In both human and veterinary medicine there is a growing understanding of the importance of gut microorganisms to overall health, and their role in various disease states.
This is an emerging science that is complex and rapidly evolving. In man, there is an increasing appreciation of the link between the gut microbiome (see glossary, opposite) and immunity and metabolic function.
This is in addition to the alterations to this microbiome that are linked to intestinal disease such as irritable bowel syndrome, as well as obesity and metabolic disorders including type 2 diabetes. In the horse, similar changes are being identified, although equine intestinal physiology is fundamentally more complex.
The horse’s stomach and small intestine lead to the hindgut, which comprises the caecum, ventral colon, dorsal colon, small colon and rectum. Within the hindgut there are differences in the microbiota — the community of microorganisms — between these regions, with the main change occurring at the junction of the ventral and dorsal colon, known as the pelvic flexure.
The microbiota from the dorsal colon through to the rectum are very similar to those found in fresh droppings. Given the importance of the dorsal colon to fermentation of dietary fibre, this allows researchers to study the impact of changes in faecal microbiota. If this relationship did not exist, more invasive techniques would be required to sample intestinal contents.
The gut of the horse has two sets of genetic material. The genes inside the cells that make up the gut wall are inherited from the horse’s sire and dam, and remain almost stable throughout his life. In addition, there’s the microbiome, acquired from the horse’s environment and dynamic in its population.
The bacterial component of this microbiome is predominantly made up of “super families” of bacteria. These minuscule organisms are able to exist within the inhospitable gut environment due to the activity of their “core community” — a “housekeeping” microbial population that creates the environment and functions necessary for microbial life and the stability of the microbiome.
The equine core community is smaller than that of humans, and is smallest if the horse is on a starch-rich diet. This less stable gut environment can increase the potential for adverse microbiotal change and disease.
Why microbes matter
Like a human, a horse is not born with a functioning gut microbiome. The uterus in which he has developed is largely sterile, so he must acquire this genetic material from his environment.
The gut microbiota are already complex within the foal on its first day of life — although individual microorganisms are relatively low in number. Marked change then occurs between two and 30 days of age as the foal encounters a wide range of maternal and environmental organisms.
These come from ingestion of colostrum (the mare’s first milk) and as the foal starts to eat the mare’s droppings and explore his surroundings.
These transient organisms quickly reduce in number as the true colonisers take hold. From 60 days of age, the foal’s microbiota remain relatively stable and are similar to that of his dam.
An established and stable microbiome brings many benefits. As hindgut fermenters, horses are reliant upon microbiotal activity to break down the fibre in their diet efficiently. This produces the short-chain fatty acids that are crucial to the horse’s daily energy requirements, along with essential nutrients.
The microbiome also boosts immune function — both locally, through production of antimicrobial products, which limit pathogenic (disease-causing) activity in the gut and neutralise toxins, and also by guiding the development and regulation of the horse’s immune system on a wider scale throughout his body.
Within adult horses, the gut microbiome shows great individual diversity and is more variable than our own: only about 65% of the equine bacterial community is retained over a six-week period — even when the horse is fed a consistent diet.
Given the importance of the microorganisms to fibre digestion, it is not surprising that dietary change alters the gut microbiota rapidly — within just four days. Stability and resistance to change are greater in horses fed plenty of fibre, compared to those on a concentrate-supplemented diet.
Starch fed in small quantities is digested and absorbed in the small intestine. If more is fed, the small intestine’s absorptive capacity can become overloaded. The starch is then delivered to the hindgut, where it undergoes rapid microbial fermentation, leading to production of lactic acid and reduction in the hindgut pH (an increase in acidity).
This alters the microbiome, increasing the population of certain bacteria and reducing those that break down fibre.
This change has been found in horses fed a high-concentrate diet and those with colic due to intestinal impaction.
To limit starch overload, the concentrate level should be gradually increased over a three-week period to allow sufficient time for increased production of a specific starch-digesting enzyme in the small intestine.
The equine gut microbiome also responds to stressors such as fasting, exercise, anaesthesia and transport, all of which lead to a reduced population of a family of bacteria that break down fibre and could have anti-inflammatory functions.
Additionally, the use of antimicrobials to treat bacterial infection, both orally and by injection, leads to reduced diversity of the hindgut microbiota and population changes that are specific to the antimicrobial given. After finishing treatment, however, the microbiota gradually return to their pre-treatment population over 30 days.
For general health, the best advice is to maintain a diverse, stable hindgut microbiota by keeping your horse on a forage-based diet with the minimum amount of concentrates needed for the work performed.
There is no evidence of a beneficial effect of probiotics to the hindgut microbiome, because it is unknown which microorganisms are beneficial and in what quantity or formulation. Neither is there evidence to suggest that they are harmful, however, so keep feeding them if you think that they help your horse.
One day it may be possible to manipulate the gut microbiome in a good way. A current example of this is faecal transplantation, where droppings from a healthy horse are given by stomach tube to one with diarrhoea — in some cases causing complete resolution.
Keep watching this space, because an increased understanding of equine microbiota in general could revolutionise vet care.
Ref Horse & Hound; 8 February 2018