Probiotics – The key to a well-balanced equine gut
/Article by Kerrie Kavanagh
It is no surprise that the health maintenance of the racehorse is a top priority for trainers. And probiotics can be used as a treatment modality to manipulate the gut microbiome to improve or maintain health. Equine studies to date have shown that probiotic strains can offer an advantageous approach to minimising disturbances in the gut microbial populations, repair these deficiencies—should they occur—and re-establish the protective role of the healthy gut microbiome. Other probiotic-associated health benefits include reducing diet-related diseases such as colic and laminitis, preventing diarrhoea, conferring host resistance to helminth infection, improving stress-related behavioural traits (e.g., locomotion) and even promote the development of an effective gut-brain communication pathway.
Probiotics have been used by humans for more than 5,000 years with their development closely linked to that of dairy products and fermented foods. Today, probiotics are seen as an excellent non-pharmaceutical way to improve the health of both humans and animals, and there are a plethora of products to choose from. But what exactly is a probiotic, and how do they work? Why would your horse need one? What types of probiotics are available for horses? These are all questions that horse trainers ask frequently, which we will attempt to answer here.
The Equine Gut Microbiome
Probiotics and the equine microbiome can benefit from a valuable symbiotic relationship; probiotics are seen as a restorative treatment modality for the gut, to re-establish the bacterial populations there and also to re-establish the protective role that the health gut microbiome confers to the host. But when we discuss the equine microbiome, what are we really talking about?
The gut microbiota/microbiome can be categorised by anatomical location such as the oral microbiota/microbiome in the mouth and the intestinal microbiota/microbiome in the intestines, etc. Therefore, the gut microbiome pertains to the microbiota in the gastrointestinal tract. This population of microorganisms (bacteria, fungi, viruses, protozoa) is referred to as the ‘microbiota’ of the gut, while the term ‘gut microbiome’ refers to the genetic material associated with these microorganisms. The microbiome can be defined as the sum of the microbes and their genomic elements in a particular environment. If we look at the definition of the microbiome having the propensity to an equation, then any equation must be balanced; to maintain that balance is key. If the microbial community exists in an environment in a balanced state, then any upset or disturbance to the microbial populations will cause the balance to shift (known as dysbiosis). To maintain the balance, we need to firstly understand the way the microorganisms exist within their community (i.e. their microorganism-to-microorganism interactions and also microorganism-to-environment interactions) and secondly, their functioning role. If we can understand their (microorganism) position and role, then we can maintain the balance or re-establish the balance if a shift occurs.
The human intestinal microbiome is now recognised as an organ and likewise, the equine intestinal microbiome is deemed an ‘organ’ of the body and is vital for the breakdown of complex food and subsequent release of energy, protection against the pathogenic bacterial colonisation and in regulating the immune system and metabolic functions. There has been much debate regarding the content of the healthy equine microbiome, and even to deduce what ‘healthy’ or ‘normal’ is requires a level of understanding of the microbiota associated with healthy horses. This question has been posed by many researchers and frankly has yet to be answered with certainty. There are many reasons why the ‘normal’ microbiota keeps eluding us; and this can be attributed to the many reasons as to why the gut microbiota (of a healthy horse) can be affected (see Figure 1). It is thought that the diversity of the human gut microbiota and the general assembly of microbial communities within the gut (with the dominant phyla being classed as belonging to Firmicutes and Bacteroidetes) is a shared hypothesis across most species (i.e., humans and animals share a similar gut microbiome structure). Firmicutes and Bacteroidetes have been shown to constitute the main dominant phyla in equine, bovine, canine and feline gut microbiome studies indicating the cruciality of the role they play in the maintenance of a healthy microbial ecology in the gastrointestinal tract. Several studies do agree that dominant phyla of the equine gut microbiota are obligate anaerobes: the gram-positive Firmicutes and the gram-negative Bacteroidetes; other phyla are identified as Proteobacteria, Verrucomicrobia, Actinobacteria, Euryarchaeota, Fibrobacteres and Spirochaetes. Ninety-five percent of the Firmicutes phyla contains the Clostridia genus in addition to genera related to gut health such as Lachnospiraceae, Faecalibacterium and Ruminococcaceae. The other main dominant phyla, Bacteroidetes, on the other hand contains a large variety of the genus.
Role of the Equine Gut Microbiota
The role of the gut intestinal microbiota serves to protect and prevent disease. The gut microbiota has several purposes: prevention of pathogen colonisation by competing for nutrients, enrichment and maintenance of the intestinal barrier—their ability to renew gut epithelial cells and repair damage to the mucosal barrier, the breakdown of food and releasing energy and nutrients, such as synthesising vitamins D and K and also conserving and restoration of the immune system by the formation of antimicrobial metabolites and blocking access to the binding sites of the mucosal wall. The gut microbiota is also thought to play some role of influencing the neuro-active pathways that affect behaviour. It is not surprising to see that gut disorders and gastrointestinal diseases can arise when gut dysbiosis occurs. The role of the gut microbiota may have even more importance than is realised and may have a role to play with developing illness or disease later in life.
The microbial colonisation of the intestinal tract begins at birth. The foal begins its colonisation through contact with the microbiota of the mare’s vaginal and skin surfaces plus the surrounding environments to which the foal is exposed and reaches a relatively stable population by approximately 60 days in age. It is perhaps a fight for dominance to achieve establishment in the gut among the bacterial populations that sees the foal’s microbiota as being more diverse and quick to change when compared to that of the older horse. The subsequent colonisation of the intestinal tract will reflect the foal’s diet, changing environment, introduction to other animals, ageing and health.
Figure 1: Factors that can lead to gut dysbiosis
What exactly is a probiotic?
The word ‘probiotic’ is of Greek origin meaning ‘for life’ and the WHO/FAO have defined probiotics as ‘live microorganisms which when administered in adequate amounts confer a health benefit on the host’. People have long believed that exposure to non-pathogenic microorganisms can benefit the health of humans and animals. The thinking behind this is that daily consumption of sufficient numbers of ‘good’ microorganisms (either bacteria or fungi) can maintain a healthy population of microorganisms in the gut and benefit overall health.
Probiotics are used to manipulate the bacterial populations of the gut in order to re-establish the delicate microbial balance there which, in turn, confers health benefits on the host. As the benefits associated with some of the ‘good’ bacteria within the gut became known, these were referred to as probiotic bacteria.
How do probiotics work?
There are 4 main mechanisms by which probiotics are thought to exert their effects.
By inhibiting pathogen colonisation in the gut through the production of antimicrobial metabolites or by competitive exclusion; in other words, they prevent the ‘bad’ bacteria from growing in the gut.
By protecting or re-stabilising the commensal gut microbiota, probiotics can be a means to re-establish the balance of the gut microbial populations.
By protecting the intestinal epithelial barrier, they maintain the health of the intestinal wall.
By inducing an immune response, probiotics can boost the immune response and help prevent disease.
If we consider the definition of a probiotic as ‘live non-pathogenic microorganisms that, when administered in adequate amounts, confer a health benefit on the host’, then this reference to ‘adequate amounts’ must be emphasised, and the dose administered is critical to ensure that the probiotic has the desired effect. For horses, we must consider the route through the digestive tract that the probiotic strains must travel to arrive at their destination is a distance over 15 metres long. It is a race for survival! The gastrointestinal system has many obstacles along the passage such as the acidic stomach environment and the dangers of exposure to bile and digestive enzymes, in which they must survive. The initial dose of ‘live’ probiotic strains is therefore crucial to ensure survival in the gut. Prebiotics are ingredients such as carbohydrates and fibre, which promote the growth of these probiotic bacterial/yeast strains in the gut. Prebiotics are essentially the food for the probiotic strains and can help form a symbiotic relationship with the probiotic to improve the overall health status of the horse.
Why would you need to give your horse a probiotic?
Gut dysbiosis is a fluctuation or disturbance in the population of microorganisms of the gut, which may be linked to a wide range of diseases in horses. Gut dysbiosis can be caused by many factors ranging from dietary changes, antibiotics, disease, intense exercise and training, age, worms, environment, travel, or even minor stress events—resulting in major consequences such as colic. Dysbiosis is generally associated with a reduction in microbial species diversity.
Diet is one of the major factors contributing to gut dysbiosis. Unlike the ruminant cattle and sheep that use foregut fermentation, horses are hindgut fermenters. The large intestine is the main area where fermentation occurs. The horse utilises the microbial enzymes of the hindgut microbial population in the colon and caecum to break down the plant fibres (cellulose fermentation) sourced mainly from grasses and hay. The horse itself does not possess the hydrolytic enzymes that are required to break the bonds of the complex structures of the plant carbohydrates (in the form of celluloses, hemicelluloses, pectins) and starch; so therefore, it strongly relies on the microbiota present to provide those critical enzymes required for digestion. The main phyla Firmicutes and Bacteroidetes possess enzymes capable of breaking down the complex carbohydrates (such as starch and cellulose).
Research has shown that forage-based diets (grasses and hay) promote the most stable gut microbiomes, but ultimately the equine athlete requires far more energy than a forage-based diet can supply. Supplementing the diet with concentrates containing starch such as grain, corn, barley and oats can affect the number and type of bacteria in the gut. Optimising diet composition is so important as carbohydrate overload—as seen with high-starch diets (>1g/kg body weight per meal)—can change the populations of bacteria in the gut, alter pH, upset digestion and the gut environment, and ultimately result in diseases such as colitis, colic and laminitis. The correct diet is essential for maintaining the delicate balance of bacterial populations. Probiotics can be used to either replace the bacteria missing in the gut and/or can help maintain the delicate microbial balance even in the face of adversity such as abrupt dietary changes, antibiotic treatment and stress.
What types of probiotics are available for horses?
There are several probiotic products on the market, and most are in powder or liquid form. There are two main categories of probiotics: generic and autogenous. Generic probiotics are off-the-shelf products that contain specific strains of bacterial or yeast, singularly or in combination. The Lactobacillus and Bifidobacterium families, Enterococci and yeasts such as Saccharomyces cerevisae and boulardii are the most common equine probiotic strains. Advantages of generic probiotics are that they are widely available, easy to administer, and they may be beneficial to horse health (if the strains are alive in sufficient numbers). Autogenous probiotics are specifically formulated using bacteria obtained from the horse’s own faecal sample and, as such, are uniquely adapted to that individual animal. These host-adapted bacteria are more likely to survive in the gut than non-adapted generic strains and can quickly replenish absent or low levels of bacteria unique to the individual horse, thus maintaining health.
