Transcript:

[0:03]

Welcome to this video podcast on how heat stress impacts your horse. Now, this is a very important podcast for any horse owner because as the world gets warmer and we are starting to see these extreme temperatures in regions throughout the planet, it is going to impact our horses.

[0:26]

And it already has. This is especially important for anybody that lives in warmer climates to understand what heat stress is and how it impacts your horse. So again, another topic for any horse owner, and this one kind of hits near and dear to my heart because a lot of my research has been looking at heat stress in horses and exercise stress impacts on fertility in the horse.

[0:51]

To kind of kick this off, why we started to investigate that a couple decades ago was because my mentor, Dr. Martha Velain at Texas A&M, when I started my PhD, she mentioned to me that her husband, Steven Vulgus, who worked at a large ranch in Texas and did a lot of embryo transfer, said that in the heat of the spring and summer in Texas, these mares were just not producing embryos — or any embryos he got were just of poor quality. So there was some sort of phenomenon going on with the heat of the day and its impacts on fertility.

[1:43]

So Dr. Vulgus and I got together and we formed a committee with Dr. Katherine Henrik, who is a world-renowned reproductive physiologist in horses, Dr. Dewey Kramer, who was one of the grandfathers of embryo transfer in large animals, and Dr. Nancy Ing, a genetics expert. So we formed this committee to investigate this further.

[2:16]

In a follow-on podcast, I’m going to talk about exercise impacts on reproduction, which, you know, you can relate some of that to humans because a lot of the research that I looked at was from the human side. To tie that up — in Texas in the spring and summer, you’re looking at days where it’s 93 to 96 degrees Fahrenheit (that’s 34 to 35 degrees Celsius), humidity is upwards of 80% in the day. I mean, it is hot, very hot. And so we were looking at the stress effects on these mares, and it got me into talking about heat stress.

[2:58]

So as we enter these hotter times of year, you keep this podcast in mind. We are going to take a journey today — we’re going to talk about how horses accumulate heat in their bodies, how they dissipate that heat, how sweat even works (especially in these horses — it’s special, it’s different), and then how you can help your horses during these hot days. And we’ll learn to recognize signs of heat stress leading up to — which can be fatal — heat stroke, and that can kill your animal. So you want to be very, very cautious of that.

[3:33]

Now, to kick this off, I thought it’d be fun to bust a myth because I heard this early in my career — that you never let a hot horse drink water, or especially never let a hot horse drink cold water, because it can lead to things like colic or laminitis. And to tell you — that is not true. That is absolutely, 100% not true. It is a myth. It does not happen.

[4:04]

Usually, what probably did happen and what started this myth was overworked horses that were given cold water — or even rinsed with cold water — did colic, or did cramp, or did suffer laminitis. And it wasn’t the cold water; it was the overwork that led to that. Horses absolutely need water, and cold water or even ice water can help your horses cool down. It helps them dissipate this heat, especially that core temperature.

[4:42]

During heat stress, or let’s say during exercise, what is going on in the horse’s body? Well, they’re shunting blood away from the core to their outer extremities because that blood flow is bringing the heat out to where these concepts of cooling happen — let’s just talk about sweating, right — and other ways that they cool down. So they need that blood to cool down to go back into the core to help that cool down.

[5:16]

The blood starts to be shunted away, say, from the digestive system, the reproductive system — because if you think about it from a biological standpoint, or even an evolutionary standpoint, why is this horse exercising? Well, out in the wild, it’s probably running from a predator. And so the last thing it needs to really worry about is slowly digesting its feed or reproducing. It’s bringing blood to the surface to keep the body running, to keep the brain cool, to keep the heart pumping, those lungs working. That’s the priority in the biological system of the horse as it runs away from a predator.

[5:55]

So when we do exercise, it’s mimicking that physiological stimulus, and so it causes that blood to shunt away from that. Now — yes, let your horse drink water after exercise. But here are some things to take into consideration: it can be cold, absolutely. The horse’s stomach is relatively small compared to the rest of their digestive system, so they can’t drink copious amounts of water unlike, say, other animals.

[6:34]

The general rule of thumb is you can give them up to, like, half a gallon or two liters every 20 minutes or so. That will allow the horse to drink and then allow them to absorb that water, digest that water, and then drink again. And another thing to remember is horses can’t vomit, so if their stomachs get full, there’s nowhere for that to go — unlike, say, dogs or cats. But yeah, absolutely let your horses drink cold water after exercise; it really, really helps them cool down.

[7:11]

Now, I talked about the evolution of all this, right — why horses, when they exercise, what their body’s doing. I mean, it’s a stress response. Exercise is a stress response, and it triggers a cascade of events in the mammalian system — in horses and humans.

[7:34]

Now, when we think of heat stress, we have to go back and understand horses’ origins. That’s why I love studying the history of the horse — the origins of the horse, their evolutionary history — because it impacts so much of what we do with them today and our understanding of them. So to understand the horse, you need to understand their history. And I’m going to use a comparison with their close cousins — the donkeys.

[8:01]

Our domestic donkeys are descended from the African wild ass — that is where our original domestic donkeys came from. Horses evolved in North America. So you had horses on the Great Plains, and you did have some in Mexico, some in southern Canada, middle parts of Canada, and then they did migrate into Asia — equids did, right — but our Equus ferus, it was this region of the planet where the weather patterns… so think about it, I think of Kansas, Nebraska, the Dakotas, the Great Plains, these prairies that were there many, many years ago.

[8:47]

You would have really warm summers — hot, humid summers, not too hot, but warm — and then you would have really cold winters, especially in North America, those Arctic blasts and storms coming down from Canada. Southern Canada — same thing — you had horses there, but the weather patterns: the summers were pleasant, not too hot, but then the winters could be really cold.

[9:14]

So you had an animal that evolved in that, right — they learned to survive — and millions of years of evolution have led us to the horse we have today. Now, the donkeys — North Africa, okay — deserts, hot all the time, humid, and they have evolved to survive in those hot, hot environments. I mean, you just look at their ears, and whenever you look at different mammal species, the arctic animals that we have up there, like polar bears, have really small ears because that is an area where they will lose heat, whereas donkeys and African elephants have very, very large ears because they dissipate heat that way.

[10:01]

They get rid of the heat. And some other things that donkeys can do compared to horses — and that’s why they’re a little bit more tolerant to heat — is even though they have fewer sweat glands than the horse, their sweat is more efficient, their evaporative cooling is more efficient, they have thinner coats so the hair of the coat doesn’t trap heat, and they’re able to dissipate heat easier than, say, our horses. Donkeys are used to living in the heat — they seek out shade more often, they’re not out as busy during the hottest parts of the day, they are more efficient at conserving water in their bodies, and they actually have a higher surface area to body mass compared to horses.

[10:51]

What I’m going to explain a little bit later — especially when comparing to humans — is that horses have a really big body, right, with spindly legs and then their long necks and their head, whereas donkeys — their bodies aren’t as robust and they have a little bit more surface area, so they can dissipate heat a little bit easier. I’ll talk about that concept more in a little while. So when we understand that, then we understand horses don’t tolerate heat and humidity as well as, say, donkeys.

[11:26]

Then, when we get into our breeds — when humans domesticated horses over 5,000 years ago, we started to mess with the genetics. And so you do see Arabs, Turks, these Oriental types — so that leads into Thoroughbreds and other related breeds — being a little bit more heat tolerant than, say, our draft breeds or our ponies, who were bred and selected in parts of Europe and the United Kingdom, where they were selecting the best animals. And the best animals were ones that could probably withstand the cold better — it wouldn’t get really hot and humid there — whereas the Bedouin peoples or other earlier humans and cultures would select horses that did well in the heat, right, the ones that could go farther, and those were the ones that were selected to be bred.

[12:30]

So there has been some human impact on horses’ ability to withstand the heat within breeds. But, you know, getting back to this — all horses and even donkeys can get heat stress. They absolutely can. It doesn’t mean the Arabs won’t get heat stress — they will — they’re just probably a little bit more tolerant than some of these other breeds.

[12:48]

So when I say heat stress, what am I saying? A good definition is when horses cannot adequately regulate their body temperature, leading to overheating. And when they can’t regulate their body temperature — and I’ve seen this in a few horses that were heat stressed — they don’t cool down, they are breathing rapidly, they are sweating rapidly, they have elevated heart rates, they’re a little bit lethargic, they might even be a little bit in distress. And I’ve noticed in my studies that at times we see the temperatures go up after exercise and spike and then maintain there — they were not coming back down. And that’s when we would step in and start cooling these horses down because we didn’t want to get to the step where it’s heat stroke.

[13:46]

And at the end I’ll talk a little bit more about heat stroke — but heat stroke is where the horse is collapsing, it has dry mucus membranes, it’s dehydrated, it’s in severe distress, and you are in danger of that horse dying. So heat stress is where you start to see these signs and you step in before you get to the heat stroke.

[14:04]

So we talk about normal body temperature for a horse — we’re looking at the range of 99 to 101 degrees Fahrenheit, which is about 37.2 to 38.3 degrees Celsius, and that’s the range we want them in — their thermoneutral range. When a horse’s body temperature gets above 105°F or 40.1°C, that’s where they’re going to start suffering heat stress. And then when they get into 107°F or 41.6°C — if they get above that — you’re starting to get into this heat stroke death zone, and that’s where you really have to intervene with the horse and get them cooled down as quickly as you can, because that is very, very dangerous to them. So we’re going to talk about that at the end — some of the things you can do.

[14:59]

Now, heat stress can impact your horses — not just overall health — so you’re looking at reduced performance, reduced reproduction (which I’ve talked about), reduced resistance to illness or disease, their immune systems get compromised, they’re very stressed, and even chronic stress. So a horse that’s chronically heat stressed can develop some serious issues. It can impact management, the way you use those horses, their behavior, their eating.

[15:29]

This is where a big concern is — when a horse is stressed, especially heat stressed, the last thing they want to do is eat. Again, that blood is shunting away from their digestive system, so it is not as effective, and they’re not going to eat as much, or their eating patterns get disrupted. And this is where you run into those issues like colic or laminitis, especially when they’re overworked. So that is why it is such a big concern, and bottom line — it’s a welfare issue. Horses — we don’t want them to suffer at all, and so we need to take steps to manage them.

[16:00]

Now, here’s an excellent, excellent article that was recently published — it’s out of Australia — but it is Heat Stress in Horses: A Literature Review, and I love it because it actually quotes a couple of my studies. It was in the International Journal of Biometeorology. So it’s talking about a changing world where especially places like Australia are seeing extremes in their temperatures. I know in Canada — northern Canada — they’re seeing temperatures of 90 to 100 degrees Fahrenheit in the far, far north, and you can only imagine how hot it’s getting around the equator. It’s getting warmer and warmer, so we are seeing increases in temperatures, and so horses that are not adapted to the heat will be suffering from this.

[17:03]

I guarantee you — horses in the north of Canada, coming out of winter with those thick furry coats, when spring hits and if they get some extreme days like they’ve seen — those horses suffer because they haven’t shed those coats yet and they’re not used to those temperatures, so their bodies aren’t adapted. And I’ve got a good study that talks about adaptation, so this is a very good paper that really touched upon how horses do suffer heat stress.

[17:32]

And again, getting back to normal temperatures — when ambient temperatures are between 5 to 25 degrees Celsius (so that’s 41 to 77 degrees Fahrenheit), that’s generally where horses are fine, they thermoregulate well. Anything outside those ranges — that’s where we start to go, okay, they might need some help. And just to jump ahead — above 77 degrees Fahrenheit or 25°C, horses need shade, right, and water. Shade and water — that’s a big one that you can do for them. If it’s under 5°C or 41°F, that’s where we start to think, okay, they need shelter from the wind, they might need blankets, or you bring them into the barn.

[18:24]

And you know, in a future podcast we can talk about cold stress, prepping for winter, all of that — how do you keep your horses warm during winter. But you know, in that range, horses generally thermoregulate well — they maintain what we call homeostasis — so they’re going to maintain those ranges of thermoneutral.

[18:45]

Now, when we ride them — say it is those temperatures but we do ride them — they’re going to increase their body temperature because you’re going to start to metabolize, your muscles are working, and you’re going to get above that thermoneutral zone. Stress, activity, things like that can cause it because stress will cause the horse to breathe more, heart’s beating a little bit faster, so it’s generating more heat in the body. And these are some generalities — like I said, I’ve already talked about a little bit with some of those breed differences.

[19:21]

Other things to keep in mind — horse temperatures do fluctuate during the day. In the mornings, they are generally lower than they are in the evenings. In winter, the lowest body temperatures horses will experience are in those early mornings, and then in summer, their highest body temperatures are going to be in the late afternoons. That’s peak temperature, peak humidity, and that’s the hottest part of the day.

[19:51]

Now, the horse, like I said, is maintaining homeostasis — so it’s either producing heat to gain heat or it’s dissipating heat to lose heat. And a couple of terms that you might hear are hypothermia and hyperthermia. So, hypothermia is when the body temperature drops lower than 99°F or that 37.2°C. Hyper — and the way I always remembered hyper was I always think, oh, my kids are hyper, they had sugar, they’re bouncing off the walls — so it’s like, ah, increased activity. So then I think, oh, increased whatever “hyper” is before. So, hyperthermia is body temperature, meaning body temperature is above that 101°F or that 38.3°C. Okay? So you’ll see that in literature or articles that you might read or come across — hyper and hypothermia.

[20:39]

Now, I mentioned donkeys have a higher body surface-to-mass ratio compared to horses. I think the easier way to describe this is between us and horses — because we can relate, right? We get hot, we exercise, and think about the horse — the way they’re built — that huge body with, again, those spindly limbs, those thin limbs, and then the necks and the head. The heat has to get from the core to the surface of the horse, and that’s the area where they’re going to lose a lot of heat.

[21:26]

Whereas we — we are bony, we have really long legs, really long arms, here’s our core, right, and we have multiple places where we can lose heat. So we have a higher body surface-to-mass ratio, so we lose heat easier than the horse after exercise or when we’re hot. You know, it’s like when you take a hot shower or bath on a cold day, or you get out of a hot tub — next time you do this, look at your body and the steam coming off all around you — you’re losing heat, right? Whereas the horse is just coming off mainly in the neck and their body. So we lose heat a lot easier than the horse — it takes them more effort to lose heat.

[22:14]

So, switching gears a little bit — talking about how horses do get hot. Heat accumulation — hot and humid days — that is one way, without any exercise. Let’s say horses are just on maintenance, they get some voluntary exercise, they’re not being pushed, but it’s just one of those hot days. And I lived in Texas, I lived in Florida — anybody that’s lived in those areas of the world where it’s hot and humid most of the year… we had some cold days, but you know, mainly it was, oh geez — especially Florida — really hot, couldn’t wait to get into the air conditioning to cool off.

[23:04]

Well, horses can’t — they don’t have an air conditioning. I mean, they might, it depends on your facility, but generally horses are out on pasture or in paddocks or even in open-air barns — at least they’re shaded there — but it’s hot, it’s humid, and they will sweat, and they will start to thermoregulate. They will drink a lot more water — that’s why you always want to make sure your horses have fresh, clean water 24/7. And in the summer or on hot days, you want to make sure that water is not stagnant, that it’s cold — you refill it often, you can put ice in it to cool it off, especially in those late afternoons when those horses need to cool down.

[23:46]

And you know, in those conditions, the horse’s body starts to work with their hormonal and nervous systems, and one of the things you’re going to start seeing the horse do is sweat. And on hot days you see horses sweating, and with that sweat, again, they will lose water — but then electrolytes. And this is again where electrolytes are — now, today, and I’d say in the last 20 years — I mean, I’ve seen research a lot on electrolytes, we always wanted to replace those in our horses, but now, even today, it’s even that much more important. We’re providing electrolytes in the diet — and I’m going to talk about horse sweat and what makes it unique and why they need electrolytes.

[24:38]

But what happens with sweat — which is so cool, this is why biology, this is why I love biology, it’s so fun — so sweat, again, as I said earlier, as that heat is pushed out, blood flow is increased to the surface of the horse, to the skin. So the internal temperatures are above that 101°F or that 38.3°C, and the horse’s body kicks in saying, hey, we need to cool down. So one of the ways we’re going to cool down is sweat. And so it starts to shunt all that blood away to the surface, and then it causes the horse’s sweat glands to produce beads of sweat that work their way to the surface of the coat. And through heat of vaporization — it’s physics — is how the horse will cool down.

[25:30]

What’s going on is that water molecule — that sweat — it takes energy to convert that to moisture in the air, and what it’s doing is using the body heat to make that sweat evaporate, so it has that cooling effect. I mean, again — biology — it’s just amazing what’s going on. So that transfer, that movement of heat from the body out to cause that sweat to dissipate into the air, that is one way they do it.

[26:01]

Now, these sweat glands, like I said, in horses are different than, say, us. They are known as the champion sweat producers of all of our domestic animals, and they have two types of sweat glands — the apocrine and the eccrine. The apocrine (or apocrine, depends on how you say it) glands — that’s the majority of the body of the horse, and this is the majority of the sweat you’re going to see. The eccrine glands are located in the frog of the foot, so the horses do sweat and they do lose some heat through their hooves, especially on cool ground. So we’ll talk about that.

[26:41]

And so, you will see sweat typically on the neck, the buttocks or the croup of the horse, the shoulders, the flanks — especially if you, if you know, they sweat a lot under a saddle pad if you have a saddle pad on, and you’ll see that. What’s also really cool about horse sweat — and that foamy white sweat that you see — is latherin. That is a special protein that the horse produces in their sweat, and what it’s doing — this is again why biology is cool, why horses are so cool — is it’s like a detergent, and it reduces the surface tension of that moisture from the sweat to allow it to go through the coat of the hair of the horse, and that allows it to cool off.

[27:33]

And then when you come to electrolytes — they actually lose more electrolytes than we do when they sweat. So that’s why electrolytes are so important with horses that sweat a lot or exercise a lot — in a good part of their diet, you can add some electrolytes that way you’re ensuring — and I’ve got a study that’s going to back this up a little bit here I’ll talk about — that’s why it’s such an important aspect of managed care.

[28:05]

Horses produce two times more sweat per square inch of skin than us. And if a horse is sweating just in mild weather, not really hot weather, they can lose up to two gallons of fluid per hour — up to two and a half gallons per hour in warmer temperatures — and then during exercise they can lose upwards of four gallons. So they can lose a lot, they sweat a lot, and again, that’s why you always give them access to fresh, clean water. And adding those electrolytes to their diet can be very, very beneficial.

[28:42]

Okay, so in those hot and humid days — here’s some generalities. If it’s mild outside — so let’s say temperatures are 80 to 90 degrees Fahrenheit, it’s getting toasty a little bit — so that’s 26.7 to 32°C, horse body temperature might get up to 102°F, which is 38.9°C, but generally can go back down, especially if they have shade, shelter, water, all that. High heat with high humidity — these are the tough days. These are the average days of Florida in June, July, August, September, October — oh gosh, even back in April, May — I mean, most of the year there, it was just hot and humid.

[29:23]

And so, you’re looking at outside temperatures 90 to 100 degrees Fahrenheit — that’s 32.2 to 37.8 degrees Celsius — high humidity, so greater than 50% humidity. That means that’s the moisture in the air, the humid atmosphere. Here, you might see again a horse at rest — this is just horses out on pasture or even in stables or paddocks — horse body temperatures can get up to 102 to 103 degrees Fahrenheit, so that’s 38.2 or 39.4°C. So again, this is where you want to ensure your horses have absolutely access to shade, shelter, water, and other ways you monitor them to see if they start to suffer from heat stress.

[30:11]

In these extreme temperatures — above 100 degrees Fahrenheit or 37.8°C — this is where horse body temperatures can rise to be over 103°F, which is 39.4°C. This is where it can be dangerous for the horse; they may need some extra cooling or be brought in during the heat of the day if they don’t have somewhere to go. But I will say, horses that live in that heat — they do adapt to it. When I was in central California, it was 100 degrees every day, but it was very dry heat, so it wasn’t as humid, and our horses were pretty well adapted to it. They had shelter, and they were fine — plenty of water to drink.

[30:59]

In the heat of Texas, these horses again were adjusted. Florida — adjusted to it. They had areas where they could get sheltered, they had plenty of cool water — we had ponds, made sure there were no gators in them — but you know, they were fine. I’m thinking, when I talk about this, that — I mean, in the Yukon Territory, I remember, I forgot the name of the town, was it Yellowknife or something? — 100 degrees Fahrenheit there, and the horses up there were like, “What is this? I’m being cooked alive!” because they’re not used to it, right? So you get those extremes.

[31:34]

So these are the people that really need to pay attention to those horses. When you live in these milder, cooler climates, like where I live now, if it hit upwards of 90 degrees Fahrenheit — holy smokes, our horses would be suffering big time around here because they’re not used to that.

[31:53]

Okay, so that’s, you know, ambient temperature, ambient humidity. Now, let’s talk about exercise. This is where you’re going to start to see horses suffer heat stress, especially on those hotter, warmer days. That is why this is such a big topic for our industry, because this is where you’re going to push your horse — because that’s what we do. We ride them, we exercise, we compete with them. And while we’re doing that in hot, humid conditions, we’re really pushing them, really pushing their body to the limit sometimes.

[32:30]

So I went back to one of my seminal papers — this was a paper that, one of the very first ones I read — it was published in 2000, but it was titled Heat Storage in Horses During Submaximal Exercise Before and After Humid Heat Acclimation, and it was Raymond Geor out of Canada, University of Guelph, and other authors. This was a study that was one of the first ones I ever pulled as a PhD student and read through, and I used it as a guide to see, you know, where I could push my horses to evaluate heat stress and then look at reproduction and things like that. So it was just a wonderful paper, and it really is relevant here.

[33:16]

Because what they’re talking about is just not how horses get hot during exercise, but can they acclimate to it? Because think about it — in the modern world today, right now as I record this, horses are getting ready for the Summer Olympics in 2024. Now, they’re in Paris, France this year, so it’s probably not going to be too hot in July, but it could be — Europe’s been getting crazy, and those of you watching from Europe know the swings in temperature, where parts of Europe just get really hot really, really quickly, and then they go back to where they normally are. So some of these horses could be impacted by this.

[34:04]

But as we’re taking these horses around the world, and if we go from a milder climate to all of a sudden we’re competing at the Olympics where it’s hot and the horses aren’t acclimated to it, their performance is going to suffer. Bottom line — I mean, that’s a no-brainer. So this study talks about that — can we train these horses under hot, humid conditions or hot, dry conditions so when they do go compete, they can recover faster and they’re not suffering while they’re competing, right? They’re not suffering heat stress.

[34:35]

And one of the things that he talks about in this paper is horses actually produce more heat compared to humans. So, while the horse is exercising, the cells — especially think of muscle cells — so you’ve got the heart and skeletal muscle that’s moving. The horse needs fuel — needs the gasoline of the cell — so what it’s doing is it’s burning the fuel or oxidating the fuel, carbohydrates, fatty acids, and that’s providing fuel for the cells. That reaction produces heat — so that’s where the heat’s coming from. And it comes from the core, and again, is getting shunted to the extremities of the horse.

[35:19]

Now, this study looked at exercise with horses in hot, dry — so their hot, dry was 90 to 92°F, so that’s 32 to 34°C with 50% humidity — then hot, humid — so this was 90 to 92°F with 85% humidity. So that’s where I was really pushing my horses to look at impacts on reproduction. It was just — it was hot, and again, I was looking out for the welfare of these animals. I didn’t want them to be heat stressed, I didn’t want them to suffer. So it was exercising them to spike those temperatures and then breed them — that was my study, and I’ll talk more about that in a future podcast.

[36:02]

This study particularly would put them on a treadmill at moderate-intensity exercise and would exercise them until their core temperatures — or it was the pulmonary artery they were measuring the temperature in — got to 106.7°F or 41–41.5°C. That is hot — that is way hot. And my research — my horses rarely got above 105°F, and they generally were in that 103 to 104°F range. I had a few that did get hotter than that, but not above 106. Like I said, the death range, that heat stroke range, was getting to 107°F.

[36:45]

Now, these horses — this was at a research university — these horses were watched by DVMs, carefully monitored getting to those temperatures. But that is really hot — that’s really hot. So this study is very, very interesting in what it showed. And what they were showing was — we’d exercise them at these intervals and look at 18 and 21 days later, how the horses acclimated, and they found some really interesting data.

[37:18]

The heat stress index — so they were looking at all the factors of heat stress — the hot, dry spiked obviously above controls, but hot, humid — it was way above, way above. I mean, that’s really pushing the horse. So you don’t want to exercise — I’m just saying this here now — you don’t want to exercise your horse in the heat of the day, especially those late afternoons or when it’s really humid. Even early mornings tend to be that way, and I would even warn you if your horses normally reside in, say, places like Florida or Texas, because even though the horses are kind of used to the heat, when you exercise them, you really push them.

[37:58]

Now, sweat rate was interesting — it was pretty similar from hot, dry and hot, humid. So when they looked at time for them to reach that temperature — so going from, say, a 101°F to a 106.7°F (and I’ll just stick in Fahrenheit) — this is what was really surprising. This data — this is what I found really, really interesting — it took these horses only, at hot, humid — so 85% humidity in those 92–94°F conditions — it only took them around 20 minutes of moderate-intensity exercise to get that hot. That’s not very long, that’s not a huge training session.

[38:46]

So it only took those horses, again, 20 minutes to get there. Now, that’s hot, humid, because humidity is a key factor here — because that really reduces the horse’s ability to sweat, and again I’ll talk about some of the other ways they lose heat. So the hot, humid conditions — you need to be very, very careful with your horses. Hot, dry — this was a little bit interesting — just under 30 minutes at the start, but after, it was about 35 minutes on average for horses to get there. So under hot, dry conditions, that was 15 more minutes of exercise compared to the hot, humid.

[39:32]

And then when you looked at exercise recovery, the temperatures after 60 minutes were just above baseline — so they were coming back down. They were still in that 102°F, 38°C range, most of them. In the hot, humid, the heart rates after 60 minutes were roughly back to baseline before they did the exercise, but respiration rates were still pretty much going — especially in the hot, humid conditions — where horses were still, after 60 minutes… and again, that’s a big way they lose heat, is breathing rapidly. It’s important to the brain — again, I’ll talk about that in a minute.

[40:22]

So, understand the cool-down period for horses that exercise in hot, humid conditions needs to be longer. And this is where you might step in and make — you know, and I’m going to talk about running cold water on them and letting them drink. The big takeaway from the study was horses do adapt. So, after the adaptation period — so, 21 days — they were not as stressed during exercise, and they did tend to recover quicker. And so that was the main finding of the study.

[40:56]

I really want to highlight the study, though, because I wanted to say — moderate exercise isn’t heavy or very heavy exercise. That’s not endurance or polo or some of these other really intense events that push horses. This was moderate exercise, and under the conditions of that, especially in the hot, humid, it only took them 20 minutes to get to the 106°F range. That’s very hot for a horse. So that is the reason I really wanted to highlight this study — to show you that horses can get really hot, really quickly. That’s why you have to be very, very cautious in hot and humid environments.

[41:40]

All right — really quickly, something so cool about horses, this again why I love them — they have an ability to selectively cool their brains. Now, remember, the brain is the most important organ of the body — no brain, no nothing. You know, the heart’s not going to pump and all that, and if the brain starts to cook, that’s where the body will shut down, right, and that leads to death — heat stroke death.

[42:05]

So, this study was really, really cool — “Selective Brain Cooling in the Horse During Exercise and Environmental Heat Stress.” And again, out of Australia — because Australia gets really hot, it gets really hot — and they’re really seeing some extremes these last few years. Now, in this study, they were just looking at comparing the brain temperature, say, versus other parts of the body, and that’s why this study is way cool — this is why I was really excited to present it.

[42:37]

They were just exercising horses around 25°C — so that’s 77°F — 65% humidity. So they were not — this isn’t a heat stress study — this is just seeing how horses cool their brains down. So they were measuring core temperatures — so they had surgically inserted measurements throughout the horse’s body when they did the study — core temperatures, the hypothalamus (which is at the base of the brain), rectal temperatures (and that’s how we normally take temperatures of a horse, right — we get a thermometer and insert into the rectum), then they measured the cavernous sinus of the horse — so that’s in their nasal cavity, near the brain — then the facial skin, and then the midside skin. And these are different ways we measure temperatures in horses — we have these thermal guns now that we can go out and look in large animals, and it shows us skin surface temperature.

[43:31]

So after they exercised these horses, this data is what is really interesting: core temperatures were 42.3°C — so that’s over 107°F — this is core, this is inside the horse. Now, they looked at rectal temperatures, which was just under 104°F — which is 39.9°C — and that’s the range that we’re talking about when we measure it, because that’s the way we routinely measure temperature. So, core temperature is going to be a little bit hotter, right, and then you’ve got to cool these horses down. So those are your ranges — but it shows you, going from close to 108°F to close to 104°F, that the rectal temperature and core body temperature don’t correlate. And we know that — we know that as scientists — that when I go in and take the temperature in the rectum of the horse, which is near the exterior and I’m using a digital thermometer, compared to the core — that’s the internals of the horse.

[44:36]

So again, that’s why when you see a horse getting 104, 105, 106°F with the rectal thermometer, you’re getting into those danger zones — you’re getting into that 107, 108°F rectal temperature death zone, okay? So keep that in mind as I talk about this. Now, the core temperature is really hot — the sinus, the cavernous sinus, was 36.2°C — it’s a huge drop-off from the core temperature. And then the skin temperatures were around 38°C. So rectal temperatures close to 40°C, core temperature 42.3°C, and the cavernous sinus 36.2°C.

[45:11]

And it’s really interesting because horses are using this to cool off their brains. Horses cannot breathe through their mouths — they breathe through their noses — and because the soft palate separates the nasal cavities and the oral cavities, horses are always breathing through their nose. And that respiration, and the increased blood flow I talked about with exercise — so there’s more blood flowing to the nasal cavities — is helping not just the body cool down, but also the brain. And horses also have guttural pouches and other functions in the venous blood flow that help the brain stay cooler while the body’s way hotter. And so, it’s really just fascinating — fascinating — horses.

[46:06]

Okay, so we’ve talked about horses accumulating heat just standing outside, you know, and then exercise will definitely cause them to get hot. Now, another interesting study I came across was hydration impact on heat stress in horses — and, you know, if you give them fluids before they exercise, does that impact their ability to perform better and suffer less heat stress?

[46:33]

So — great study, again Dr. Geor out of Guelph — “Hydration Effects on Physiological Strain of Horses During Exercise Heat Stress.” And he was looking at giving these horses hydration therapy to see if that improves their ability to thermoregulate and not suffer heat stress. So they were exercising horses at 34.5°C, or 94°F — so hot day — but only about 50% relative humidity, so it wasn’t as humid. But they were — you know — it’s hot, that’s a hot day.

[47:24]

Then, they were giving horses either 10 liters of water or a carbohydrate-electrolyte solution two hours before exercise, or no fluids — and then they looked at their performance. Really, the big takeaway from the study is the horses that had hydration therapy — whether it was water or electrolytes — body temperatures didn’t get as hot because these horses sweat more. So there was more fluids available for them to sweat, and when you gave them electrolytes, they sweat even more. And so, hydration was really big.

[48:10]

And just some takeaways from the study were that giving them water or that carbohydrate-electrolyte solution was effective in reducing physiological strain associated with exercise-induced dehydration and heat stress. So hydration, even with electrolytes, can help alleviate heat stress. And before exercise, you know, you want your horses to drink if they can — give them some electrolyte therapy, let them absorb that, and then go off and do what you’re doing.

[48:46]

Now, the final consideration is age and age impacts on a horse’s ability to thermoregulate. I’ve done multiple podcasts on aging — just last week I was talking about, you know, how aging can impact horses’ health, their viability in retirement, when to retire your horses. I talked a lot about that, and you can listen to that podcast to learn more about how age impacts the horse’s body.

[49:21]

But when it comes to thermoregulation and cardiovascular function — and this was, again, one of my friends, Dr. Ken McKeever out of Rutgers — he published a study recently in the Equine Veterinary Journal, “Age-Related Decreases in Thermoregulation and Cardiovascular Function in Horses.” And, in essence, he was taking horses of the same body type, same weight, exercising them, and then seeing how quickly they got hot, and then afterwards, right? So he stopped exercise once their core temperatures reached 40°C.

[50:02]

The overall findings of the study — just to get through this one quickly — was the older horses, it took half the time to get that hot. So they couldn’t exercise as long. The younger horses could go longer before they hit that 40°C mark, whereas the older horses did it in half the time. And he concluded that this increased susceptibility of older horses to overheating — exemplified by this study — should enable veterinarians, owners, and riders of horses to identify certain horses that could develop hyperthermia — remember, that’s overheating — during exercise. So keeping that in mind with your older horses — they’re going to get hotter quicker.

[50:46]

Okay, so we know our horses can get hot. Now — how do they get rid of it? How do they get rid of that heat? What do they do? Well, there’s thermal radiation, convection, conduction, and evaporation. So we’ve talked about sweat a little bit, but let’s talk through these concepts.

[51:05]

Thermal radiation — this is just where the horse radiates heat. Remember earlier I said, you get out of a hot bath on a cool day or night at your home, or you get out of a hot tub and it’s cold outside, and you just see the steam coming off you, radiating heat — you feel it. That’s why we take hot baths — because our body absorbs that, we get a little bit warmer, and then we radiate that heat out. That’s what’s going on — they’re losing, they’re radiating that heat out.

[51:36]

And, you know, in cold environments they’ll still radiate heat but lose less, because one of the adaptations we all have as mammals is less blood flow to the extremities or to the outer surface area. So the horse will vasoconstrict — less blood flow there — it’s like your hands and feet on a cold day, they get cold, right, because there’s less blood flowing down there, so they’re not as hot. And you’re probably picking up, especially like your feet on a cold floor, you’re losing heat there too — so you cool off.

[52:12]

Now, this thermal radiation — if it’s hot outside and humid outside, it’s really difficult for the horse to radiate that heat, especially in those extreme heats when it’s, say, 105 to 110°F and the horse’s body temperature is there. I mean, it’s like the gradient’s the same, so there’s not a lot for the heat to go, and horses will go through other things to lose heat if they can’t do it very well.

[52:42]

Very fun and interesting fact — zebras. Black and white stripes — the black stripes in zebras are warmer than the white stripes, because the white’s reflecting, the black’s absorbing. So, that heat radiation — they know there’s a temperature difference. I’m going to talk a little bit more about this in a second — where the horse is absorbing heat from the sun and the white is reflecting that. Also impacts horse coat colors — so our darker coat colors may absorb more heat from the sun. So, again, that’s why shade’s important, but then, say, our lighter-colored horses… so keep that in mind.

[53:27]

Now, convection and conduction — these are physics, I’m just trying to make them as simple as I can. Convection is when you get wind or water running over the skin of the horse. Breathing — they’re losing through convection. Again, hosing your horse down with cold water — the heat from the horse will transfer to either the air or the liquid or the gas — so the air or the water — through conduction. Conduction is where the molecules actually exchange and you’re transferring that heat, but convection is the air movement over the body or the fluid over the body of the horse.

[54:12]

What’s interesting, getting back to zebras, is the stripes — they’ve studied this because of the heat — and the white’s cooler than the black, it causes air movement over those stripes. So it’s probably an evolutionary adaptation that zebras have gotten that makes them able to survive in the heat of Africa, where there’s some air movement between the black and white stripes that’s causing this convection, and they will lose heat that way. That’s how they dissipate some of this. Again, that blood flow to the surface, that cool wind — that’s why fans (I’m going to give you that as a tip here in a minute) are a good idea for horses. It gets that air moving over them. And they all work together — I mean, all of this works in conjunction — but if one doesn’t work, hopefully the others are working so they dissipate that heat.

[55:04]

Then conduction is just the actual transfer of heat to the outside environment. So convection’s that movement of liquid or gas, but conduction’s the actual transfer of that heat. So horses’ hooves on the ground — like I said, that cold feet — when you put your feet on cold surfaces, they get cold, right? Every winter, if you live in an area with cooler winters, you’re losing heat through your feet. Well, horses lose heat through their feet — remember, that frog can sweat a little bit and they can lose heat through the ground too, but most of it’s all surface — I mean, they’re losing a lot of heat through their body, that’s where most of that’s taking place — but they can do it there too.

[55:46]

So that myth about running cold water over your horse is terrible — that’s terrible. No — that’s how you cool them down when they’re suffering heat stress. When I had a couple horses that were suffering and they just weren’t cooling down properly — because we’d watch them very carefully — we’d hose them down with cold water and help speed this process up. And for those of you that ride endurance or know people that ride endurance — during the breaks they wash them off with cold water and encourage them to drink. And if that caused serious health problems in our endurance horses, they wouldn’t be doing that — those horses couldn’t survive or compete in those events. They’re cooling the horse down — so you want to do that to help your horse cool down.

[56:28]

And then we get to evaporation — so they all, again, all working together. We’ve talked about the sweat producing — you know, most of the heat lost during exercise for a horse is through sweat — up to 70% of it, as long as that humidity is low. Because when that — again, you’re getting that evaporative cooling. But if there’s really high humidity, there’s already a lot of moisture in the air, and it’s a lot harder to lose heat that way.

[56:59]

So keep that in mind when you do that. All right — signs of heat stress — we’re going to start wrapping this up a little bit. Rapid, shallow breathing — not deep, but rapid. Flared nostrils — you see it. Their gait, their behavior’s off a little bit — they’re weak, they’re lethargic. High body temperatures — again, you want to take these multiple times, you want to see them going down. If it’s not — like I said, I saw it spike in those horses — that’s when I started washing them down, I was like, “Okay, you’re not cooling down properly, you’re stressed.” But you want to start seeing that rectal temperature start to go down.

[57:37]

High heart rate, sweating — sweating, sweating, sweating. The mucous membranes start to get dry because they’re getting dehydrated, so you check their hydration status — skin pinch test. Dark urine — that’s how we know we’re dehydrated. You know, when we’re hydrated, we have really light yellow urine. When we’re severely dehydrated, it’s very dark yellow. Same thing with your horses — really dark urine means they need more fluids.

[58:08]

So, how do you do this? That cool water over the skin — not just the legs, I see people doing that — but over their backs, that’s where you want to cool them off. Fan the horse — put them under fans. That combo works really well, and in human studies, that’s actually — water with fanning works way better than ice. So that is one thing that these studies have talked about. Another one — they would wet the horse down and then scrape it off, and then wet them again — and they found that was a good way to reduce temperature. In this one study, blood temperatures dropped by 4.1°C in 11 minutes, and the rectal temperatures and muscle temperatures dropped 1.1°C and 2.2°C respectively. So when they did that study, they dropped their temperatures really quickly.

[59:09]

So, they’re looking at ways to dissipate heat stress. Then — shade — horses need shade. I had a mare in that study early on — she was suffering from anhidrosis, meaning she had the inability to sweat. We see this happen in horses that live in warm temperatures year-round — again, they didn’t evolve that way, so you get these obscure ones that just can’t sweat. So while all the other mares ran off to the trough to get a drink, she ran straight to the shade, and she’d sit there forever. And I watched her, and we finally — we took her off the project — she couldn’t sweat, she couldn’t thermoregulate properly. But shade really helps, because that radiation from the sun — the horses will get hot in the sun. Typically, most horses are fine in the sun, you don’t find them under shade a lot, but when they are hot they will seek out shade, because that radiation from the sun will cause them to warm up.

[1:00:00]

Then — make sure again — cold water, electrolytes, and look at modifying your exercise or when you exercise. And those of you that live in hot environments — you probably already do — but you don’t want to exercise when it’s hot and humid, or in the late afternoons — you know, the peak temperatures of the day. A lot of my friends — we’d exercise in the mornings, or we didn’t exercise in outdoor pens, we were under the arena where it was shaded, and we had big fans blowing, and we monitored our horses carefully so they weren’t getting really hot. And that was in Florida.

[1:00:40]

So, you know, tailor what’s good for you, and just understand — heat stress is a serious issue for horses all around the world. Like I said — far north in Canada, obviously in Australia, from all around the world — Europe is seeing extreme temperatures, Asia is seeing extreme temperatures — it’s something that we need to keep in mind for our horses. So understand how they get hot, how to recognize those signs, and how to cool them off and how to help them. You do that, the horse is going to thank you for it — and so will I. You know — thank you for caring about them. If you haven’t hit the follow link, please do. If you like this video, give us a thumbs up, and we’re going to keep producing this content as long as you keep consuming it. Take care.