Evidence of probiotic efficiency in horses is weak, review team says

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Evidence of probiotic efficiency in horses is weak, according to a just-published review, despite several supposed clinical applications, including acute enterocolitis and diarrhea in foals.

A German review team, which set out to examine current research on the gut microbiome in horses, noted that products classified as probiotics had reached the commercial market, not only for humans but also for horses.

In 2001, experts defined a probiotic as “live strains of strictly selected microorganisms which, when administered in adequate amounts, confer a health benefit on the host”.

In the United States, probiotics can either be classified as a drug needed to gain approval from the Food and Drug Administration, or as a feed supplement “generally regarded as safe” based on information provided by the producers, which removes the need for FDA approval.

In the European Union, probiotics are regarded as feed additives and gut flora stabilizers for healthy animals.

The EU applies very strict regulations for products labeled as probiotics. Producers need to prove product identity, safety and efficacy to a scientific committee.

So far, bacteria such as Lactobacillus, Enterococcus, Bacillus, Streptococcus and Bifidobacterium are considered as putative beneficial probiotics for horses, Anne Kauter and her colleagues reported in their review, published in the journal Animal Microbiome.

“Probiotics should be able to survive the extreme gastric environment, have an antimicrobial property against pathogens and adhere to mucus and epithelial cells,” they said.

Probiotics for horses are designed to reach and establish themselves in the large colon, where many diseases occur.

The review team noted that a recent study investigated the effects of multi-strain probiotics on the bacterial microbiota of foals during and after administration. Limited changes were found concerning the relative abundance of bacterial families, with an enrichment of Lactobacillus in the probiotic group only at week six.

“Yet, evidence of probiotic efficiency in horses is weak despite several putative clinical applications including acute enterocolitis, diarrhea in foals, as well as fecal sand clearance,” they wrote.

The authors noted that understanding the complex interactions of microbial communities including bacteria, archaea, parasites, viruses and fungi of the gut with states of either health or disease is still an expanding research field in both human and veterinary medicine.

Gut disorders and their consequences are among the most important diseases of domesticated horses, but current gaps of knowledge hinder adequate progress in terms of disease prevention and microbiome-based interventions.

“Current literature on enteral microbiomes mirrors vast data and knowledge imbalance, with only a few studies tackling archaea, viruses and eukaryotes compared with those addressing the bacterial components.”

Today, molecular-based sequencing technologies open a new door to researchers.

They say knowledge expansion around the beneficial composition of microorganisms within the equine gut creates fresh possibilities for diagnosing problems earlier, as well as opening the door to innovative therapeutic approaches.

The scientists, who cited 187 papers, found that the current understanding and progress in equine microbiome research is clearly not yet at eye-level with the latest vast progress in human medicine.

“Nonetheless, important first research initiatives have been kicked off, and fields worth investigating have been addressed clearly.

“Although microbiome research is considered an emerging science, with some areas of research still in their infancy, the field is progressing rapidly.

“Nowadays, the most important research task is to gain a deeper understanding of the complex relationships between the gut microbiota, wellbeing and disease.”

In humans, some diseases are marked by the presence of potentially pathogenic microbes in the gut, whereas others are characterized by a depletion of health-associated bacteria.

Only recently, the first study investigating changes in the fecal microbiota in healthy horses over 52 weeks was published. Throughout all seasons, Firmicutes and Bacteroidetes dominated the fecal microbiota, but supplementary forage, season and weather conditions were linked with change in the fecal microbiota composition.

This, they said, provided an excellent starting point for further microbiome research investigating changes associated with metabolic disorders, infectious diseases or the effects of drugs, now that the first framework for microbial composition associated with healthy horses has been set.

“However, disturbance of gut microbiota leading to or indicating illness still needs to be defined more precisely for horses.

“Similar to the current trends in human medicine, it might be possible to develop individual treatment opportunities for certain kinds of equine diseases which were marked through a certain and distinct pattern of microbial composition like equine grass sickness, laminitis or colitis.”

Further research is also warranted around the effects of different antibiotics on the equine gut microbiome, they said.

The full review team comprised Anne Kauter, Lennard Epping, Torsten Semmler, Esther-Maria Antao, Dania Kannapin, Sabita Stoeckle, Heidrun Gehlen, Antina Lübke-Becker, Sebastian Günther, Lothar Wieler and Birgit Walther. Most were affiliated with either the Robert Koch Institute or the Free University of Berlin.

Kauter, A., Epping, L., Semmler, T. et al. The gut microbiome of horses: current research on equine enteral microbiota and future perspectives. anim microbiome 1, 14 (2019) doi:10.1186/s42523-019-0013-3 https://doi.org/10.1186/s42523-019-0013-3

The review, published under a Creative Commons License, can be read here

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