09/28/2024
Let's talk about stress, baby (Part I)
We'll start with the kind of stress that the horse's body is actually physiologically adapted to cope with. A horse in a herd is alerted to a threat. The nervous system processes the threat and induces a stress response that can include increased respiration, blood pressure, heart rate, and temperature. The herd flees, and puts a comfortable distance between themselves and the threat. One horse goes back to eating, then another. This kind of short, intense burst of stress is exactly what the horse is built to handle. In the wild, a horse is also capable of "closing the stress loop" by having the freedom of movement to allow to body and nervous system to move through stress.
So how is the stress response of our domesticated horses different than the example above? Chronic stress and acute stress take two different biological pathways to the adrenals, and the processes are modulated by different hormones. In that way, chronic stress is physiologically different than acute stress. Prolonged activation of that chronic pathway is what can lead to long term health consequences for our horses. We know that long term over-production of cortisol in particular can cause "aggressive behavior, decreased growth and reproductive capability, inhibition of the immune system, increased risk of gastric ulceration, colic and diarrhea" (Rutgers).
We should include that even measuring cortisol in our horses, though, is not always a reliable indicator of chronic stress. Studies have found that abnormally low cortisol levels could also be an indicator of stress and welfare issues. It could be that horses exposed to chronic, prolonged levels of stress experience a breakdown in the stress response system as a whole. In theory this leaves the horse less capable of coping with even the acute stress it's body is better equipped to deal with.
So why does all this matter? It matters for welfare, but it also has huge implications for our goals with horses. In the next part of this series, we'll talk about how a horse's stress impacts the expectations we have for them, and why prevention (as usual) is so critical!