The gastrointestinal (GI) tract of the normal adult horse contains a complex community of microorganisms. Most of these microbes are located in the large intestine, where they assist in food digestion (especially fiber) and synthesize compounds (such as some vitamins) that are important to the horse’s health. Most of the helpful microbes in the gut of the normal horse are nonpathogenic, and they are typically called the normal flora. Some pathogenic organisms also reside in the GI tract, but these organisms are kept in check because they have to compete with the normal flora.

In foals the colonization of the GI tract by pathogenic organisms can lead to diarrhea. Considerable research has focused on identifying the organisms responsible for neonatal diarrhea, but less effort has been made to identify the factors that allow the pathogenic organisms to become established. Additionally, very little is known about how or when the normal GI flora becomes established in the foal.

Researchers at the University of Kentucky have been studying the development of digestive capacity in foals for several years. An initial project indicated that very young foals have a low capacity for fiber digestion but older foals have a capacity similar to their dams. Because fiber digestion is performed by specific bacteria in the large intestine, these observations suggest that foals develop a normal microbial population in their GI tract by at least 1 or 2 months of age.

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