Exhaustion occurs in most equestrian sports, but it is more frequent in events that require sustained endurance work such as endurance racing, three-day eventing, trial riding, and hunting. Exhaustion is also more likely when an unfit, unacclimatized, or unsound horse is exercised. Mechanisms that contribute to exhaustion include heat retention, fluid and electrolyte loss, acid-base imbalance, and intramuscular glycogen depletion. Clinical signs include elevated temperature, pulse, and respiratory rate; depression; anorexia; unwillingness to continue to exercise; dehydration; weakness; stiffness; hypovolemic shock; exertional myopathy; synchronous diaphragmatic flutter; atrial fibrillation; diarrhea; colic; and laminitis. Treatment includes stopping exercise; rapid cooling; rapid large volume intravenous or oral fluid administration; and nonsteroidal anti-inflammatory drug administration.
Horses that are inadequately trained or have other systemic or musculoskeletal problems or those that are competing in hot conditions may develop a variety of metabolic problems. It is hoped that knowledge of the fluid and electrolyte losses may help in treating horses with metabolic problems and, more importantly, in improving management of horses in training and competition to prevent any loss of performance.
It happens often at endurance races and is, therefore, commonly viewed as a problem of the endurance horse. But heat exhaustion happens in polo ponies, steeplechasers, eventers, trail riders - any horse exerting himself for more than just a quick sprint, and particularly when it's hot and humid outside. At endurance races, the scenario is common to the point of being a stereotype: one of the horses has been pulled from the race for failing to recover adequately during a vet check, and now his condition is worsening despite several minutes of rest, and suddenly the reality hits his worried owner like a freight train: his horse might not make it. The event's on-site veterinarian is summoned, and the drama unfolds.
But what if your horse is stricken and you're at an unsanctioned event, where there's no veterinarian available? Or you're on a training run, rather than at an official event? Or it wasn't intentional exercise at all - your show horse somehow got out of his stall and has been running, terrified, with snarling dogs at his heels, for the past hour as the outside thermometer creeps up to 95°F and the relative humidity is close behind? What do you do?
The hallmark of advanced heat exhaustion is lack of interest in eating or drinking after a workout, despite the absolute certainty, after the rigors of his exercise, the horse desperately needs the liquids and electrolytes that water and food would provide. The full list of signs includes:
The goal in dealing with this dangerous condition is catching it in its earliest stages, before the diagnosis is obvious and before the horse's cardiovascular system breaks down.
This is much more than just a bad case of fatigue. It's a dangerous buildup of heat in the horse's body which, if unresolved, can lead to cardiovascular instability, collapse and death.
It is caused by electrolyte deficits in the exhausted horse that can be astounding, and since the sensation of thirst is linked to the detection of excess salt in the system, the salt deficient horse feels no desire to drink. This means that, if left to his own devices, he would be unlikely to recover because despite his dehydration he has no natural urge to replace the liquids he so desperately needs.
To naturally replace electrolytes, the horse must eat. However, of his mineral deficiency is severe enough to cause his gut motility to slow down, he's got a belly ache - it might be very mild and subtle, or it might appear as full-blown colic - but even the mildest colic pain can squelch his desire to eat. He must eat, but he won't.
Meanwhile, he continues to pant in an attempt to blow off the heat that is killing him from the inside. But with each breath he can become more dehydrated, and the muscular activity involved in panting can actually stoke the fire, creating more heat.
As his cardiovascular system collapse, blood vessels to the muscles and skin begin to constrict. This keeps his too-hot blood close to his core, away from the body's surface where it had the best chance of dissipating the extra heat. It also sets the horse up for myositis, because those muscles, which are sorely in need of nourishment after their workout, are suddenly being cut off.
Early in the course of heat exhasution, the horse might appear bright and normal, but he needs to be pushed to keep up the pace, and if you checked his rectal temperature it might be as high as 104° F. Horses that are stricken with heat exhaustion while performing some short-term, high-intensity exercise such as 3-day eventing or polo are likely to have less critical need for fluids and are simply in need of help of lowering their core temperature. For these horses, repeated cool (not cold) water baths and slow, relaxed walking in a light breeze is likely to do the trick. Getting the horse to eat and drink is rarely a challenge, so he'll probably replenish his fluids and electrolytes on his own.
The endurance horse is a different case - his fluid and electrolyte debts are likely to be heavy and possibly life-threatening, and without the option of administering intravenous fluids and electrolytes, you're severely limited. However, even though you're in serious trouble in this situation and should move heaven and earth to get a competent equine veterinarian to come to your aid, you should still be able to help the horse if you're prepared.
Prevention. Heat stroke can be prevented by limiting the horse's exposure to predisposing situations and by insuring that hard-working and endurance horses drink frequently during prolonged exercise.