Do Probiotics Help Prevent Foal Diarrhea?

Do you give your foals probiotics to prevent diarrhea? New research suggests that perhaps you shouldn’t. It appears that the substance meant to put a halt to diarrhea isn’t stopping it and, in fact, might even encourage it.

Designed to enhance the body’s defense, probiotics can produce antimicrobial compounds that target intestinal pathogens and their toxins. In theory, and even in in vitro (laboratory dish) tests, the concept seems to work. However, results from a new study by scientists in Switzerland suggest that in the living foal, the probiotic’s effects are disappointing.

“Our results are a bit frightening, particularly because probiotics are often given under the assumption that ‘even if they don’t work, they won’t cause any harm,’” said Angelika Schoster, DrMedVet, DVSc, PhD, Dipl. ACVIM, ECEIM, senior clinical lecturer at the University of Zurich’s Vetsuisse Faculty in the Clinic for Equine Internal Medicine, in Zurich.

However, that doesn’t mean all probiotics are ineffective, Schoster added. “There are some studies where the product used did not cause a negative effect,” she said. “But ours did. What this means is that every product has to be tested.”

Currently, many of the over-the-counter probiotics on the equine market have not been tested in clinical trials, she said.

In their study, Schoster and her fellow researchers investigated the efficacy of a probiotic they designed based on promising in vitro study results. The scientists selected the specific bacteria strains—which, based on the preliminary results, should have been even more potent and effective in preventing diarrhea than currently available probiotic products, she said.

They followed 72 weanling foals—mostly Thoroughbreds—over a four-week period at several different Canadian breeding farms. For the first three weeks, half the foals received the specially designed probiotic, while the other half received a placebo (an identical-looking product but with no active ingredients). While the scientists kept track of which foal received which product package, the foals’ handlers were unaware of which product they were giving to each foal. Twice-weekly fecal analyses provided useful data about the microbiota (the whole “community” of microorganisms in a certain environment) of the horses’ intestines.

The researchers found that 41 of the 72 foals (59%) developed diarrhea during the study period. While the treated foals and untreated foals had about the same rate of diarrhea, it was the treated foals who were more likely to need veterinary intervention for their bouts of diarrhea, Schoster said.

“We were fairly surprised about the negative impact,” she told The Horse. “We were hoping for a positive effect of the probiotic; however, given results from other studies, a lack of effect would not have been unexpected.” Several previous studies have shown poor levels of efficacy in other strains of probiotics for preventing diarrhea.

Furthermore, fecal samples showed disappointingly little effect of the probiotic on Clostridia (such as C. difficile or C. perfringens), organisms that can cause diarrhea in foals, she added. “We thought we would see more impact on Clostridial shedding, as the strains we chose were selected based on their ability to inhibit selected Clostridia,” she said.

“It is likely that we need to move away from the current practice of using probiotics which contain one or a few strains of bacteria,” Schoster said. “The future likely lies in products containing many bacterial strains, or even complete fecal bacterial transplantations.”

In the meantime, letting nature take its own course might be best. “Foals are very smart and usually consume mare’s feces through the first weeks of life,” she said. “This aids in colonizing the gastrointestinal tract with ‘good bacteria,’ and foals should certainly not be prevented from consuming the mare’s feces. Maybe we can use this practice therapeutically in the future. However, scientific studies are needed before recommending this practice to the breeder or veterinarian.”

The study, “Effect of a probiotic on prevention of diarrhea and Clostridium difficile and Clostridium perfringens shedding in foals,” was published in the Journal of Veterinary Internal Medicine.