, behavioral stress, and specific physiological markers.
Horses exposed to high physical and mental loads exhibit microbial alterations compatible with dysbiosis, which may be reversible.

The study suggests that the microbiota may not only reflect well-being status but also actively contribute to it through neuroendocrine and immune mechanisms.
Butyrate-producing bacteria (Roseburia) or H₂S-producing taxa (Desulfurispora) could play a protective role against chronic stress effects.

Conclusions and Perspectives

This pioneering study highlights the importance of the intestinal microbiota in regulating both behavioral and physiological well-being in athletic horses.
It opens promising perspectives for:

• The development of microbial biomarkers of stress and well-being

• The use of targeted probiotics to enhance stress resilience

• The integration of microbiota analysis into sport horse breeding and management programs