New Method for Scoring Sweat Losses in Horses Proposed
Horses mainly lose heat through sweating and heavy breathing. | Anne M. Eberhardt/The Horse

If your horse sweats and appears outwardly comfortable, he must be managing the high temperatures just fine, right? Not according to Clair Thunes, MS, PhD, equine nutrition consultant and owner of Clarity Equine Nutrition. During her presentation at the 2024 EquiSummit virtual conference, held Dec. 3-4, Thunes said horses suffering heat stress might appear outwardly unaffected. However, their gastrointestinal (GI) tracts struggle, which affects their health and performance.

How Horses Manage Heat

Horses heat up not only from the sun shining on them but also the heat generated during their metabolic processes. For example, the chemical reactions muscle cells use to contract large muscle groups during exercise all produce heat. Horses’ bodies need to manage heat to stay in a thermoneutral zone.

The key way horses lose heat is through evaporation of sweat, said Thunes, “and when that doesn’t work, they rely on respiratory losses through heavy breathing.”

Heat Stress, Injury, and Shock in Horses

Thunes defined heat stress as an increase in body temperature that causes any change in physiology that can impair other body functions. “Heat stress may not cause obvious harm or injury, but the body is nonetheless too hot to do everything it’s supposed to,” she said.

When heat stress progresses to heat injury, noticeable problems start to appear. Horses with heat stroke develop overt clinical signs, including GI tract damage.

Heat-Induced Leaky Gut in Horses

When horses get hot, their bodies reduce blood flow to the GI tract, redirecting it to the large muscle groups to facilitate cooling. This can weaken the gut lining, potentially leading to heat-induced leaky gut—a condition where the cells lining the GI tract lose their tight connections. Instead of maintaining an impermeable layer, gaps develop between the cells, allowing harmful compounds to seep into the intestinal lining and potentially the horse’s bloodstream.

“The presence of endotoxin from bacteria in the intestinal lining and bloodstream initiates an inflammatory response because they are not supposed to be there,” said Thunes. “Degraded food products and digestive enzymes may also leak into the circulation with similar consequences.”

Healthy intestinal tract is lined with villi (fingerlike projections) that absorb nutrients. These can become blunted due to heat stress, resulting in less surface area to absorb nutrients. Horses tend to eat less when heat stressed, which further reduces nutrient availability. Fluid movement in the GI tract can also become impaired, resulting in diarrhea.

In addition to the leaky gut, dysbiosis (microbiome imbalance) occurs in heat-stressed horses. The bacterial population in the intestinal tract can change in diversity, composition, or even the way it functions. “For example, there could be a decrease in the butyrate-producing bacteria, which normally help maintain the GI tract lining,” said Thunes. “With this decrease in butyrate, we see an increase in GI tract inflammation.”

In addition, the digestive tract plays a large role in a horse’s immunity. A leaky gut can trigger the innate immune system, worsening systemic inflammation, and compounds passing out of the gut could result in an allergic reaction.

Supporting Horses in the Heat

In addition to physically cooling horses and minimizing environmental sources of heat, owners can also combat the side effects of heat stress (predominantly leaky gut) by choosing feeds that contain the following ingredients:

  • Butyric acid and zinc, which support the cells lining the GI tract. These ingredients will help maintain tight junctions and decrease inflammation; and
  • The probiotic Bacillus subtillis PB6, which has been shown to kill endotoxin-releasing pathogens (disease-causing organisms) and help maintain the beneficial population of bacteria in the GI tract.

Take-Home Message

Heat stress negatively affects horses’ gut health, along with their immune system and metabolic function. In addition to reducing sources of heat, both environmental and metabolic, horse owners can provide electrolytes and offer feeds using ingredients known to counter the effects of heat stress, such as butyrate.

“It’s important we’re cautious about working our horses in hot weather and really understand what is happening at a cellular level when horses get hot,” said Thunes.

Additional 2024 EquiSUMMIT Coverage: