Hindgut Issues, Dysbiosis and the Effect on the Functioning of the Horse’s Digestive System
The intestines of the horse are a crucial part of the digestive system. In the intestines, fibre from roughage is converted into volatile fatty acids that provide the horse with energy. These fibres are fermented by microorganisms in the intestines. These microorganisms are also referred to as the gut flora or the microbiome. Nutrition affects the composition of the microbiome and therefore influences the health and function of the intestines, as a healthy microbiome is part of a well-functioning gut. If the diet does not meet the horse’s needs and is out of balance, it can lead to various health problems. This blog discusses several gut-related issues and the impact of nutrition on the horse’s digestive system.
The Microbiome: A Summary
The microbiome is located in the horse’s large intestine and caecum. It consists of bacteria, fungi and other micro-organisms. The microbiome is responsible for fermenting the fibres in the roughage, thus providing energy for the body¹. As 60 to 70% of the horse’s daily energy intake comes from this fibre fermentation, a healthy digestive system and microbiome are vital for the horse². When the microbiome becomes imbalanced — for example, due to an unbalanced diet — it negatively impacts the health and functioning of the intestines. This imbalance, or "dysbiosis", allows certain types of bacteria and fungi to dominate, reducing the population of ‘good’ micro-organisms³. This ultimately harms the gut and can result in various digestive issues.
Digestive Problems in Horses Caused by Dysbiosis
After food passes through the horse’s stomach, it enters the small intestine, large intestine and caecum. As mentioned earlier, fibre fermentation occurs in the large intestine and caecum. When the composition of the microbiome changes, problems tend to occur primarily in this part of the digestive system.
1. Acidosis in the Horse’s Gut
In dysbiosis, the number of lactic acid-producing micro-organisms increases. This leads to higher lactic acid levels, making the gut environment more acidic and lowering the pH⁴. The beneficial bacteria and fungi cannot survive in this acidic environment and die off. As they die, these micro-organisms release toxins. The acidic environment damages the intestinal wall, allowing toxins to enter the bloodstream. These toxins can cause inflammation throughout the body⁵. Acidosis can also impair nutrient absorption due to intestinal damage and increases the risk of ulcers and further digestive issues. It may also negatively affect the horse's immune function. Symptoms of acidosis include: Reduced appetite, ulcers in the intestines, signs of colic, poor performance, greater susceptibility to laminitis and colic.
2. Colic
Colic, a common digestive issue in horses, is described as (severe) abdominal pain. There are various types of colic with different causes.
Gas colic is caused by gas build-up in the intestines. Normally, gas is expelled through the digestive system, but in gas colic, a blockage prevents this. Gas colic can also occur when a horse is fed a diet high in non-structural carbohydrates (e.g., sugars and starch)⁶. As the digestive system cannot fully process large amounts, some carbohydrates reach the intestines undigested and are fermented by micro-organisms. This results in increased lactate production, a lower pH, and excessive gas⁶ — leading to gas colic. Symptoms of colic include: Loss of appetite, lethargy, looking at or biting/kicking at the belly, rolling, sweating, elevated heart and breathing rate.
3. Free Faecal Water Syndrome (FWS)
Faecal water is a condition seen more frequently in horses. In this condition, horses produce normal droppings, but water is also excreted either alongside or after defecation. This is not the same as diarrhoea, where the consistency of the faeces changes. Though not life-threatening, it is abnormal and can cause discomfort for the horse. The exact cause is not always clear, but studies suggest it’s linked to diets that don’t meet the horse’s needs and cause a microbiome imbalance⁷. A study by Lindroth et al. (2021) examined the effect of diet on faecal water. The diets of horses with and without faecal water were compared, revealing clear differences in the levels of structural and non-structural carbohydrates. Horses with faecal water consumed more sugars and starch than the control group. The type of roughage may also play a role. The study suggested that fibres which poorly absorb water may lead to FWS, whereas horses with this condition benefit from fibres that retain more water.
Other Negative Effects of Dysbiosis
1. Dysbiosis and Horse Behaviour
Beyond gut function, dysbiosis can affect horse behaviour⁸. Studies on other species have shown increased anxiety-like behaviour following microbiome changes. Research by Destrez et al. (2015) found a link between microbiome changes and stress-related behaviours in horses.
2. Dysbiosis and Laminitis
As previously mentioned, dysbiosis leads to the release of toxins into the bloodstream. Researchers suggest these toxins may play a role in the development of laminitis, although the precise mechanism remains unclear. However, diets high in sugar and starch are known to increase the risk of laminitis due to the associated dysbiosis, toxin release, and potential damage to the lamellae in the hooves⁵.
Supporting a Healthy and Functional Microbiome and Digestive System
First and foremost, ensure your horse receives enough roughage. The standard guideline is to feed at least 2% of the horse's body weight in roughage daily. The fibres in roughage are essential for a healthy microbiome and properly functioning gut.
Regularly review your horse’s diet to ensure it continues to meet their needs and avoid overfeeding starch and sugars. It's not necessary to eliminate these entirely, as they are important energy sources, but make sure the amount aligns with your horse’s activity and condition.
Adding a pre- and probiotic supplement can also support the microbiome and gut health. Prebiotics are fibres that are fermented by the microbiome into short-chain fatty acids, which are vital for gut health and function. Probiotics are live micro-organisms that support a healthy microbiome when ingested. Synovium Immunomodulator contains a high-quality prebiotic yeast to support gut and immune health. Synovium Gastrosafe includes the prebiotic fructooligosaccharides (FOS) and, through its ingredient blend, supports the gut and stomach of the horse.
If your horse suffers from digestive issues and you're looking for tailored nutritional advice, it's advisable to consult an equine nutritionist.
References
1. Dougal, K., Harris, P.A., Edwards, A., Pachebat, J.A., Blackmore, T.M., Worgan, H.J., Newbold, C.J. (2012) A comparison of the microbiome and the metabolome of different regions of the equine hindgut. FEMS Microbiology Ecology, 82(3):642-652.
2. Bergman, E.N. (1990) Energy contributions of volatile fatty acids from the gastrointestinal tract in various species. Physiological Reviews, 70(2): 567-590.
3. Boucher, L., Leduc, L., Leclère, M., Costa, M.C. (2024) Current Understanding of Equine Gut Dysbiosis and Microbiota Manipulation Techniques: Comparison with Current Knowledge in Other Species. Animals,14(758):1-23.
4. Julliand, V., Grimm, P. (2017) The Impact of Diet on the Hindgut Microbiome. Journal of Equine Veterinary Science, 52. doi:10.1016/j.jevs.2017.03.002
5. Tuniyazi, M., He, J., Guo, J., Li, S., Zhang, N., Hu, X., Fu, Y. (2021) Changes of microbial and metabolome of the equine hindgut during oligofructoseinduced laminitis. BMC Veterinary Research, 7(11):1-13.
6. Shirazi-Beechey, S.P. (2008) Molecular insights into dietary induced colic in the horse. Equine Veterinary Journal, 40(4):414-421.
7. Lindroth, K.M., Lindberg, J.E., Johansen, A., Müller, C.E. (2021) Feeding and Management of Horses with and without Free Faecal Liquid: A Case–Control Study. Animals, 11(2552):1-14.
8. Destrez, A., Grimm, P., Cézilly, F., Julliand, V. (2015) Changes of the hindgut microbiota due to high-starch diet can be associated with behavioral stress response in horses. Physiology and Behavior, 149: 159-164.
The Microbiome: A Summary
The microbiome is located in the horse’s large intestine and caecum. It consists of bacteria, fungi and other micro-organisms. The microbiome is responsible for fermenting the fibres in the roughage, thus providing energy for the body¹. As 60 to 70% of the horse’s daily energy intake comes from this fibre fermentation, a healthy digestive system and microbiome are vital for the horse². When the microbiome becomes imbalanced — for example, due to an unbalanced diet — it negatively impacts the health and functioning of the intestines. This imbalance, or "dysbiosis", allows certain types of bacteria and fungi to dominate, reducing the population of ‘good’ micro-organisms³. This ultimately harms the gut and can result in various digestive issues.
Digestive Problems in Horses Caused by Dysbiosis
After food passes through the horse’s stomach, it enters the small intestine, large intestine and caecum. As mentioned earlier, fibre fermentation occurs in the large intestine and caecum. When the composition of the microbiome changes, problems tend to occur primarily in this part of the digestive system.
1. Acidosis in the Horse’s Gut
In dysbiosis, the number of lactic acid-producing micro-organisms increases. This leads to higher lactic acid levels, making the gut environment more acidic and lowering the pH⁴. The beneficial bacteria and fungi cannot survive in this acidic environment and die off. As they die, these micro-organisms release toxins. The acidic environment damages the intestinal wall, allowing toxins to enter the bloodstream. These toxins can cause inflammation throughout the body⁵. Acidosis can also impair nutrient absorption due to intestinal damage and increases the risk of ulcers and further digestive issues. It may also negatively affect the horse's immune function. Symptoms of acidosis include: Reduced appetite, ulcers in the intestines, signs of colic, poor performance, greater susceptibility to laminitis and colic.
2. Colic
Colic, a common digestive issue in horses, is described as (severe) abdominal pain. There are various types of colic with different causes.
Gas colic is caused by gas build-up in the intestines. Normally, gas is expelled through the digestive system, but in gas colic, a blockage prevents this. Gas colic can also occur when a horse is fed a diet high in non-structural carbohydrates (e.g., sugars and starch)⁶. As the digestive system cannot fully process large amounts, some carbohydrates reach the intestines undigested and are fermented by micro-organisms. This results in increased lactate production, a lower pH, and excessive gas⁶ — leading to gas colic. Symptoms of colic include: Loss of appetite, lethargy, looking at or biting/kicking at the belly, rolling, sweating, elevated heart and breathing rate.
3. Free Faecal Water Syndrome (FWS)
Faecal water is a condition seen more frequently in horses. In this condition, horses produce normal droppings, but water is also excreted either alongside or after defecation. This is not the same as diarrhoea, where the consistency of the faeces changes. Though not life-threatening, it is abnormal and can cause discomfort for the horse. The exact cause is not always clear, but studies suggest it’s linked to diets that don’t meet the horse’s needs and cause a microbiome imbalance⁷. A study by Lindroth et al. (2021) examined the effect of diet on faecal water. The diets of horses with and without faecal water were compared, revealing clear differences in the levels of structural and non-structural carbohydrates. Horses with faecal water consumed more sugars and starch than the control group. The type of roughage may also play a role. The study suggested that fibres which poorly absorb water may lead to FWS, whereas horses with this condition benefit from fibres that retain more water.
Other Negative Effects of Dysbiosis
1. Dysbiosis and Horse Behaviour
Beyond gut function, dysbiosis can affect horse behaviour⁸. Studies on other species have shown increased anxiety-like behaviour following microbiome changes. Research by Destrez et al. (2015) found a link between microbiome changes and stress-related behaviours in horses.
2. Dysbiosis and Laminitis
As previously mentioned, dysbiosis leads to the release of toxins into the bloodstream. Researchers suggest these toxins may play a role in the development of laminitis, although the precise mechanism remains unclear. However, diets high in sugar and starch are known to increase the risk of laminitis due to the associated dysbiosis, toxin release, and potential damage to the lamellae in the hooves⁵.
Supporting a Healthy and Functional Microbiome and Digestive System
First and foremost, ensure your horse receives enough roughage. The standard guideline is to feed at least 2% of the horse's body weight in roughage daily. The fibres in roughage are essential for a healthy microbiome and properly functioning gut.
Regularly review your horse’s diet to ensure it continues to meet their needs and avoid overfeeding starch and sugars. It's not necessary to eliminate these entirely, as they are important energy sources, but make sure the amount aligns with your horse’s activity and condition.
Adding a pre- and probiotic supplement can also support the microbiome and gut health. Prebiotics are fibres that are fermented by the microbiome into short-chain fatty acids, which are vital for gut health and function. Probiotics are live micro-organisms that support a healthy microbiome when ingested. Synovium Immunomodulator contains a high-quality prebiotic yeast to support gut and immune health. Synovium Gastrosafe includes the prebiotic fructooligosaccharides (FOS) and, through its ingredient blend, supports the gut and stomach of the horse.
If your horse suffers from digestive issues and you're looking for tailored nutritional advice, it's advisable to consult an equine nutritionist.
References
1. Dougal, K., Harris, P.A., Edwards, A., Pachebat, J.A., Blackmore, T.M., Worgan, H.J., Newbold, C.J. (2012) A comparison of the microbiome and the metabolome of different regions of the equine hindgut. FEMS Microbiology Ecology, 82(3):642-652.
2. Bergman, E.N. (1990) Energy contributions of volatile fatty acids from the gastrointestinal tract in various species. Physiological Reviews, 70(2): 567-590.
3. Boucher, L., Leduc, L., Leclère, M., Costa, M.C. (2024) Current Understanding of Equine Gut Dysbiosis and Microbiota Manipulation Techniques: Comparison with Current Knowledge in Other Species. Animals,14(758):1-23.
4. Julliand, V., Grimm, P. (2017) The Impact of Diet on the Hindgut Microbiome. Journal of Equine Veterinary Science, 52. doi:10.1016/j.jevs.2017.03.002
5. Tuniyazi, M., He, J., Guo, J., Li, S., Zhang, N., Hu, X., Fu, Y. (2021) Changes of microbial and metabolome of the equine hindgut during oligofructoseinduced laminitis. BMC Veterinary Research, 7(11):1-13.
6. Shirazi-Beechey, S.P. (2008) Molecular insights into dietary induced colic in the horse. Equine Veterinary Journal, 40(4):414-421.
7. Lindroth, K.M., Lindberg, J.E., Johansen, A., Müller, C.E. (2021) Feeding and Management of Horses with and without Free Faecal Liquid: A Case–Control Study. Animals, 11(2552):1-14.
8. Destrez, A., Grimm, P., Cézilly, F., Julliand, V. (2015) Changes of the hindgut microbiota due to high-starch diet can be associated with behavioral stress response in horses. Physiology and Behavior, 149: 159-164.