Transcript:

[0:00]

Hi everyone, welcome back to Mad Barn Academy, and if you are new here today, then welcome — we hope to earn your subscription today. I’m Dr. Fran Rowe, one of the veterinary nutritionists here at Mad Barn, and today I’m going to be talking about heat stress in horses. With summer in full swing and higher-than-normal temperatures being reported in many places across North America over the past couple of weeks, we thought this would be a great topic to review. So, let’s get started.

[0:32]

Heat stress is a condition that occurs when the body’s ability to thermoregulate — or maintain its internal body temperature — becomes challenged or even completely fails. As a result, body temperature rises beyond safe levels, which can lead to muscle damage and organ dysfunction if not addressed promptly. In severe cases, heat stress can progress to fulminant heat stroke, which is a life-threatening form of heat-related illness.

[1:09]

Horses can develop heat stress during exercise, particularly in hot weather, because they generate a large amount of heat from their large muscle mass — they are truly metabolic machines. But they can also become overheated at rest when they’re not working, if conditions are right.

[1:35]

There are several mechanisms by which horses thermoregulate. The most important mechanism is evaporative cooling via sweat — approximately 70% of heat loss during exercise occurs through evaporation under normal ambient conditions. The second most important mechanism is through conduction, or when heat is exchanged along a temperature gradient. A good example of conductive cooling is heat transfer from the skin surface to water, such as when you hose down your horse after exercise. Other, more minor methods of heat dissipation include infrared radiation and convection.

[2:30]

As you might expect, there are numerous risk factors for heat stress — some are intuitive, like coat, exercise, humidity, and anhidrosis, while others may be more surprising, such as age, body condition, diet, and the presence of other health conditions.

[3:00]

Let’s go through each of these factors in more detail. For age, foals are especially sensitive to heat, as they’re still developing normal thermoregulatory mechanisms. It’s important to monitor very young foals in direct sunlight and ensure they have access to shade when it’s hot. For body condition, overweight horses are less heat tolerant because fat is insulating.

[3:49]

Coat is another factor — horses with thick hair coats, such as senior horses with Cushing’s disease, or horses that are blanketed when it suddenly becomes hot, will have a harder time thermoregulating due to the extra insulation.

[4:10]

Diet can impact heat tolerance, particularly diets high in protein. Digesting protein produces a lot of heat, so horses in hot climates on high-protein diets may struggle to stay cool, especially if other risk factors are present.

[4:35]

Health conditions also play a role. Any illness that causes a fever will increase the risk of overheating if the horse is in a hot environment. Respiratory disease and anhidrosis are also significant. While respiratory cooling is a minor thermoregulatory mechanism in horses, compromised respiratory function can still reduce the horse’s ability to maintain body temperature. With anhidrosis — the inability to sweat, or decreased sweating — the main thermoregulatory mechanism is severely compromised, making it difficult for the horse to cool down.

[5:55]

Exercise generates a lot of heat, and high-intensity or long-duration exercise in hot conditions can lead to dehydration and electrolyte imbalance, further increasing the risk of overheating.

[6:18]

Humidity is another major factor — high humidity prevents evaporation of sweat, so that primary cooling mechanism becomes severely challenged.

[6:41]

The clinical signs of overheating in horses fall on a sliding scale, from mild signs of heat stress to severe signs of fulminant heat stroke. Even though heat stress is a milder form of heat-related illness, it’s still serious and warrants intervention. The good news is that most horses recover well once cooling measures are instituted — and we’ll talk about those cooling measures shortly.

[7:27]

Signs your horse may be overheating include abnormal vital signs — elevated heart rate, respiratory rate, and rectal temperature — even when at rest. You might notice the horse is huffing, puffing, and panting without activity, or is struggling to recover after exercise in hot weather. Other signs include dehydration (tacky mucous membranes, sunken eyes, delayed skin pinch over the shoulder) and excessive sweating, especially if it’s very humid outside.

[8:32]

So we had talked about that humidity decreases the effectiveness of sweat — or the sweat’s ability to evaporate and cool the body down — so the body sweats more, trying to compensate for that. Sometimes in these really humid conditions, after exercise, these horses will be sweating excessively. A couple of other things to look for would be a dull attitude, some lethargy, and signs of exhaustion, as well as an unwillingness to eat or disinterest in eating.

[9:13]

Heat stroke is a more serious heat-related condition, and it can be life-threatening. If your horse’s signs of overheating are this severe, you should institute cooling measures as immediately as possible and call your veterinarian for help. You’re going to notice severely abnormal vital signs — heart rate, respiratory rate, and rectal temperature will be way elevated. That horse may be stumbling, showing incoordination, reluctance to move at all, acting agitated, or showing signs of distress that might even look a bit colicky. In really severe cases, those horses can collapse, become recumbent, and even die.

[10:14]

If your horse is showing signs of heat stress, we recommend instituting the following cooling measures, which you can remember with this handy acronym: SHADE. First, stop whatever exercise you’re doing and remove as much tack as possible. Move to a cool or shaded area if that’s available. Offer your horse water for drinking — even if they’re not initially very interested, it’s important to have that option available. Hose off with water continuously and, if possible, apply a fan to the horse as well. These are our two most effective cooling methods to help the horse cool down quickly. Continue these measures until your horse’s temperature and other vital signs begin to stabilize. If your horse’s signs worsen or do not improve within 20 minutes of starting to cool them down, call your veterinarian for assistance.

[11:33]

Avoiding the heat really isn’t possible this time of year, especially for those of us living in hotter, drier, or more humid climates. But let’s cover some precautions you can take to help your horse manage the heat and reduce the risk of overheating. First, let’s talk about dietary changes. The first is focusing on hydration — adding extra salt or electrolytes to your horse’s meals will replenish any electrolyte losses from sweat and encourage them to drink and maintain good hydration. You can also soak meals to increase water intake — mash grain meals, add a meal of soaked beet pulp or forage cubes, etc.

[12:30]

We can also look at reducing the amount of protein in the diet, especially if the horse is already on a high-protein diet such as a lot of alfalfa or other high-protein feeds. We want to ensure the horse is getting adequate protein intake but limit excessive amounts — this is especially true for mature horses at maintenance or in very light work. Lastly, if you have a horse that’s a little bit of a “chunky monkey,” instituting a weight loss diet can help improve heat tolerance in hot climates.

[13:22]

Other prevention strategies to reduce overheating risk include reducing activity — modify your exercise routine or avoid forced exercise on particularly hot and humid days. We recommend using heat index as a guide for planning exercise in summer. Second, institute cooling measures as much as possible — use fans and ensure access to shade when turned out. Body clipping is also helpful for horses with thick or shaggy coats, especially if they’re new to hot or humid climates, as it can take time to acclimate. This leads into our next point — travel and acclimatization. Monitor horses closely during trailering to ensure they’re eating and drinking well, and remember it can take several weeks to acclimate to a new climate. If you’re traveling for shows, especially down south, recognize that if the horse isn’t used to that heat, it can affect them.

[15:19]

As I mentioned, in conjunction with modifying exercise, we recommend monitoring heat index to guide decisions about when or how hard to exercise in hot weather. The heat index measures how hot it feels outside by factoring relative humidity into the actual air temperature. This gives a better estimation of overheating risk than temperature alone, because humidity makes it harder for the body to cool itself down.

[16:00]

We have a heat index tool on our website that allows you to determine the heat index for your location in real time. There’s a link on this slide and in the description below. We’ve also provided a breakdown of how cautious you should be in different humidity conditions. Once the heat index reaches about 90°F (32°C), exercise caution and modify your plan. If the heat index goes above 100°F (37°C), forced activity should be avoided.

[16:52]

Here’s another visualization of our heat index recommendations. Both this and the summary chart from the previous slide are available in the blog article linked in the description below.

[17:09]

All right, here are the references for today, and thanks so much for listening! Don’t forget to like and subscribe, and explore the other videos on our channel — we have new content coming out all the time. Be sure to check out the additional links in the description below. If you want to use our heat index tool, I’ve also linked another article on heat stress in horses. All right, thanks so much — until next time!