mare-and-foal
Heat stress can affect any horse, but older horses and young ones, especially foals, are particularly vulnerable. • Photo by Soledad Lorieto via Unsplash

Is it Hot Enough for You?

“Is it hot enough for you?” 

That is very likely the most annoying question anyone could ask during the summer.  Yes, yes it is hot enough for me.  It is certainly hot enough to cause issues with our horse’s health that we need to be aware of, regardless of the region of the country we live in. 

So far in Texas it has been a relatively mild summer, in that, it is just now hitting the 100s at the beginning of August.  The X factor though, is the unusually high amount of rainfall we have had through the end of July.  Having tall, lush pastures this late in the summer is a blessing for sure, but the accompanying increased humidity is what has lessened the value of temperatures we would otherwise be pleased to have.  Heat stress is a primary limiter of activity for people and horses.  There are multiple different ways we feel the effects of heat stress, which are compounded by the factor of time.  

Heat Stress in Horses

In the simplest of terms, heat is generated by two things within a horse’s body.  Muscle contraction is the first.  Physical exertion and muscle contraction generate the most heat within a horse.  The other source of heat within a horse’s body is digestion.  The heat generated by the break down, combustion and assimilation of nutrients within the gut creates a continual source of heat.  A great example of why not to feed your horse like grandpa did with oats and grass hay is because approximately fifty percent of the energy in oats is released only as heat.  Carbohydrates create more heat than you think. 

With this increase in heat the body is stimulated to cool the engines.  The innate mechanisms of vasodilation push blood through muscles out to the skin surface to allow for evaporative cooling to increase.  Easily seen on the inside of the thighs, forearms and jugular grooves of the neck are large superficial blood vessels that dissipate heat well.  The lung capacity of the horse with large bore nostrils and primary airways serve as heat exchangers by blowing off hotter air than they take in.  Of course, sweating is a major contributor to decreasing an elevated body temperature through evaporative cooling as well.

These processes work well in the healthy, adult horse.  However, the burden of heat stabilization can be made more difficult when deviations from ideal occur.  Cushing’s Disease is an obvious contributor as hirsutism (abnormal hair growth/inability to shed hair) is the equivalent of wearing your winter coat during summer vacation at the Grand Canyon.  If you needed another reason to not have an overweight horse is that they too poorly dissipate heat by the insulating effect of fat.  Heavier muscled horse will obviously generate more heat than adequately muscled ones. 

The ones you may not think of as being vulnerable to heat stress are the very young, or older horses.  Foals, in particular, do not efficiently cool themselves as well as adults.  So, a stressed mare running the fence line can inadvertently overheat her foal as he struggles to keep pace with her.  We have all seen the effects of cumulative heat stress on older horses.  It is almost like you can see the weight melt off them as the days of August go slowly by. 

It’s the Humidity

To better understand how that happens we can look at the environmental effects on equine physiology.  It’s not the heat – it’s the humidity, right?  Another annoying summer one-liner.  Well, it’s true.  Temperature is central, but the relative humidity must be taken into effect because it greatly compromises the horse’s cooling mechanisms.  Combining the temperature and humidity will give us a number – say 70ºF and 50% humidity equals 120.  The ability of a horse to cool himself is most effective when that number stays below 130.  Combinations from 130-150 result in a decreased capacity for cooling.  A number higher than 150 greatly reduces this process, and anything over 180 is considered fatal if virtually any stress is applied to the horse over time. 

How this translates to equine physiology is in the function of enzymes.  Enzymes are proteins that control and drive chemical reactions within the body.  These enzymes, just as all proteins, are heat sensitive.  By heating enzymes beyond a certain temperature, they become what is called – denatured.  They are changed in structure, permanently.  Denaturing a protein is the equivalent of cooking it.  Try to unscramble a cooked egg.  So, if they change in structure, they, by definition, will change in function.  This is when heat stress/heat exhaustion turns into heat stroke.  The permanent damage of tissue due to excessive heat.  Some tissue may repair and regenerate such as liver or muscle tissue.  Other tissue may not, such as the kidney, or brain tissue.

Know The Signs

Symptom recognition is paramount in prevention of heat related illness in horses.  Rarely will you have a rectal thermometer in your back pocket, unless you are a vet tech.  So, picking up on increased respiratory effort, and higher heart rates are crucial.  Weakness, stumbling, and dull responsiveness accompany cardiac and respiratory symptoms.  This may be exhibited in a horse’s refusal to work when they would otherwise gladly comply.  Dry skin is a give-away for sweat gland depletion where they would otherwise be dripping wet.  Slow capillary refill time of greater than 2 seconds adds to evidence of dehydration, and if you do have a rectal thermometer a temperature of as high as 105-107 ºF confirms the diagnosis. 

First aid treatment should be obvious to us all. Ending any physical activity immediately should be followed by cooling the horse off with large volumes of water over the large blood vessel areas mentioned before, as well as over the large muscle groups of the body.  Using fans to expedite evaporative cooling greatly helps this process. 

I mainly want to dispel the myths I hear related to cooling hot horses.  Yes, you can let your horse drink cold water – as much as they want to drink, and it will not cause them to colic.  They will stop on their own before they do.  You can use cold water, even ice water to pour over their large muscle groups to cool them off faster without causing them to tie-up.  Do not use ice packs directly on their muscles due to focal muscle damage, but I would hope that is an obvious point.  Offering water with electrolytes added is another way to replace the salts lost through sweating.  The assessment of immediate and potentially permanent damage should be done by your veterinarian through physical examination and hematological evaluation of blood electrolytes and systemic organ function. 

The older you get the more you will realize the negative effects of heat on human and equine physiology.  You may get hot and tired working your horse in the summer, but nowhere close to what they do – not even on a bad day.  I have fortunately only seen one horse in my career that was literally run to death catching cattle in the summer heat.  He was a big, pretty, bay stallion that had no quit in him, whatsoever.  The man that rode him was the same way, minus the physical eye appeal.  He caught every single, yearling heifer that got out that day.  When he brought him to me late that night he was halfway colicing, and in full kidney failure, because he was so dehydrated and tied up to the point he could barely walk.  His heart rate was 97 (normal of 32-36), his skin was dry and rough, and I quit counting after 5 seconds when checking his capillary refill time.  They could only get his rectal temperature down to 104 with ice baths on the way in. 

This man thought I would run the horse a little IV fluids, give him a few shots of B12, then be on the road to do it all again the next morning.  The only way I could get the point across to him that his horse had a very good shot at dying was when I showed him the tubes I had drawn his blood in.  That stud’s blood was so thick from dehydration there was only a small amount of orange fluid sitting on top of a stack of dark, red blood cells clumped together.  His packed cell volume was 72%, meaning his blood solids were 72%, while the fluid part was only 28%.  If you reversed those numbers you would have an anemic horse. Most horses sit about 35-40% solids with a 60-65% fluid component. 

I worked non-stop on that horse all night.  As painful as he was the horse never fought me.  He never offered to do anything but stand there and help.  I ran him IV fluids, passed a nasogastric tube multiple times to give him mineral oil and supplemental electrolytes by myself. He just stood there and took it all like the soldier he was.  By daylight, he was unable to stand with muscles harder than a rock, and urine the color of black coffee.  At this point, there was no amount of fluids, medicine, or prayer that would change this horse’s outcome. I had tried it all.

That horse was plenty cool, and plenty tough.  There is nothing wrong with that. That is what I remember about him.  Being tough and stupid is not okay, and that is what I remember about the owner.  That is what killed the horse.  We should all work hard to know the difference.