I am going to call those living things that act against the best interest of the horse pests and have picked some to talk about. Yes, I am making a value judgment here by labeling some of nature’s creatures as pests and nature itself does not make these kinds of value judgments. Nature is a great student of opportunities for exploitation. Chances are if an opportunity can be exploited that it will be and my horse in nature’s eyes is one big opportunity.
So, naturalists observe, a flea
Has smaller fleas that on him prey;
And these have smaller still to bite 'em,
And so proceed ad infinitum.
Thus every poet in his kind
Is bit by him that comes behind.
Jonathan Swift, (1667-1745)
The contest between horse and those living things that pester horses is ages old. There were flies and parasites associated with dinosaurs and when they died out their descendants hung out with early mammals and finally horses through all their stages of evolution. A species and its entourage of other associated living things evolve together. For example, horses have developed measures against flies. Movements of a horse’s tail, mane and forelock effectively swish flies away and there are wonderful muscles under a horse’s skin that twitch and cause flies to take flight. Often when I try to swat a fly that has landed on my horse I find that the horse itself was a lot faster and has already caused the fly to take flight long before my hand has come down on the spot where the fly was. There have been a few times when my horse has come up to me and turned to present a particular pesky fly that will not leave it alone as if to say, “Here, swat that one if you will.”
Horses have what nature has given them to cope with pests but we humans are not satisfied. We are study the nature of pests and try to control them with rational countermeasures. Often we are surprised that the pests have become “immune” to our measures. We should not be so surprised since nature in its own way is a lot smarter than we are. Certainly, regular worming has done much to extend the life expectancy of domestic horses over what it was only a century ago. So, our cause is just but in the end not so different that the "missile race." We need to keep a step ahead of the competition in the interests of owning a happy healthy horse.