To aid the digestion of the large quantities of fibre ingested, the horse's digestive system is home to a huge community of micro organisms, known as gut microflora. These include bacteria, protozoa, fungi and yeasts, which may exsist as over 400 different species and strains within the gut at any one time. One ml of the horse's hindgut content may contain over 100 billion individuals.

The community is made up of friendly bacteria and bad bacteria. The friendly bacteria or beneficial bacteria have a principle role in breaking down the food that the horse itself cannot digest into products that can be used to provide the horse with energy. A number of species are also responsible for the synthesis of protein and for the production of essential vitamins such as B vitamins. The bad microflora or pathogenic microflora contribute to disease, for example Salmonella, which may cause digestive disturbances such as colic and scouring.

The relationship between microflora and the horse is mutually beneficial, with the horse acting as a host, but the relationship between the individual species that make up the microflora is elaborate and together they form a complex and dynamic ecosystem within the gut. This dynamic ecosystem is constantly turning over and the microflora, which have died or been passed out in the faeces are replaced by new ones.

The role of the microflora extends beyond that of being in house food processors' and their health exerts a fundamental influence on the well being of the horse. Imbalances in the ecosystem can be affected by numerous factors including diet composition, meal size, and changes in diet, stress and the application of antibiotics. The one factor that we are all probably most familiar with or at least aware of is the effect of meal composition and size. Diets that are low in fibre and high in starch increase the potential of undigested starch reaching the large intestine. Rapid fermentation causes a rapid reduction in the pH level in the gut creating an acidic environment, which has a direct effect on the balance of microflora within the gut.

There are now many products available on the market known as 'digestive aids', which aim to help maintain a healthy microflora ecosystem within the horse's digestive system. Two of these are known as Probiotics and Prebiotics.

PROBIOTICS:
At the beginning of the century it was suggested by Metchnikoff (1907), that the ingestion of live bacteria could have a beneficial effect on health. It was not until 1989 that the term probiotic was defined by Fuller as A live microbial feed supplement, which beneficially affects the host animal by improving its intestinal microbial balance. Today probiotics contain a range of microorganisms that would be found naturally in the horse's gut.

When the horse ingests probiotics they need to firstly survive the acidic conditions of the stomach. The very acidic conditions in the stomach means that microbial concentrations are low. However the acidity provides a critical barrier to avoid the entry of pathogenic bacteria ingested with the feed to the lower part if the intestinal tract. Probiotic supplements therefore need to provide large numbers of microflora to increase the probability that some will survive and reach their site of action. The rate of flow of digesta through the small intestine and the environment limits the bacterial growth, however, towards the end of the small intestine the environment and the flow rate decreases, creating conditions that are more suited for microflora growth. Probiotics are also likely to be active in the hindgut where the conditions are ideal for bacterial proliferation.

Probiotics exert their effect in a number of ways.

1. Competitive Exclusion: One of the most important roles of the microflora in the small intestine is the inhibition of pathogenic bacteria by competitive exclusion to the gut wall or lining. Probiotics provide additional support by further reducing possible 'gaps' in the microfloral lining of the gut, so that pathogenic bacteria cannot gain a foothold and cause digestive disturbances.
2. Nutrilites: The probiotic microflora also produce naturally occurring acids, which help to maintain the ideal environment to promote the growth of friendly bacteria, but one that is hostile to pathogenic bacteria.
3. Toxins: Many of the strains of bacteria included in a probiotic produce compounds that actually inhibit the reproduction and growth of invading bacteria.
4. Immunity: Probiotics have also been reported to increase the horses resistance to disease by promoting the production of cells in the bloodstream that help to destroy foreign cells. They may also help to optimise the production of immunoglobins and Lysosomes both of which are associated with the immune system.

There is no evidence to suggest that feeding a probiotic to an apparently healthy horse on a long-term basis will effect the horses own microfloral population to multiply. One could argue that there is little point in feeding a probiotic to a horse or pony where there is no reason for concern. On the other hand regular use in apparently healthy horses may help to prevent against possible future problems.

Horses that have been seen to really benefit from regular probiotic supplementation are those that are exposed to stress. Stress can have very serious effects on the horses gut microflora. Travel, fatigue, worry, sudden diet changes, diet composition, work level, worming and antibiotics can all induce stress. Stress triggers a complicated series of reactions involving hormones such as Cortisol, and the ultimate end result is the alteration of the gut microflora, leading to problems such as scouring, loss of condition and colic etc.

PREBIOTICS:
These are also known as Fructo-Oligosaccharides (FOS) and are short chains of simple sugars, glucose and fructose that are extracted from sugar beet or chicory. The bonds that join the fructose and glucose molecules are unable to be broken by the horse. This means that they cannot be digested in the small intestine and so are utilized entirely in the hindgut. An FOS provides a meal or substrate to the beneficial microflora that are already living in the hindgut for example Bifidobacteria and to those that were introduced as a probiotic. They are unable to be used as a food source by the pathogenic bacteria such as Salmonella. This therefore provides the friendly bacteria with the optimum nutrients and environment to reproduce and multiply thus maintaining a stable gut environment. Prebiotics can be used in a wide variety of situations similar to probiotics even though their mode of action is very different! They are particularly useful for older horses and ponies to help compensate for a less efficient digestive system, scouring foals, those that are convalescing and horses that are on a high starch diet.

Prebiotics can be fed as a supplement to your horse if you know that he is going to be put in to a stressful situation like travelling. Fed for a couple of days before the journey and for a few days post journey can help to reduce some of the symptoms of stress. Alternatively some feeds on the market already contain prebiotics for example Saracen Bio-Life 2000. Providing that these feeds are fed at the manufacturers recommended feeding rates they will normally provide a maintenance dose of prebiotic. This will depend upon the level of inclusion of prebiotic per kg of feed and if it is not displayed on the declaration label, then do not be afraid to ask the manufacturer.

While 'digestive aids' can certainly help to maintain a happy and healthy microbial population it is still important to remember that horses in the wild did not require such 'aids' as their diets were consistent and fibre based. Always provide your horse with access to plenty of good quality long fibre (never less than 1% of bodyweight), as this goes a long way towards keeping happy microflora and ultimately a satisfied horse.