Hind Gut, Colonic Ulcers & IBD

Gastric diseases such as ulcers and EGGD have received greater attention and more research focus given their high prevalence. Issues affecting the small and large intestine in horses are poorly understood by comparison and present considerable management challenges.

Inflammatory Bowel Disease (IBD) and Inflammatory Bowel Syndrome (IBS)

In humans the issues are clearly differentiated but the terms are still often used inter-changeably. Crohn’s disease and ulcerative colitis are examples of IBD where the immune system attacks the bowel causing inflammation and considerable discomfort. Steroids, other medications and occasionally surgery are used to treat them. The exact cause of IBS, in contrast, is unknown and so unsurprisingly there is no cure. Contributing factors include stress and certain food types and so changes to lifestyle and diet are recommended to try and help control the symptoms.

What do we know about IBD and IBS in horses?

Inflammatory Bowel Disease (IBD) is recognized as an infiltration of the intestinal mucosa caused by components of the immune system (Vitale, 2021). This can result in changes to gut motility and the absorption of nutrients. If this occurs in the small intestine, it can result in more sugar and starch reaching the hind gut where it is fermented relatively quickly resulting in gassiness in the horse. This may result in discomfort and colic symptoms.

Inflammatory Bowel Syndrome (IBS) is poorly defined in horses but, if we keep the definition in humans in mind, it could be caused by certain feed types seeming not to suit the horse to the point where behavioural changes or discomfort are apparent. Stress should be considered as a likely contributing factor and the meaning of stress in this context, is anything that stops the horse displaying normal behaviours.

Maintaining a healthy intestine is a delicate balance between diet, the microbiota and the host’s immune system. When the gut barrier is compromised through stress or dietary change, even normally beneficial bacteria can stimulate an immune response. The mucosal immune system represents the largest component of the immune system containing around 75% of all lymphocytes (white blood cells). Researchers highlight how historically the microbiota in the small intestine has often been overlooked or dismissed as it is smaller and less diverse than in the large intestine. However, it is likely to change more over time because of fluctuations in oxygen and digestive secretions entering the small intestine.

A fundamental principle to consider in the context of inflammatory conditions in the gut is that the initial trigger may not necessarily be what perpetuates the problem. This is one of the reasons that managing and resolving issues is so difficult. As more is understood about risk factors for EGGD, it becomes apparent how many parallels it has with IBD/IBS. On that basis, the management advice for EGGD can sensibly be applied to those with IBD/IBS too:

• Ensure the horse has a companion that they are happy with
• Try to provide a quieter environment
• Keep the number of different handlers to a minimum
• Provide plenty of rest days
• Allow them to display natural eating/foraging behaviours by providing a range of forages

Keeping a feed and management diary is key when trying to identify things that trigger the issue. It can be a long process to identify what does and doesn’t suit each horse so taking a systematic approach is key.

Step 1: ensure minimum forage and fibre requirements are met – too little fibre can be an issue

Step 2: reduce or remove starch from the ration as much as possible – this is beneficial for overall health and well-being and will help to reduce the risk of excessive amounts of starch reaching the hind gut

Step 3: Introduce different types of forage – whilst it may seem counterintuitive, the most digestible forms of fibre are not always the most suitable and in some cases, forages like straw might be more appropriate. This is why working with your vet and nutritionist is really important.

Step 4: Digestive aids – these include yeast, prebiotics and post-biotics. Discuss your horses with a nutritionist who will help you select the most appropriate digestive aid.

Unfortunately there are no live bacterial probiotics approved for use in horses (yeast is often referred to as a probiotic but it is yeast rather than bacteria). This means that the only way to put live bacteria into the horse’s digestive tract is by carrying out a faecal transplant. This technique is used in humans with some success and needs to be carried out a number of times – a one-off transfer is not likely to be effective. It can sometimes be a challenge to find a healthy donor and not all vets offer this service. It is something you should discuss with your vet who may refer you.

Hind Gut or Colonic Ulcers

Diagnosing ulceration of the large colon is very difficult and surgery is defined as the gold standard method for diagnosis but is clearly largely impractical for most cases. For this reason, studies that report the prevalence of colonic ulcers are most often conducted in horses that have been euthanised and in some cases in abattoirs. This has to be taken into account when considering factors that may be contributing to those horses having colonic ulcers. One study in the USA reported a prevalence of 44% (Pellegrini, 2005) but no indication of the cause of the ulcers was given. In a study carried out in the UK (Kerbyson et al. 2017), most cases of colonic ulceration were as a direct result of parasitism.

The normal pH (level of acidity) in the hind gut is between 6.5 and 7 which is near to neutral. If significant amounts of starch reach the hind gut then it can greatly reduce the pH (increase the level of acidity). This clearly has the potential to cause damage in the same way that increased acidity can result in gastric ulceration. However, the evidence-base doesn’t exist yet to conclude that hind gut ulcers is as prevalent a problem as gastric ulcers.

Applying the same best practices for feeding management will help to reduce the risk of issues occurring in the hind gut.

The lymphatic system and its relationship with the gut

What is the lymphatic system?

It is probably fair to say that the lymphatic system is not very well understood and yet it is crucial to the horse’s health and performance. The lymphatic system consists of an extensive network of vessels and nodes that help to maintain fluid balance and cellular health. As with most systems in the body, the lymph interacts with other systems including the immune system. As waste material passes through lymph nodes it is analysed for potential pathogens and if present, an immune response is triggered.

Movement of lymph around the body

The lymphatic system doesn’t have one large pump like the heart in the circulatory system; instead lymph builds up in bead-like collector vessels that keep filling up until the pressure is sufficient to push the valve open. The flow of lymph is also stimulated by muscle contraction, the arterial pulse and crucially, movement of the gut which is also known as peristalsis.

The horse has a lot of lymph nodes – 8000 compared to 600 in humans. Half of the horse’s nodes are in the colon and so will function best when plenty of peristalsis occurs. This is one of the anatomical differences between an herbivore that is designed to spend most of its time eating, and an omnivore that tends to meal feed. This is another reason to feed horses plenty of fibre as it helps to stimulate the lymph system.

Filled legs – the problem of pushing water up hill!

Physical movement is beneficial to stimulate flow which is why horses that are sedentary for long periods can end up with filled legs. After exercise, the lymphatic system has lots of work to do clearing the debris from cells that have been working hard to supply fuel. Even with some time dedicated to “cooling down” the performance horse is sedentary again relatively quickly which can compromise the efficiency of the lymph system. Turning a horse out after exercise is really beneficial as the horse is moving around and eating, both of which help to stimulate the flow of lymph.

References

• Kerbyson et al (2017), Idiopathic Colonic Ulceration: Prevalence, Gross Pathology and Clinical History in 56 Horses, Equine Veterinary Journal, vol. 49, no. S51, pp. 13-16.
• Pellegrini (2005), Results of a Large-Scale Necroscopic Study of Equine Colonic Ulcers, Journal of Equine Veterinary Science, vol. 25, no. 3, pp. 113-117.
• Vitale (2021) Inflammatory bowel diseases in horses: What do we know? Equine Veterinary Education. https://doi.org/10.1111/eve.13537