Jennifer Little BSc Hons MSc RNutr PgCert
Equinutrition
Independent Equine Nutritionist

The horses gut and its interaction with other body systems.
The horses digestive system has evolved to survive on a diet of forage, functioning on a relatively poor nutritional value but high volume intake. This adaptation is derived from the structure of it’s digestive system (figure 1).

Figure 1: The structure of the horses digestive tract. Image sourced:
https://horseislove.com/wp-content/uploads/2019/04/horse_digestive_system.jpg

The Foregut

The foregut comprises of the stomach and small intestines. The stomach produces hydrochloric acid and enzymes to aid digestion. This then leads onto the small intestines, which are divided into three regions, the duodenum, jejunum and the ileum. A combination of bile and enzymes digests fats, proteins and some carbohydrates. The products of which are then absorbed across the intestinal wall.

The Hindgut

The hindgut, or large intestines, comprises of the cecum, the large colon and the small colon. Up to this point in the digestive system food digestion has been primarily driven by enzymes, but in the hindgut digestion is driven by the microbes housed within it. These microbes are populations of bacteria and protozoa, collectively known as the gut-biome. In a healthy gut the gut-biome work symbiotically (for the benefit of the horse), digesting fibre, through fermentation, to produce energy for the horse, in the form of Volitale Fatty Acids (VFA’s) and short-chain fatty acids (SCFA’s). These bacteria are critical for fibre digestion, overall health and performance [1].

The Hindgut & other functions

In addition to converting fibre into a viable energy source, the hindgut carries out multiple other functions, and influences organs and processes outside of the digestive system. This inter-organ association is referred to as the gut-organ/system axis, and this interaction can be influenced by the number and diversity of the beneficial bacteria within the gut-biome. The more extensively understood and researched examples include the gut-brain axis and the gut-immunity axis.

The Gut-Brain Axis

The composition of these beneficial microbes within the horses’ gut-biome can influence behaviour. A horse experiencing stress produces stress hormones, which causes the gut to respond by synthesising cytokines, neurotransmitters and other stress hormones. Unfortunately, this can alter the gut-biome, reducing the beneficial microbe activity and diversity, and even promoting the growth of intestinal pathogens [2;3]. This is associated with changes in behaviours, including increased fight and flight responses, and a reduction in horse:handler interaction seeking behaviours [4;5].

The Gut-Immunity Axis

The micro-organisms within a horses’ gut biome also influence the levels of systemic inflammation and the cells making up the immune system. Disturbances within the gut-biome can result in increased inflammation and infection rates, with reduced immunity and slower healing rates [4;6].

A lesser known and researched gut-organ axis is the gut-lung axis.

The Gut-Lung Axis

Current research on the role of the gut-lung axis in horses is currently limited, but it does pose some interesting insights. Especially in the association of a beneficial gut biome with an improved lung function in performance horses and those suffering from conditions such as Chronic Obstructive pulmonary disease (COPD) or equine asthma.

In horses the inhalation of irritants or allergens can result in an immune response, increasing inflammation and mucus production, potentially causing bronchoconstriction. Reducing lung function in performance horses and increasing symptoms in COPD or asthma suffering equines [7]. The association between the gut and lung function is thought to involve the ability of the gut-biome to produce short-chain fatty acids (SCFA’s), enhance local mucosal immunity, and promote immunotolerance to airborne irritants or allergens [8].

In performance horses increased strenuous exercise can cause the level of airborne allergens and irritants inhaled to be greater than in leisure horses. Posing an increased risk of pulmonary inflammation, reduced lung function, impeding athletic performance. In leisure horses low levels of exercise and a heightened prevalence of equine obesity can also reduce pulmonary function, due to inactivity and increased fat tissue stimulated systemic inflammation [7]. Horses suffering with COPD or equine asthma experience an exaggerated response to airborne irritants and allergens, which healthy horses could seemingly tolerate with no ill effect.

Supporting lung function & gut biome

Managing a horse to help ensure pulmonary health and lung performance requires consideration of their management. Measure can be taken with regard to the horses environment to reduce the levels of airborne irritants inhaled. Stable and feed room hygiene measures to ensure a clean and dust or feed residue free environment can reduce the levels and types of irritants inhaled. Increased stable ventilation and turn-out time can also help. Opting for persevered forages with higher water content (haylage vs. Hay), or steaming has been shown to reduce both COPD and equine asthma symptoms [7]. Simple measures such as removing a horse from the stable while it is mucked out can also support pulmonary health.

Horses with responses to airborne allergens can have a gut-biome that is slower to adapt to diet and management changes than their counterparts [7]. Again highlighting the link between the gut-biome and the gut-lung axis. Further research into the gut-lung axis is required to understand this interaction better. But, the initial findings suggest that supporting the gut-biome with the dietary addition of Pro and Prebiotics maybe a suitable course of action.

Providing a probiotic is supplying the actual beneficial microorganisms to support their population within the hindgut, restoring balance to the gut-biome. One example is Saccharomyces cerevisiae, which is proven to support the immune system in the horse, modulate inflammatory processes, and breakdown toxins within the hindgut [7]. The supplementation of the probiotic Saccharomyces cerevisia also provides a stress-mitigating response within the gut, and has been shown to support the gut-biome during periods of change [9]. In other species the supplementation of Saccharomyces cerevisiae has been shown to significantly reduce inflammation of the airways [10].

Providing prebiotics is supplying a nutritional support package for the microorganisms already within the hindgut, helping to maintain a healthy population. The Prebiotics Fructooligosaccharides (FOS) and Mannanoligosaccharides (MOS) prevents non-beneficial micro-organisms from establishing in the gut, and increases the production of SCFA’s. SCFA’s can attenuate inflammatory responses, helping to modulate allergic responses and support pulmonary health [7]. The inclusion of FOS within a horses diet has even been shown to limit the negative impact of changes to feeds and forages [9]. Whereas MOS supplementation has been proven to improve recovery, overall health and performance [11;12].

Irrespective of a horses’ performance levels and if it either has or does not have a pulmonary disease, such as COPD or equine asthma support lung health should be considered part of their daily management. Ensuring a hygienic and well ventilated environment provides the fundamentals for supporting a horses overall and pulmonary health, as does providing a suitable diet. While the research into a horses diet and the functioning of the lungs is in its infancy, the understanding so far suggests that the addition of pre and Probiotics could support the gut-lung axis and promote overall health and performance.

References

[1] Muhonen, S. & Julliand, V. 2023. Fibre composition and maturity of forage-based diets affects the fluid balance, faecal water-holding capacity and microbial ecosystems in French trotters. Animals. 13: 328

[2] Freestone, P., Lyte, M. 2010 Stress and microbial endocrinology: Prospects for ruminant nutrition. Animals (4) 1248-1257

[3] Lyte, M. 2013 Microbial endocrinology in the microbiome-gut-brain axis: How bacterial production and utilization of neurochemicals influence behaviour. PLoS Pathog (9) e1003726

[4] Chaucheyras-Durand, F., Sacy, A., Karges, K., Apper, E. 2022 Gastro-Intestinal Microbiota in Equines and its role in health and disease: the black box opens. Microorganisms (10) 2517

[5] Mach, N., Ruet, A., Clark, A., Bars-Cortina, D., Ramayo-Caldas, Y., Crisci, E., Pennarun, S., Dhorne-Pollet, S., Foury, A., Moisan, M.P, 2020 Priming for welfare: gut microbiota is associated with equitation conditions and behaviour in horse athletes. Sci. Rep. (10) 8311

[6] Tench, M., Bobel, J.M., Dolly, C.B., Hansen, N.K., Lopez, C., Warren, L.K. 40 dietary Saccharomyces cerevisae fermentate affects mucosal immunity in young stress challenged horses in training. Journal of Veterinary Science 2022 (100) 103503

[7] Leduc, L., Costa, M., Leclere, M. The microbiota and equine asthma: an integrative view of the gut-lung axis. Animals 2024 (14) 253

[8] Robinson, N.E., Chairperson, W. International workshop on Equine Chronic Airway Disease. Equine. Vet. J. 2001 (33) 5-19

[9] Geor, R.J., Harris, P.A., & Coenen, M.,  2013 Equine Applied and clinical nutrition. Health welfare and performance. Saunders Elsevier, 575-576

[10] Milani, T.M.S., Sandy, C.M., Calazans, A.P.C.T, Silva, R.Q., Fonseca, V.M.B., Martins, F.S., Borges, M.C. Dose-response effect of Saccharomyces cerevisae UFMG A-905 on the Prevention of Asthma in an Animal Model. Probiotics Antimicrob. Proteins 2020 online ahead print.

[11] Zhao, W., Chen, L., Tan, W., Li, Y., Sun, L., Zhu, X., Wang, S., Gao, P., Zhu, C., Wang, L., Jiang, Q. 2023 Mannan Oligosaccharides promoted skeletal muscle hypertrophy through the gut microbiome and microbial metabolites in mice. Foods, 12, 357

[12] Martin, R., Chamignon, C., Mhedbbi-Hajri, N., Chain, F., Derrien, M., Escribano-Vazquez, U., Garault, P., Cotillard, A., Pham, H.P., Chervaux, C., et al 2019 The potential probiotic lactobacillys rhamnosis CNCM I-3690 Strain protects the intestinal barrier by stimulating both mucus production and cytoprotective response. Sci Rep 9, 5398

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