May 2021

In this VETgirl online veterinary continuing education blog, Dr. Patrick Barko, MS, DACVIM (SAIM) reviews the role of the intestinal microbiome in patient health. This blog is sponsored by Purina® Pro Plan® Veterinary Supplements. Please note the opinions in this blog are the expressed opinion of the author, and not directly endorsed by VETgirl.

Keeping Bugs in Balance: The Role of the Intestinal Microbiome in Patient Health

By Dr. Patrick Barko, MS, DACVIM (SAIM), Research Specialist at the College of Veterinary Medicine at University of Illinois at Urbana-Champaign

Bring up the topic of dysbiosis, and gastrointestinal (GI) disorders typically come to mind. From transient conditions such as acute diarrhea to chronic disorders like inflammatory bowel disease, veterinarians are quick to link the imbalance of “good” and “bad” bacteria in the intestinal microbiome with enteropathies.

Dysbiosis: more than a gut feeling
However, while dysbiosis definitely is associated with GI disease, it also has been associated with an array of other conditions. The microbiome and the host animal’s metabolic and immune systems interact in important and interesting ways, both to promote the health of the host or—when these interactions go awry—to cause abnormal stimulation of mucosal immune responses. The bacteria in the GI microbiome interact indirectly with almost every organ system in the body, where they can both start and stop inflammatory processes.

How does this work? Previous studies have estimated that approximately 70% of a mammal’s immune system is located in the intestinal tract. As food is digested with the help of gut bacteria, metabolites produced by microbes from dietary nutrients enter the bloodstream and make their way to organs like the liver, pancreas, lungs and heart. Meanwhile, nerve endings in the intestinal tract connect directly to the brain via the vagus nerve. Scientists today hypothesize that disorders in humans as diverse as diabetes, obesity and cardiovascular diseases, as well as neurocognitive conditions such as schizophrenia, autism and anxiety (the latter in dogs as well as humans), can all be associated with dysbiosis.

It’s interesting: Several thousand years ago, Hippocrates said that “all diseases begin in the gut.” While he may not have known anything about the microbiome, his intuition may have been correct!

The origin of the [bacterial] species
The intestinal microbiome is essential to long-term health. In studies conducted in laboratory animals, mice were grown in a sterile environment where their intestinal microbiome didn’t properly develop. Researchers learned that their organs, including the heart and the brain, as well as their immune systems, also didn’t develop properly and their lifespans were significantly shortened.

Like other mammals, companion animals acquire their first enteric microbes from their mothers and their environment. The uterus is a sterile environment, but once animals are born and enter the outside world, they are quickly exposed to a variety of microbes. During the neonatal period, the developing microbiome is unstable and it undergoes significant changes with the introduction of solid food and weaning. As a puppy reaches maturity, so does his GI microbiome. By the time a dog is around 6 months of age, his intestinal microbiome will have become relatively stable.

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Intestinal interventions
While the adult microbiome tends to be stable in mammals, it can change in response to environmental, nutritional and pathogenic stimuli. If the intestinal microbiome is imbalanced, we have the challenge of trying to correct the imbalance and address health issues that may result. Probiotics, prebiotics and synbiotics provide ways to intervene.

• Probiotics are living bacteria that are administered to promote host health. In dogs and cats, probiotics are typically administered via oral supplements. The goal with probiotic therapy is to introduce populations of specific microbes that are known to have health benefits. However, not all probiotics are alike. Some probiotics stimulate the immune system while others suppress immune responses. Meanwhile, the effects of probiotics can be patient-specific. As this field matures and we learn more about the microbiome, the goal will be to have markers that can inform clinicians about which probiotics to use for specific conditions—and even in specific patients.

• Prebiotics are dietary substances, typically fiber sources, that are metabolized by GI microbes. This produces two main effects: (1) to provide nutrition for gut bacteria; and (2) to generate beneficial byproducts via the metabolism of these fibers. These include short-chain fatty acids like butyrate that help maintain the mucosal barrier and regulate the enteric immune system, generally in a way that suppresses inflammation. They also interact with the metabolic and endocrine systems. Prebiotics can be fed in either food or as supplements.

• Synbiotics are combinations of probiotics and prebiotics, typically fed as supplements. In theory, providing a probiotic with a source of nutrients (prebiotics) can enhance survival and increased activity of the probiotic species, potentially resulting in more benefits to the recipient.

Veterinarians have become familiar with using these supplements but continue to have questions. Which patients will benefit? Which products are effective for which conditions? How long should the supplement be fed? More research is needed to be able to fully answer these questions. For now, my advice is to base product selection on the data that is available along with clinical experience.

A final caution: We are living in the Wild West of supplements. Walk into a health food store or local megamart, and you’re likely to find aisles dedicated to every kind of vitamin and supplement. Unfortunately, many of these have not been tested for safety or efficacy. I strongly advise veterinarians to stick with veterinary supplements made by companies known to engage in research and development with facilities that are routinely inspected. That way, you can be sure the products are safe and have more confidence in the claims they make.

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