One of the many, many amazing things about the body is how it maintains a very narrow temperature range so that all the body systems function optimally. In people this range is about a degree above or below 98.6 F. Horses have a somewhat wider range, which is generally given as 99.5 to 101 degrees F.
A horse that has been standing in a stall on a 0 degree F. New England day might be expected to have a body temperature of down around 99.5. On a summer day when the outside temperature reaches 100 the same horse‘s temperature may run up to 101. Think of it. A huge environmental difference of 100 degrees but the body makes adjustments to keep the temperature within a very narrow range.
How does all this happen, and what happens when things seem to get off kilter?
There are nerve cells in a part of the brain that are temperature detectors. Some sense cold, others heat. Those cells act like the thermostat in your house to keep things within a normal range. Too cold and the furnace kicks on, too hot and it shuts down.
If you take a ride on a summer day, your horse’s muscular activity produces heat. The warmed blood is carried to the brain and the heat sensitive cells fire. Messages are sent to the blood vessels under the skin to dilate and to the sweat glands to contract. The horse starts to breathe more rapidly to throw off heat. The body’s activity has made the temperature go up a few degrees, but it won’t get too hot because of all the cooling systems that are working. Within an hour after the ride the temperature will be back within the normal range. On that cold day with the horse just hanging out, the cold sensitive nerve cells in the brain sense the cooler blood. Hormones are released; blood vessels under the skin contract, the hair stands up, other heat preserving systems kick in, and finally involuntary shivering will start to keep the body warm. Normal body temperature is maintained.
Things seem to break down when a horse is sick. The body recognizes the bacteria or viruses as invaders and defense mechanisms are turned on. The horse’s white blood cells attack the organisms. As the white blood cells engulf them their own lives are sacrificed. Toxins from the degenerating white cells are carried to the brain by the blood and the “thermostat” in the brain sets the body temperature at a higher range. Instead of 100 degrees the horse’s temperature may climb up to 106 degrees. The body senses that this higher temperature should be maintained for awhile so the cooling system is not activated. This is actually a good thing. The invading viruses or bacteria have their own ideal temperature range, and the horse’s high temperature starts to destroy them. The body’s fever is acting like a “cooker” to kill the organisms.
Horses with fevers usually go off feed and act a little “out of it.” It depends on the situation, but I usually hesitate to give a horse medicine to reduce the fever for awhile unless it gets towards the 105 or 106 range. The higher body temperature is helping to get rid of the bugs. You might have heard that a high unrelieved fever in people will cause brain damage. This doesn’t seem to happen in horses. When you do give Bute or Banamine the body’s thermostat gets reset, and the horse may sweat profusely or sometimes pant like a dog. This is usually scary if you haven’t seen it before. It is just the cooling system kicking in to make the fever break.
If you have a horse that isn’t eating and seems dopey, by all means, take his temperature. Call your vet and report in with the temperature reading. Don’t be surprised if the advice given is to hold off on the Banamine or Bute for awhile.
Let the body’s cooker work.
-David A. Jefferson, DVM