I am told that kids as young as 2 years can look at a horse and know that it’s not a cow. Just a glance and our brain takes in all the features, and says: “horse.” There are also even more internal differences that we can’t see. The first article in this series featured some differences within the horse’s head. This article is focused on other parts of the horse hidden to our eyes. The emphasis will be on things that are of significance in our care of horses.
One interesting survival mechanism peculiar to horses is the huge capacity of the equine spleen to store red blood cells. Whenever the horse is alarmed, the spleen contracts, and millions of red blood cells are released into the blood stream. This reserve system is one of the reasons that horses can lose a great deal of blood before bleeding to death. It is also why veterinarians prefer to take a blood sample from a horse that is not excited. When a horse is anxious, the spleen contracts, and if a blood sample is pulled during that time, the blood will show a much higher red blood cell count than it would if the horse were relaxed. If your horse is sick and you think a blood test may be in order, have your vet pull the blood before he or she does a physical exam which may cause anxiety and spleen contracture.
Mares do not go through anything like menopause. Right up until death they keep forming new eggs and having regular cycles which run from mid spring until early fall. Even though your mare may not show outward signs of being in heat, the cycles are still going on. People will frequently ask why their mares seem to be coming into heat so frequently. The fact is that you can expect them to be in heat from 4-8 days, and out for about 14, which means that every 2 weeks you can expect another heat. Although they do cycle all their lives, it’s unusual for an aged bred mare to carry to term. Older mares often do not maintain the proper hormone balance to get them through an entire pregnancy. If you want to breed an older mare, ask your veterinarian for help with assessing and supplementing hormones to help her go to term. Just be aware that you are fighting Mother Nature.
You have probably heard that horses can’t vomit. This is one of the reasons that horses are so apt to colic. If a horse’s stomach doesn’t agree with what was just eaten, it’s not easy to get rid of it. A very strong muscle surrounds the entrance to the stomach and acts as a one way valve to prevent vomiting. We also have a muscle there, but the heaving of our stomach can overcome it. That strong sphincter muscle in horses means that they are destined to bear stomach pain. Occasionally so much pressure can build up that a horse’s stomach will rupture. This is one of the reasons your vet may pass a stomach tube and release the fluids and gas in the stomach as part of a colic treatment.
When we see our winter coated horses we can understand how they can look comfortable in a New England blizzard. But how do they stand on that frozen ground with no foot protection? The quick answer is that horses have an amazing blood supply to the foot. Equine hoof expert Dr Gene Pollitt from Australia calls it a bioengineering miracle. Blood to the foot is carried by two large arteries that run behind each fetlock down into the hoof. Have your vet or farrier show you how to take the pulse there. This is a great skill to learn and is handy when you are assessing a lame horse.
Deep within the foot is a network of veins, unusual in that they do not have valves to prevent blood from flowing back down to the foot. Instead, when a horse is in motion and the foot hits the ground, all its tissues are squeezed and virtually all the blood is pushed back up the leg. This works because the hoof capsule is relatively rigid. Once weight is on the foot the blood has nowhere to go except back into the circulation. When the used blood has been squeezed out a space is created for all the blood pumping down from the arteries to fill the foot again. The fill is immediate and the new warm blood has totally replaced the old. This constant turnover keeps the internal structures of the foot toasty warm, even in subzero conditions. One take home from this is to be sure that your horse has room to move around in the winter. I often see horses just standing in one spot because it hurts their legs to break through the crust on snow. Break it up for them, and put the hay in different spots in their paddock so they have to walk and keep that blood flow going more effectively.
Horses have the unusual ability to “lock” their hind legs so that they can stand for long periods of time using minimal energy and no muscle exertion. There are 3 ligaments that connect the patella (knee cap) to the tibia (the bone between stifle and hock). One of the ligaments can slide over a knob on the femur (the big leg bone) and literally lock that leg in a standing position. This is handy because with one leg locked the horse can rest the other. This neat feature of horses can be a problem when the hind leg locks or catches as the horse is moving. This doesn’t cause any pain, but it bothers the horse to have a hind leg go straight out behind and not flex. We see this most often seen in horses that aren’t getting enough exercise. The intermittent locking can affect horses of any breed but is by far more prevalent in ponies. Programmed exercise including hill and trot pole work to strengthen the quads often corrects the condition. If not, there a new simple surgery that can be done on the standing horse that will usually fix the problem.
These are just some of the interesting differences that make horses what they are. Knowing a little bit about these hidden workings of horses helps us in understanding and managing them better.
–David A. Jefferson, DVM