Page 93 - May 2022

‘wonderful net’ to describe the vascular complex that helps cool the blood going to the brain. This is a complex of arteries and veins lying close to each other, where the blood spreads out into smaller vessels as it comes to the brain. It does this in an area close to a lot of airflow to help cool the blood. Cattle have a well-developed rete mirabile, but horses do not,” he says.
“The only thing horses have in the head region is the guttural pouch which provides some air exchange, and a little area where the carotid artery is exposed inside the guttural pouch. Perhaps one reason it’s there may be
to help cool the blood a little as it goes up
to the brain. If the horse is breathing rapidly and drawing in a lot of air, this can move the
air around a little. The pouch is a dead-end, however, with only one opening (air comes in and swirls around and has to go out the same opening), so it’s less efficient for cooling. Horses depend more on sweating to cool the body. Each species has its own way to regulate body temperature,” says Randall.
SIGNS OF TROUBLE
Any horse that just sweats under the mane and saddle pad should raise suspicion, because this isn’t enough to cool the body. If a horse at pasture is staying in the shade during the day, or playing in the water all the time, you need
to look more closely to make sure he is not developing a problem. “These are signs that horses are trying to do whatever they can to cool themselves and stay out of the heat,” says Randall. Some of those horses’ feet become soft and unhealthy because they are standing in mud and water all the time trying to stay cooler.
Anhidrosis usually develops during the
hottest part of the year and may appear quickly
or gradually. In a mild case the horse may still sweat, but not enough to cool himself, and may have exercise limitations. The rider or trainer
may notice performance slipping as weather gets hotter and more humid. If the horse must sweat continually to cool himself, as when working hard or living in a hot stall, sweat glands work overtime and eventually shut down, and the horse becomes dry-skinned. He may have fast respiration and flared nostrils, trying to create more air exchange in the lungs, and an elevated temperature.
“Sometimes the first thing that’s noticed is that the body temperature is higher than normal because the horse can’t cool himself,” says Mallicote. “Even if a person doesn’t notice that the horse isn’t sweating as much as the other horses in the barn, if it’s a barn where they are checking temperatures daily, they might identify a horse with a fever. Then when we look closer at that horse, with a good history, we usually realize this horse is anhidrotic. First, however, we look for other possibilities and make sure
the horse does not have something else, like an infection, that could cause the fever,” she says.
The horse’s sweat patterns may change. This can be subtle and sometimes people don’t realize that the horse is not sweating enough. The horse may sweat a little, with a few patches of sweat behind his ears, under his mane or saddle pad, at the elbows and flanks, but no moisture over his body.
Other signs might be dry, flaky skin and hair falling out, especially around the eyes and on the forehead. “As a horse becomes chronically affected, hair quality changes,” she says. Oils from the sebaceous glands are no longer taken out onto the skin by sweat. The dry skin may itch, and there is often hair loss on the forehead.
“This is the one place that they all seem to get scurfy skin and hair loss. It’s a fairly reliable sign; I look at horses that have patches of hair loss on the forehead and when I ask the owner or trainer if this horse sweats, they usually say, ‘Not really.’”
Generally, the most noticeable sign is heat stress; the horse can’t handle the heat and has poor performance, and it takes a long time to cool out after exercise. “In severe cases the horse is just standing there panting with respiration rate of 100 or higher. These horses can also look like they have heaves because they are breathing so hard, but not sweating. You take the temperature and it’s high, but the first thing you see is increased respiratory rate,” Mallicote says.
There are diagnostic tests that can be done, but these are generally not needed for clinical diagnosis because the signs (particularly lack of sweating) are obvious.
Anhidrosis may be acute or chronic. “Many of these horses just quit sweating, and then a month later the sweating turns back on--an immediate reversal. It doesn’t always affect
the horse for the rest of its life, especially if
the owner/trainer can make changes in the management and environment to give the horse a break from the heat.”
In acute cases, the horse may have a problem for a summer or two, but then can revert to normal. “But once a horse becomes chronic, which we find in cases that continue for the third summer and beyond, we see changes in structure of the sweat glands. They not only suffer from whatever the original problem
might be (causing the inability to sweat) but also start to atrophy and can’t work properly. Once a horse slips into that chronic situation, it’s much harder for them to ever switch back to normal,” she says.
“Even severely anhidrotic horses may often revert during winter, however. They improve in cooler weather and may be able to sweat a little, but then the anhidrosis returns the next summer.”
TREATMENT
“There are multiple ways to treat these horses, and I tell people that if there are lots of ways to treat a certain condition, this means none of them work reliably,” Randall explains. A certain treatment might help one horse but not another.
“People have tried everything from vitamins to beer. There are some horses that respond to beer, and it helps them, but there’s no consensus on what kind of beer! If a horse owner wants to go that route, it’s worth a try to see if it helps, and if it doesn’t, try another kind!”
There are also some products on the market aimed at trying to manage the non-sweating horse. One of the classic treatments that’s been in use a long time is One AC, a powdered supplement containing L-Tyrosine, Choline Bitartrate, Niacin, Pyrodoxine Hcl, and d-Calcium Pantothenate, to be added to the horse’s feed. “For some non-sweating horses this product seems to work, while for others it doesn’t make any difference. Adding regular electrolytes to the feed doesn’t work very well, either, but adding potassium chloride (Lite salt) does seem to help,” says Randall.
“Another thing I’ve had a fair amount of luck with is acupuncture. It definitely helps in early cases of anhidrosis. In longstanding cases and horses that have completely shut down (not sweating at all), it doesn’t work
as well. I haven’t had any luck treating those horses. It works best when the horse has just started having a problem, especially if you are very aggressive with the acupuncture in the beginning. I use this method two or three daysinarowandthenonceaweekfora while,” he says.
It also helps if you diligently try to keep these horses cool so they don’t have to sweat when you first notice the problem, giving the sweat glands a break. “If ambient temperature cools off at night, this also helps. Continuously elevated temperature and humidity is the big problem for these horses,” says Randall.
Using fans and misters can help. “With my own horses in the summer, I may keep them indoors with fans during the day (out of the hot sun) and turn them out at night. My barn is pretty open, so they still get some sunlight, but they are not baking in the sun, or getting really fat being out at pasture 24/7,” he says.
EQUINE HEALTH
SPEEDHORSE May 2022 91
While people sweat to cool off, we don’t have as many sweat glands as horses.