Those of us who live in northern climes have felt it: The chilly, tingly, numbing feeling of ears, nose, fingers or toes exposed to cold. If exposure to frigid temperatures continues, frostbite can occur. But Mother Nature has better provided for horses than for humans against the perils of frostbite.
"Horses evolved as temperate animals and actually are more comfortable in cold temperatures than in warm," explained Dr. Janice Sojka, associate professor of large animal medicine at Purdue University. "One source states the horse's neutral zone - the temperature where the animal is most comfortable and is not expending any energy to keep himself cool or warm - is between 20 and 40 degrees Fahrenheit."
That's why healthy horses, when left to their own devices, don't suffer the same discomforts as humans when exposed to chilling temperatures.
What is Frostbite?
Frostbite occurs when tissues become frozen and ice crystals form inside cell membranes. Frostbite can be superficial, affecting only the outside layers of the skin and marked by discoloration when healed, or it can be more serious, extending to deeper fascial (connecting) layers.
"When the cell gets cold enough, its contents expand and damage the cell membrane. This results in dehydration of tissue and cells, and damage to small blood vessels in the region affected. Ischemia (lack of good blood supply) of the affected tissues leads to necrosis (death) of those tissues," explained Dr. Andris J. Kaneps of American College of Veterinary Surgeons, and assistant professor, large animal surgery, Oregon State University. Frostbitten tissues don't recover with thawing: Once they're dead, they're dead.
"Ice crystals inside the cells cause those cells to rupture and die," said Dr. Gerald E. Hackett, Jr., professor of Animal and Veterinary Science, California State Polytechnic University, Pomona. "This is why frozen tissue typically turns black after it thaws. The thawing process cannot restore the integrity of the cells, so if frostbite is severe and the tissue dies, that is a permanent situation and the affected tissue will slough off and the area will scar over."
Although frostbite in healthy horses is uncommon, certain conditions can place horses at risk. Newborn and old horses are more susceptible to frostbite as are horses that have lost a lot of weight, are lame, have heart problems, are dehydrated or suffer from aberrant sweating.
"Horses out in extreme cold that are unable to find shelter from the wind, or are unable to stay dry, or are unable to take in adequate calories and forage to generate normal body heat are mostly likely to become victims of frostbite," said Dr. Hackett.
Horses kept inside metal sheds or plastic-covered wooden barns that do not have adequate ventilation are also at increased risk for exposure and severe respiratory disease. Wind-proofed metal sheds, old barns or wooden sheds wrapped in plastic are not suitable living quarters.
Dr. Hackett also notes that certain molds or plant toxins occasionally found in feeds may cause peripheral vasoconstriction and make an animal much more susceptible to frostbite and/or exposure. "While it is a good idea to increase the amount of hay a horse is fed in cold weather, it is never a good idea to feed moldy hay or grain," he said.
When Frostbite Strikes
Clinical signs of frostbite often are masked by hair coat or skin coloration, but the condition is most likely to hit the ear tips. However, stallions and geldings that have been sedated and subsequently can't retract their penises may be in danger for frostbite of the external genitalia.
"In most cases, horse owners don't discover their horses had frostbite until after the ear tips fall off," says Dr. Kaneps. "But if you have a really astute eye and you're looking very closely, you should find a sharp demarcation between normal and frostbitten tissues. If skin coloration is in your favor, the area will be very, very pale compared to the normal tissue around it. As the situation progresses, the affected area becomes swollen because of the edema of the damaged tissue, and it can redden as the small blood vessels try to continue to get blood into the area. Eventually, the tip of the ear dries up like a piece of beef jerky, shrivels, and falls off." Aside from cosmetics, this usually doesn't pose a problem for the horse.
Treatment - aimed at minimizing the damage - is based on medical practices for humans.
"Frostbite should be treated by rapid thawing in warm (100 F to 109 F) water," said Dr. Kaneps. He cautions against using hair dryers because there is less control over the heat. "It's better if you use a warm bucket of water, wetting a towel and putting in on the affected area."
Although people usually rub cold hands to warm them, frostbitten areas should never be rubbed because that can cause further tissue damage.
Nonsteroidal anti-inflammatories may be administered to reduce pain and inflammation in horses. And vasodilators, such as acepromazine, or NSAIDs may be administered to inhibit blood clot formation.
Prevention and Natural Protection
Although mammals protect their vital organs against severe cold by shunting blood away from the extremities to the core, leaving the extremities vulnerable to frostbite, horses can shunt a lot of blood away from their feet and still have a very functional foot.
"We don't understand blood shunting of the horses' feet very well," said Dr. Kaneps. "But there is some type of protective role to the feet in cold weather. It's empirical information because we know a horse can stand all day in a snow bank and not get frozen feet, whereas if you or I stood in a snow bank, we'd have frozen feet pretty quickly. The hoof capsule helps protect, and many of the tissues in the foot can sustain, some level of decreased blood flow naturally without being damaged."
The best advice in preventing frostbite is to use common sense horse care. Horses that can stay dry, find shelter from the wind, have adequate energy and forage intake, and are allowed to acclimate to the cold gradually - as would normally happen with the change of seasons - can survive bitter cold (minus 20 F to minus 40 F) temperatures quite nicely, even for extended periods of time.