Angus the Boston terrier stayed warm in this past week's frigid temperatures. — Photo by Jamie Stringfellow

When Mother Nature first graced us with two feet of snow, we diligently kept our animals inside through the actual storm, but afterward, nature called in a different way. My husband gallantly shoveled the back porch, and my daughter cleared a trail for our dog to make her way to the woods. The cats we confined inside with a litter box until there were pathways they could safely negotiate without drowning in drifts. By the second big snow, the animals were a bit wiser about how to manage, as were we, but we must remain vigilant to keep our pets safe through this unusually harsh winter. Now we are facing not only snow but exceptionally frigid temperatures. Let’s talk about hypothermia.

For those of you who have dogs or cats who essentially live outside, let’s skip the discussions about whether that is right or wrong, ecologically and philosophically, and stick to the medical. As long as a pet is acclimated gradually to cold, provided with adequate shelter from wind, rain, and snow, and is of appropriate age, breed, and robustness, outdoor living is usually fine — admittedly not the middle-class suburban vision of the life for a family dog, but nonetheless an acceptable option, embraced by working dogs and barn cats for centuries. But even for such rugged animals, spells of extraordinary weather can be life-threatening.

What happens when Chilly the chow gets cold? First, she alters her behavior to conserve heat, by seeking shelter or curling up. Her fur puffs up (called piloerection), trapping a layer of air close to the skin that serves as an insulator. She will shiver, the tiny muscular contractions generating internal heat. Her body protects core functions by constricting peripheral blood vessels, focusing circulation of her warming blood to the command centers of heart and brain. If all these mechanisms fail to maintain normal core body temperature, hypothermia results.

Any condition that impairs heat production or conservation predisposes Chilly to hypothermia. Smaller animals are more susceptible because of the larger skin surface in proportion to body mass. Short coats provide less insulation than heavy ones. In the very young and very old, thermoregulatory mechanisms may simply not function effectively. Thin animals have less heat-conserving body fat, and also reduced muscle reserves needed to generate heat. Dogs with arthritis or any disability, injury, or illness that impairs mobility, move around less, and as anyone who does outdoor winter activities knows, physical activity keeps you warm. Cardiac disease and endocrine abnormalities such as hypothyroidism also increase risk. A scrawny, elderly Chihuahua invalid who never leaves mother’s lap will quickly become a pupsicle in this weather. Chilly, the fat, fluffy, healthy, young chow with a sturdy insulated doghouse, who has lived outside all fall, may not even notice the cold, but unless he’s a sled dog acclimated to arctic conditions, even Chilly should come inside when the wind-chill factor is in the negative numbers.

Hypothermia often occurs because a pet is injured or lost: the dog who falls through the ice on a pond, the stray cat stuck in a snowdrift. But it doesn’t even have to be winter. Consider Gramps, the old terrier. Thin, arthritic, partially blind, he was sunning himself on the deck while his owner raked leaves on a crisp fall day. Busy doing yard work, no one noticed until dusk that Gramps had wandered off. Calling him was fruitless — Gramps was completely deaf. The neighbors and the animal control officer all joined the frantic search, but it wasn’t until mid-morning next day that he was found half a mile away. He had waded across a little creek, then stumbled into a ditch, injuring his leg. Wet and confused, unable to extricate himself, his night outside had led to significant hypothermia.

Early signs of hypothermia include mental depression, stiff gait, and lethargy. Shivering may be present, but ceases as hypothermia worsens. Pupillary responses become sluggish. Breathing is shallow and irregular. Heart arrhythmias may develop, as well as a profoundly slow heart rate. Blood pressure plummets. Eventually reflexes disappear, pupils are fixed and dilated, and the individual becomes stuporous or comatose. Severe cases may actually be mistaken for death. In human medicine they say about hypothermia cases, “You’re not dead until you are warm and dead.”

Moderate to severe hypothermia is life-threatening, but treatment must be handled appropriately to avoid worsening the situation. Too much movement may precipitate lethal heart problems, so patients must be transported slowly and carefully. Then rewarming can begin. In mild cases, “passive rewarming” may be sufficient, simply wrapping the patient in blankets and letting the body’s natural heat-producing abilities correct the problem. “Active external rewarming” adds heat sources like hot-water bottles or heating pads. These should not be applied directly to the skin, and should be concentrated around the chest, focusing on restoring core temperature first, not extremities. “Core rewarming” involves using things like warm intravenous fluids to increase body temperature from the inside. The complicated interplay of circulation, fluid balance, and heat transfer involved can occasionally result in sudden death called “rewarming shock.”

Large animals are also susceptible. Back when I still worked on horses, I treated a gelding that had fallen at the top of a steep hill in a blizzard. By the time I arrived, he was stiff and stuporous, his extremities icy cold. The wind was so fierce the intravenous fluid line kept freezing. We managed to warm and rouse him sufficiently that, after several hours and multiple attempts, we got him to his feet, but after several stumbling steps down the snowy incline, he cast himself again. Ultimately the owners opted to euthanize him as both his condition and the storm worsened. Gramps, too, did not survive. Although we restored him to normal body temperature, the leg injury was severe. This, his age, and other disabilities, led to the decision for euthanasia. So keep your pets close to home and toasty warm during this bitter weather. And take heart. Mud season is just around the corner.