How do high temperatures and humidity levels affect our horses? Most of the research has been done on exercising horses and has universally shown that horses exercising in both high temperatures and high humidity have higher core body and blood temperatures, higher heart and respiratory rates, lower cardiac output, worse sweat response and a decreased ability to dissipate heat from their bodies. And while the horses in these studies were being run on treadmills at moderately increased temps (90F) you can imagine a horse just standing in a field at 102F would experience the same changes.

Signs

• High rectal temperature (>103F with no exercise)
• High heart rate
• High respiratory rate
• Excessive sweating (inappropriate for the amount of work being done)
• No sweat despite ambient heat and exercise
• Lethargy
• Colic
• Collapse

Causes

While every horse is at risk for heat stress, there are some that should be given special considerations. Older horses suffering from Cushing’s Disease will not shed out their winter coats. This extra fuzz traps heat in and prevents cooling. These horses should be body clipped to help them keep cool (don’t worry, clipping the hair now won’t affect its ability to grow back in once the weather turns cooler). Horses with known respiratory problems such as Inflammatory Airway Disease or Heaves should also be monitored closely. Hot dry conditions can exacerbate their airway disease and decreased air exchange may contribute to higher body temps. Finally, foals who are being treated with a macrolide antimicrobial (erythromycin, azithromycin or clarithromycin) for Rhodococcus pneumonia should be kept in out of the sun. There have been reports of idiosyncratic hyperthermia in foals on erythromycin (though we should be cautious with all three) and drug induced heat combined with high ambient temps is a recipe for heat stroke.

Diagnosis

Diagnosis is based on clinical signs and rectal temperature.

Treatment

Treatments include many of the same steps as prevention (see below). You should immediately stop whatever activity you are doing and get the overheated horse out of the sun, under a hose and in front of a fan. Offer fresh water. Extreme cases, or cases that do not respond to the above measures should be seen by a veterinarian immediately. Prolonged high body temperatures can lead to systemic inflammation, denaturation of proteins and even multi-organ failure. Your veterinarian can help lower your horses core temp by lavaging the stomach with cool water (via nasogastric tube) and administering cool fluids intravenously.  Bute or Banamine can also be administered, not necessarily to lower the body temperature but to prevent some of the ill effect of the inflammation induced by excessive heat.

Prevention

While we can’t bring our horses into our homes to enjoy the cool refreshment of central air (or even a window unit), we can take some simple steps to keep our horses cool and prevent heat stress.

• Shade - Temperatures in direct sunlight are obviously going to be much higher than the shade, so make sure your horses have the option to get out of the sun. Run in sheds, large trees or even keeping them in the barn can keep your cool.
  
  
• Water - In high temperatures, and even more so in high humidity, more water is lost through sweat and increased respiration. Horses should always have access to fresh water but it’s even more important in this weather. So make sure your water troughs and buckets are full (and clean) at all times. If you’re not able to make it to the barn to refill them throughout the day, think about adding extra buckets to your stalls or paddocks.
  
  
• Electrolytes - Offering a salt or mineral block, placing an Elyte powder on their feed or administering an electrolyte paste will not only replace important electrolytes lost through sweat but can also increase your horses’ drive to drink plenty of the above mentioned water.
  
  
• Baths - Cold hosing a horse and allowing them to air dry can help lower body temperature. Their wet hair will also cool the skin through evaporation. For extreme conditions, water can be mixed 50/50 with rubbing alcohol to increase the evaporative effects. ** but make sure never to get any alcohol mix anywhere near your horses eyes!!**
  
  
• Ventilation - We all know how good a cool breeze feels. Placing fans in your barn aisles and in front of stalls can help keep things cool. Make sure barn doors and windows are open to let the air flow.
  
  
• Rest - Though we spend all winter dreaming of the days when we can ride across the fields, basking in the sunshine, extreme temperatures may be a good excuse to catch up on your favorite summer TV and give your horse a break. We discussed earlier that horses exercised in heath and humidity have much higher stress markers and much slower recovery. If you don’t have to train, don’t risk it. If you are getting ready for a big event, or use your horse in your work, plan activities early in the morning or in the evening when the temperatures are cooler.
  
  
• Protection from the Elements - The hot weather is caused by the sun’s rays, which also causes sunburns. Make sure to apply sunscreen to white noses which can become burnt just as easy as I turn into a lobster at the mere mention of UV rays! Warm weather also brings along biting insects which irritate and agitate your horses. Use fly sprays, fly masks and sheets to help keep your horse comfortable and bug-free.

So when the numbers on the thermostat are high, take some simple precautions to help keep your horses cool and healthy and keep an eye out for any signs of the detrimental effects of heat stress.