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restrictive management conditions. These behaviours are repetitive and appear to serve no clear purpose. They are most commonly associated with environments that limit a horse’s ability to meet natural needs, such as continuous foraging, free movement, and social contact. Although stereotypies are widely used as indicators of reduced welfare, the biological processes behind them are not yet fully understood.
In human medicine, stereotypic behaviours are linked to long-term stress and some psychiatric conditions, including obsessive–compulsive disorder. In these cases, saliva is often used to measure stress-related biomarkers because it can be collected easily and without causing discomfort. Similar approaches are increasingly being used in veterinary and equine science research to assess stress in animals.
A study led by Marilena Bazzano, with colleagues from the University of Camerino, Italy, and Sapienza University of Rome, examined whether salivary stress biomarkers differed between Thoroughbred horses with and without stereotypic behaviours. The study focused mainly on horses trained for racing, which are exposed to both physical and mental demands, and included a comparison with non-competing leisure horses.
Cortisol is the most commonly used hormone for measuring stress, as it reflects activation of the hypothalamic–pituitary–adrenal (HPA) axis. However, other salivary markers are now being studied, including alpha-amylase (sAA), which is linked to activity of the sympathetic nervous system, and butyrylcholinesterase (BChE), an enzyme that may also be involved in stress responses. The researchers expected that levels of these biomarkers would vary depending on whether horses showed stereotypic behaviours and whether they were involved in high-level competition.
Saliva samples were collected while the horses were at rest. Three groups of Thoroughbreds were included: ten high-level competition horses without stereotypies, eleven high-level competition horses showing stereotypic behaviours, and five non-competing leisure horses that also showed stereotypies.
The results showed clear differences between the groups. Cortisol levels were highest in the competition horses that did not show stereotypic behaviours. This suggests that the demands of training and competition may increase baseline stress hormone levels, even in horses that appear behaviourally normal.
In contrast, alpha-amylase activity was significantly higher in the non-competing leisure horses with stereotypies. Higher sAA activity is usually linked to increased activation of the sympathetic nervous system, which plays a key role in stress responses. This may indicate that stereotypic horses kept outside of competition experience a different type of stress, possibly related to long-term management or environmental conditions rather than athletic workload.
No significant differences were found in butyrylcholinesterase activity between the groups. This suggests that BChE may be less sensitive to differences in stereotypic behaviour or competition level, at least when measured at rest.
Overall, the study suggests that both competition demands and stereotypic behaviours influence how horses respond to stress at a physiological level. It also shows that using more than one salivary biomarker can provide a clearer picture of equine welfare. However, the authors note that the small number of horses involved and the lack of repeated measurements mean the results should be interpreted with caution.
Despite these limitations, the findings support the use of saliva as a practical, non-invasive method for assessing stress and welfare in horses, and they provide useful direction for future research in equine behaviour and physiology.
For more details, see:
Bazzano M, Marchegiani A, La Gualana F, Petriti B, Petrucelli M, Accorroni L, et al. (2024) Competition and stereotypic behavior in Thoroughbred horses: The value of saliva as a diagnostic marker of stress.
PLoS ONE 19(10): e0311697.
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