Perinatal Stress in Immature Foals May Lead to Subclinical Adrenocortical Dysregulation in Adult Horses: Pilot Study.

This research explored whether early birth stress in horses might lead to unnoticed issues regarding the function of their adrenal glands when they reach adulthood. The findings support this possibility, as horses that were born prematurely showed unusual responses to a hormone that stimulates the adrenal glands.

Research Purpose and Approach

• The aim of this pilot study was to examine whether gestational immaturity, or being born prematurely, might result in persistent adrenocortical dysregulation in horses. This term primarily refers to long-term problems with the regulation of the adrenal glands, which are responsible for the production of certain hormones in the body, including cortisol.
• To determine this, the team measured the amounts of salivary cortisol, a hormone linked to stress, in response to a synthetic form of another hormone, adrenocorticotropic hormone (ACTH), which stimulates cortisol production. This hormone was injected into the horses’ bodies, and the researchers subsequently assessed their cortisol responses.
• The study comprised three categories of horses: 1) cases, i.e., horses that were gestationally immature at birth (10 in number); 2) positive controls, horses related to the case horses and similarly stimulated with the ACTH (7 in number); and 3) negative controls, horses also related but that received no ACTH (5 in number).

Key Findings

• The salivary cortisol levels of all horses at the start of the experiment (baseline) fell within the normal published ranges. However, the case horses exhibited significant differences in their peak and AUC (area under the curve, a measure of the overall response to the injected hormone) values compared to the positive controls.
• Case horses had either higher or lower cortisol concentrations than the positive controls. Those with higher concentrations are termed ‘H-cases’ and those with lower as ‘L-cases’. These values fell outside the normal range for a reference population, suggesting some level of adrenocortical dysregulation.
• The study found no substantiated effects of breed, age, sex, the month of testing, and location on these outcomes. This observation emphasizes that the results seem tied to the horses’ early birth stresses, rather than these potential confounding factors.

Conclusion

• In conclusion, the study suggests that gestational immaturity in horses can potentially induce subclinical adrenocortical dysregulation. Despite having normal baseline cortisol levels, prematurely born horses may exhibit abnormal responses, either elevated or blunted, to ACTH stimulation. This could point towards altered adrenal gland functionality in these horses, brought about due to birth stressors experienced at an early life stage.