Leaky gut is a digestive issue is horses that occurs as a component of a broader gut health issue such as dysbiosis, inflammatory bowel disease, or gastric ulcers.
It is said that 70% of the horse’s immune system resides in the gut. The horse’s gastrointestinal tract breaks down food and absorbs nutrients while blocking toxins and microbes from entering the body.
In horses affected by leaky gut, the contents of the gastrointestinal tract are not appropriately contained due to impaired intestinal barrier function.
Toxins. allergens, and pathogens can penetrate the gut wall. If they are not stopped by the local immune system, they will circulate through the blood to the rest of the body. This can lead to an aggressive immune response and may cause systemic inflammation; at which point the horse is obviously ill. 
Circulating bacterial toxins and the systemic immune response can directly trigger laminitis.
Performance horses have a higher risk of this condition due to the effects of high-intensity exercise and feeding a high-starch diet.
Consult with your veterinarian if you suspect your horse has gut health issues. Submit your horse’s diet online for a complimentary evaluation by our nutritionists to optimize nutritional support of gut health.
What is Leaky Gut?
Equine leaky gut is typically not an isolated condition. It is a symptom or component of a broader gut health issue. Leaky gut can be a component of:
- Dysbiosis (microbial imbalance involving overgrowth of a pathogen)
- Twisted gut
- Heat stroke
- Severe hindgut acidosis caused by extreme simple carbohydrate overload or experimental fructan overload
Leaky gut is a general term used to describe impaired intestinal barrier function. Normally, the intestinal mucosal barrier blocks toxins and microbes from entering the body while enabling nutrients to be absorbed.
In horses affected by leaky gut, the gut wall is semi-permeable and allows foreign substances to move past the level of the cells lining the intestine. There these substances encounter the local immune system – known as the gut-associated lymphoid tissue (GALT).
Foreign substances involved in leaky gut include the following pro-inflammatory molecules: pathogenic microorganisms such as E.coli (some strains), Salmonella and Clostridrium, bacterial endo- and exo-toxins, and antigens.
If the GALT is overwhelmed, these substances can trigger a strong immune response in the body. In particular, lipopolysaccharides (LPS) which are structural components of bacterial cell walls are one of the most pro-inflammatory molecules found in the gut. The systemic inflammation that is triggered by microbes and other toxins entering the body can lead to endotoxemia (circulating bacterial toxins) or septicemia (circulating bacteria in the blood).
Common Signs & Symptoms
Horses may experience loss of intestinal wall integrity in three general categories:
- Diffuse and contained by the GALT
- Diffuse and not contained by the GALT
Focal loss of integrity
Focal loss of integrity is caused by parasites. This is likely a universal experience for horses as all horses are likely to be affected by internal parasites during their lifetime.
Common signs of parasite burden and associated leaky gut include:
- Weight loss
- Poor thrift
- Poor growth
In addition, the GALT becomes exposed to a wide diversity of food antigens which can result in a true food allergy or intolerance. This might also generate antibodies which can cause false positives on allergy testing.
Other examples of conditions that involve the GALT and local tissues only include:
- Severe gastric ulcers (grade 3 or higher)
- Inflammatory bowel disease
- Right dorsal colitis
- Infections from pathogens such as Salmonella or Clostridia
In these cases, the inflammation is limited to the bowel. The horse will show varying degrees of colic. If the hindgut is involved they may also show diarrhea. There may be fever when there is infection.
Diffuse loss of integrity
The most severe grade is when the GALT is overwhelmed and a systemic (body-wide) inflammatory response occurs. This occurs with bowel deprived of normal blood supply, severe infectious diarrhea, heat exhaustion and in the acute phases of grain or experimental fructan overload.
As above, this is accompanied by endotoxemia and/or septicemia. Septicemia is most likely in individuals with poor immune responses, such as foals. Foals can develop joint infections (“joint ill”) with septicemia.
With both endotoxemia and septicemia, the horse is obviously ill and can show the following clinical signs:
- Pale or infected mucus membranes
- Rapid heart rate
- Rapid respiratory rate
If you notice these signs in your horse, contact your veterinarian immediately. In severe cases, shock and even death can result.
In research studies, several techniques have been used to identify increased gut permeability.
Methods include measuring how well specific sugars that are not normally absorbed intact, such as sucrose or D-xylose, are absorbed after they are delivered directly into the stomach by nasogastric tube. 
Researchers have identified biomarkers of increased intestinal permeability. Biomarkers are factors in the blood that are suggestive of a condition but do not specifically identify the precise condition.
- Components of gut microbes – Lipopolysaccharide (LPS)
- Components of damaged intestinal cells – intestinal fatty acid binding protein (iFABP)
- D-lactate – the lactate produced by bacteria
- Indicators of an activated immune response – inflammatory cytokines (TNFa, interleukin-6), serum amyloid A (SAA)
Your veterinarian may choose to perform an oral glucose or xylose absorption test to measure how much of these are absorbed from the gut. This can indicate the malabsorption in inflammatory bowel disease involving the small intestine.
Leaky gut is multifactorial, meaning it can arise from a wide range of stressors including aspects of your horse’s diet and exercise program.
Below are the top five causes of leaky gut in horses.
1) High-intensity exercise
During exercise, blood flow is redirected to skeletal muscles and the extremities.  This results in decreased blood flow to the gut.
Extended periods of restricted blood flow to the digestive tract can increase the risk of intestinal cell damage, potentially increasing mucosal barrier permeability.
Furthermore, the high metabolic rate needed to support exercise produces heat. Extended periods where the core temperature exceeds 42°C (107°F)can result in exercise-induced hyperthermia.
This can damage cells that line the intestine which increases intestinal permeability and activates an immune response. 
Caution should be taken when exercising horses in hot and humid weather. Keep your horse hydrated, provide electrolytes, and cool your horse effectively after exercise to reduce the risk of intestinal barrier damage from exercise and heat stress.
2) Starch Overload
Starch from the diet is not fully digested in the stomach and small intestine. Diets high in starch can result in excess starch reaching the hindgut.
In the hindgut, starch fermentati