Probiotics have been described as a live microbial feed supplement which beneficially affects the host animal by improving its intestinal microbial balance and exert health effects beyond inherent basic nutrition. The earliest published report of this concept was by Elie Metchnikoff in 1907 who conjectured that ingesting fermented milk products were responsible for longevity of certain ethnic groups by improving the quality of the intestinal microflora. The term was coined in the 60's.

A related product but not truly a probiotic are killed Saccharomyces culture and byproduct nutritional supplements. These have come to be termed prebiotics. Saccharomyces is baker's yeast and many of the organisms are yeasts used in fermentation processes. Though often packaged live the survivability of these products is very short and therefore should be thought of as a nutritional supplement. However we will discuss them here do to their similarity to probiotics.

There is a growing body that certain live cultures of particular yeast may be useful in the treatment of colitis in horses. There has been an explosion of pre and probiotic products available and a concomitant rise in the use of these products. This article describes the principles of probiotic therapy, the research available into efficacy, and which products have solid research behind their use.

There is a growing body of evidence that certain live culture probiotic preparations lessen the severity of colitis, particularly that caused by Clostridia. There are many products currently on the market and it is likely many are not effective. However the yeast organism Saccharomyces boulardii given as a live lypholized oral treatment at the rate of 25 gm, every 12 hours for 14 days significantly improved the clinical signs and shortened the duration on acute colitis of unknown causes. Saccharomyces boulardii is a nonpathogenic yeast used in the treatment of C. difficile diarrhea and colitis in humans. It remains uncertain what exactly is the nature of this treatment but the yeast releases a protease that specifically degrades C. difficile toxins A and B, and this has been shown to be protective in experimental C. difficile colitis in rats and to prevent damage to human coionic epithelium by C. difficile toxins A and B in vitro. The yeast has mild virulence in mice, and its use in immunocompromised patients may not be entirely benign.

Research in humans suggests that probiotics may have a place in equine medicine and nutrition. In humans the GG strain of Lactobacillus rhamnosus has had the most research done on it and shown to be effective in:

• Acute diarrhea in children
• Travellers'diarrhea in adults
• Crohn's disease (an autoimmune disorder of the digestive tract.
• reduced incidence of antibiotic-associated diarrhea in infants.

Other strains studied Lactobacillus rhamnosus, L. salivarius, L. reuteri, L. johnsonii, L. acidophilus, Bifidobacterium lactis, B. bifidum, B. longum, Enterococcus faecium, and Saccharomyces boulardii.

It is important to understand that this positive research was done with laboratory cultures and not with commercial products. Research with commercial products has been disappointing in both horses and humans and a close look at the products suggest why. Misidentification of probiotic organisms, lack of viability of the organism, and fewer organisms present than were listed is the rule, not the exception. The numbers of viable organisms are critical to the success of probiotic treatment. In one study of human products fewer than 20% matched their label claims with some having no live organisms at all. Initial looks at veterinary products suggest the same problem. Another problem with commercial products is the lack of strain specificity. It has been found that different strains of the same organism have different properties that effect efficacy. It is not enough to know what organism is present but also what strain of the organism it is.

Commercial Probiotics

Yogurt Culture

Baker's Yeast