Horses can become dehydrated easily, and after prolonged periods of exercise can develop what is called the exhausted horse syndrome. Clinical signs of the exhausted horse include depression and disinterest in their surroundings and in eating and drinking. Dr. Emma Adam indicates in the AAEP proceedings that some of these horses will stop sweating, and this causes other problems as they are unable to cool themselves.
They typically have elevated heart rates and elevated respiratory rates even after exercise has stopped. Usually their rectal temperature is increased as they cannot cool down, but with severe disease the temperature can actually be below normal. The pulse may be weak when palpated and may be difficult to feel. The mucous membranes of the mouth may be tacky and dry with a prolonged capillary refill time. Place your finger on the horse's gums and pull it away; if your finger temporarily sticks to the gum, this is the tacky feel. Also, placing your finger with pressure over the gum and then removing it will cause the tissue to turn white temporarily and then the pink color will return. The capillary refill time is the time in seconds it takes for the pink color to return and anything over 2.5 seconds is abnormal and indicates poor circulation. However, if the membranes are brick red and capillary refill is less than 1 second - which is very fast - this may indicate endotoxemia in which toxins are produced and have been absorbed into the blood stream. If your horse develops any of these symptoms, stop all exercise and have your vet examine your horse immediately as this is a serious condition. Intravenous fluid therapy and other treatment may be necessary.