Overheating and the Microbiome

We’re all aware of the risk of dehydration and heat stroke with a hot horse but did you know it can also affect the intestinal microbiome?

Probiotic supplements are a good idea after exertion in the heat

 While the research in horses is in its infancy, studies in humans and other animals have  demonstrated that moderate exercise improves intestinal health and changes the bacterial community in the intestinal microbiome.  One study in horses has demonstrated a similar phenomenon https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5147854/ . However, overdoing it in the heat can have negative consequences.

In both humans and horses, heatstroke following heavy exercise in hot environments is accompanied by severe endotoxemia – the presence of bacterial toxins  from the gut in the bloodstream. The process actually begins long before the point of life-threatening heat stroke is reached.

With both severe exercise of short duration and more moderate activity of long duration, intestinal motility becomes altered, the abdominal wall begins to lose integrity and bacterial products leak into the bloodstream.  This is accompanied by a simultaneous shift in relative proportions of  bacterial species with  a drop in the more beneficial strains. The process is worsened during exercise in the heat. Prolonged shipping in hot weather could have similar effects. Mild cases may have only a temporary disruption in intestinal motility with absent or decreased gut sounds. Others actually experience colic or diarrhea.

These changes occur regardless of the level of fitness but horses that have been well conditioned for the exercise they are doing have been found to have high titers of anti-endotoxin antibodies.  As training increases in intensity, they begin to experience these episodes of increased intestinal permeability that exposes them to “doses” of endotoxin and acts like an internal vaccination. As a result they are better protected from the severe illness that endotoxins in the bloodstream can cause.

Unfortunately, while training has many benefits, protecting the gut from increased permeability and disrupted bacterial populations isn’t one of them.  The horses with high levels of antibodies can better neutralize the endotoxin and are more likely to perform well without getting sick but their intestinal microbiome is still disrupted.

Both human and veterinary researchers agree that repeated disruptions of the microbiome likely put the person or horse at higher risk of heat related illness and compromise gut function.  They propose giving probiotics to restore a more beneficial population of organisms could be very helpful.

Give a daily dose of 30+ BCFU (billion colony forming units) of a mixture of probiotic bacteria and Saccharomyces yeast, with an extra feeding after speed work or more moderate intensity exercise lasting more than 2 hours.

Eleanor Kellon, VMD




About Dr. Kellon

Graduate of University of Pennsylvania Veterinary School. Owner of Equine Nutritional Solutions, www.drkellon.com, industry and private nutritional consultations, online nutritional courses. Staff Veterinary Expert at Uckele Health and Nutrition https://tinyurl.com/vdxfex5h .
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3 Responses to Overheating and the Microbiome

  1. Allô Valérie,

    Un oui partout!😀

    Envoyé de mon iPhone


  2. Claire says:

    Does 24/7 exposure to high heat and humidity (no exercise) have the same effect? Here in East Texas, daytime temps are high 90s with humidity that adds 10 degrees to the heat index, lows are around 80 with even higher humidity, so we never get a break.


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