Hospitalized geriatric horses that successfully recovered from severe bouts of colitis—intestinal inflammation often associated with diarrhea—showed positive changes in their gut microbiota after receiving diluted feces from healthy horse donors, said Daniela Bedenice, Dr. med. vet. Dipl. ACVIM, ACVECC, of the Department of Clinical Sciences in Tufts University’s Cummings School of Veterinary Medicine, in North Grafton, Massachusetts.

By pumping a “soup” of fresh feces blended with water directly into the patients’ stomachs via a nasogastric tube, the clinicians seem to have modified the composition of microorganisms living in the horses’ intestines, they revealed.

“Specifically, we noted that there was a greater variety of the different kinds of bacteria in the gut after the transfer of healthy feces,” Bedenice said. They also noted a greater abundance of Verrucomicrobia (also known as Kiritimatiellaeota), a bacterial species that lives in the mucous lining of the intestines and contributes to increased mucus production, which potentially helps ward off disease agents.

In general, after three days of fecal transplant treatment, these horses had microbiomes that more closely resembled those of the healthy horses whose manure was used in the study, she said. Their gastrointestinal health status also improved.

Analyzing Microbiomes in Healthy and Diarrheic Horses

The scientists treated five senior horses, all hospitalized with diarrhea that wasn’t responding to treatment, with nasogastric-pumped healthy feces. Veterinarians had found no specific cause (such as a virus or bacterium) for the diarrhea, so they diagnosed it as “undifferentiated” colitis—meaning the disease’s origin was unclear.

They ran genomic sequencing on the five patients’ microbiomes as well as on those of 30 healthy horses (15 young horses, 15 seniors), including three horses randomly selected as donors. Although microbiomes among horses with diarrhea varied considerably, the researchers noted two trends distinguishing healthy horses from those with colitis: the number of different kinds of bacterial species and the total number of Verrucomicrobia were both lower in horses with diarrhea in this population.

Three of the five senior patients recovered from the diarrhea after treatment, and all three had improvements in their microbiomes following the fecal transplants, Bedenice said. Two of the horses did not recover and had to be humanely euthanized, one due to a large intestinal stone causing diarrhea. The other horse received simultaneous treatment with antibiotics during the study period to manage pneumonia, which might have affected the repopulation of bacteria in the gut following the transplant, she said.

Young and Old Microbiomes Compared

Interestingly, the microbiomes of healthy younger horses and healthy older horses didn’t differ significantly, Bedenice said. This suggests age alone doesn’t make the microbiome less healthy. In fact, older horses could probably act as good donors for fecal transplants in horses of any age.

What’s more important for fecal transplants is diet and probably genetics, she said: “We may actually need to ‘match’ donor horses to recipient horses not by similar age, but by similar diet, in order to acquire a microbiome that is most appropriate for the patient’s environment.”

Although the study focused on senior horses, as it was part of a larger research project on geriatric equids, fecal transplant therapy isn’t limited to older patients, said Bedenice. “I wouldn’t limit this treatment to aging horses; it applies to all groups,” she said.

Still, older horses might have a significantly increased risk of mortality from colitis—nearly 12% per year of increasing age compared to younger horses, Bedenice said, referring to data from a study at Tufts that has recently been accepted for publication. Further information about that risk will be released soon, she added.

“The procedure of fecal transplant itself isn’t novel, and in fact it dates back to at least 4^th-century China, when it was used in humans,” she said. “We’ve been treating horses with fecal transplants in our clinic for several years now, with good results. But this is the first time, thanks to genetic sequencing, that we’ve been able to see what’s actually happening in the microbiome when these transplants occur.”

Home Remedy? Not Really

The success shown in this study doesn’t mean people should collect fresh manure and mix it into their sick horses’ feed, however, Bedenice said. Donors need to go through rigorous screening for the presence of infectious diseases and parasites. And owners should always consult their veterinarians for treatment advice.

“So far everything we’ve looked at has been very supportive of the idea that fecal transplant has efficacy in regard to potential for improving or shortening the duration of diarrhea,” Bedenice said. “There’s a positive impact on the fecal microbiome, because we’re seeing that in the horses that are treated, their microbiota look more similar to their donor, which is what you want to achieve. So the data continue to support that fecal transplant has clinical utility.”

The next step in fecal transplant research is to home in on why it works and how to streamline the procedure so field veterinarians can perform it in a more user-friendly way, she said, adding, “We still have a lot to learn about the gut microbiome, and the work that has been done so far has relied on the teamwork and expertise of many researchers.”