Horses rely on a community of micro-organisms to digest their high fibre diets.
Food passes through their stomach and small intestine before reaching the expansive chambers of the cecum and colon, where fermentation occurs. During fermentation, a delicate interaction exists between bacterial strains and their mixing vessel.
An optimum balance of various micro-organisms, water content and acid buffering are needed for it to work.
The colon is also the site of many important digestive tract diseases in horses, including diarrhea.
It is teeming with bacteria, and it’s the good ones that occupy and prevent establishment of the bad ones.
However, the bad bugs get a chance to proliferate and potentially cause illness if something disturbs the beneficial bugs, such as antibiotic treatment, diet change and stress.
A growing understanding of the role that gut bacteria play in health has stimulated interest in probiotic therapy to re-establish healthy bacteria as a treatment for diseases such as diarrhea. Probiotics also hold promise as prevention for conditions such as neonatal diarrhea in foals.
The World Health Organization says probiotics are “live micro-organisms, that when administered orally at adequate concentrations, provide a beneficial effect beyond that of their nutritional value.”
They are typically formulated as oral pills or powders that contain one or more strains of “healthy” bacteria.
The ideal probiotic must be able to be produced at commercial scales and remain viable during production, packaging, shipment, storage and in the acidic environment of the stomach. Once in the right location, it must adhere to, proliferate and colonize the intended target site in the intestines. As well, probiotic strains should not be resistant to antibiotics because this could be a source for bacteria already within the gut.
There are several ways probiotics can modulate disease.
One way is through their effect on the immune system. Some strains dampen down the immune system and are generally anti-inflammatory, while others may increase inflammation.
As well, these healthy strains may colonize the gut lining and prevent harmful bacteria from attaching. Some probiotic types can wage chemical warfare on other bacteria through the production of antimicrobial particles. They also may be able to block the effects of harmful bacteria toxins, which in some cases diminishes their ability to cause disease.
More research needed
There are few studies on the effects of probiotics for treating digestive tract disease in horses. Properly designed and controlled studies are needed to evaluate the efficacy of specific bacteria strains.
Another criticism of probiotics that are marketed to treat horses is centred on the strains of bacteria they contain.
Most products contain the same probiotics that are common in human and pet probiotics, including Lactobacillus, Bifidobacterium and Entercocci. However, this is problematic for treating horses because research into the microbe community in the horse colon suggests that these probiotic strains are not that important. As a result, it may be pointless to add more to the colon.
As well, probiotics are not regulated in the same way as medications, which can result in a lack of rigid quality control.
For example, a Canadian study found that only eight percent of probiotics intended for use in animals had proper labelling.
Other studies have found wide discrepancies between the concentration listed on the label and what was actually in the product. Some contained none of the listed active bacteria, while others had more than 200 times the amount specified. This lack of consistency makes it challenging to trust the products, prescribe doses and test them for effectiveness.
It might be biologically naive to think that the complex microscopic universe in the gut can be distilled down to a few or even a single bacteria species. Rather than trying to pin down one or two important strains to include in probiotics, fecal transplants aim to capture the whole microbial community.
The theory is that you provide the horse’s intestine with a complex mixture of a multitude of species to better approximate the microbial mosaic that is found in the gut of healthy horses.
Published research in animals is lacking, but fecal microbial transplants in human medicine have been used with great success to treat the intractable diarrhea associated with Clostridium difficile.
No studies have evaluated the use of fecal transplants to treat digestive tract disease in horses, but the treatment holds promise for cases with chronic diarrhea.