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The animal’s microbiome
Before exploring the role of probiotics in the animal’s health, we should understand the importance of a well-balanced gastrointestinal microbiome. The gastrointestinal microbiome is comprised of bacteria, fungi, protozoa, and viruses which coexist and can be mutually beneficial in a healthy individual. The microbiome is established at birth and influenced by both the birthing process and the environment. It then becomes colonized and continues to be shaped by nutrition.^1,2

The gastrointestinal microbiome is impressively large. In fact, the number of bacterial microbes present is 10 times that of the cells which are found in the host. This immense size also means the genes encoded by these microorganisms greatly outnumber those of the host.¹ Each individual has their own unique microbiome within their gastrointestinal tract. In dogs and cats, the predominant bacteria present in the gastrointestinal tract are Bacterioidetes, Firmicutes, Proteobacteria, Fusobacteria, and Actinobacteria. The quantity of each bacteria is variable between these species.² Studies in humans indicate that diet and environment are the most influential factors in the maintenance of the microbiome. This is similar in dogs and cats, where diet influences both short and long-term maintenance of the fecal microbiome.² Other factors which can influence the microbiome include age, genetics, medications, and disease.

A well-balanced microbiome is crucial for an animal’s overall health. It is involved in immune system regulation, nutrient absorption, synthesis of vitamins, metabolism, and neurodevelopment. The microbial population varies greatly between the different segments and layers of the intestines. Studies have shown that the microbial population is important in the development of gut associated lymphoid tissue, also known as GALT. This tissue is found within the intestinal wall and involves a variety of cells which respond to microbial signaling to produce the appropriate anti or pro inflammatory response.² Not only is the microbiome an important player in immune system regulation, but it is also responsible for producing numerous metabolites. It contributes to the production of short chain fatty acids (SCFA) which are essential for gut health as they aid in strengthening the gut wall, regulating intestinal motility and contributing to the production of anti-inflammatory compounds.²

Any disruption within the microbiome which negatively impacts the host, also known as dysbiosis, has been linked to several disease processes in both animals and humans. Dysbiosis can occur due to shifts within the microbial community and diversity or due to a lack of commensal organisms. There is often associated inflammation present, and it is commonly seen in animals with enteropathies.²

The role of probiotics
Supporting the microbiome is important in both healthy patients and those with concurrent illness. One way to support the microbiome is through supplementation with probiotics. Probiotics are live microorganisms which when consumed have beneficial effects for the host. They have been shown to enhance overall health through various proposed mechanisms such as strengthening of the intestinal barrier wall, immunomodulation via cross talk with the host immune cells, and production of SCFAs.² There is great potential for probiotic supplementation to support and improve the health of our companion animals. They have been shown to be beneficial in the management of certain diseases. A common presenting complaint in veterinary medicine is diarrhea. While there are numerous causes for this clinical sign, there is a high likelihood that the gastrointestinal tract is also impacted. Acute cases of diarrhea may be treated with antibiotics. While this can result in resolution of the clinical signs of diarrhea, it does raise the question of how this may impact the microbiome. One study examined the effect of metronidazole, an antibiotic commonly used to treat diarrhea in dogs. It determined that while the diarrhea did resolve, there were significant changes to the microbiome which did not resolve within four weeks of cessation of the antibiotic.³

Could probiotics be of possible benefit in uncomplicated cases of acute diarrhea? Another study examined dogs which were experiencing acute hemorrhagic diarrhea syndrome (ADHS). There were two groups total, and both groups were treated symptomatically, while only one group was administered a high potency probiotic. The study revealed that the group which received the high potency probiotic had a microbiome which normalized faster than the group which received symptomatic treatment alone.⁴ In a study evaluating the effects of probiotic administration on dogs affected by inflammatory bowel disease (IBD), dogs were administered either the standard treatment for IBD or the standard treatment with the addition of a probiotic. While both groups improved clinically, there is evidence that the probiotic supplementation may help enhance mucosal homeostasis.⁵

Unlike dogs, the feline population tends to encounter more cases of constipation and/or megacolon. One study examined the effects of the supplementation of a multi strain probiotic administered to cats suffering from chronic constipation or megacolon. The sample size of this study was small as it examined 10 cats with a previous diagnosis of constipation, three of which had megacolon. This study examined not only the clinical signs of the cats, but also the microbial and histological parameters of the gastrointestinal tract. The study concluded the cats had significant improvement of clinical signs associated with constipation when supplemented with the multi-strain probiotic. It was also determined that there was improvement within the gut mucosal lining indicating there could be anti-inflammatory effects associated with the supplementation of the probiotic.⁶

While we tend to think patients with gastrointestinal disease benefit the most from probiotic supplementation, there are also systemic diseases where probiotic supplementation has also proven beneficial. Chronic kidney disease (CKD) is commonly encountered in our dog and cat patients in veterinary medicine. When kidney function is impaired, the kidney’s ability to filter out excess waste declines. This decline can lead to an accumulation in the bloodstream which can then cause alteration to the microbiome.⁷ Glomerular Filtration Rate (GFR) is a measurement of how well the kidneys are functioning. In a two-month study examining dogs in various stages of CKD, both groups were fed a renal diet, while only one group was administered a probiotic. Any additional conditions secondary to the CKD were managed appropriately. At the end of the study, it was concluded that the group administered the probiotic had a higher GFR compared to the control group which had a significantly lower GFR.⁸

Another commonly encountered concern in veterinary medicine is arthritis, a condition which can develop in a multitude of species. Multiple factors can contribute to the development, though there is speculation that chronic low-grade inflammation within the body may be a contributing factor. Managing the discomfort and inflammation associated with arthritis is the goal in keeping the affected patient comfortable. This can be achieved through multiple approaches. Some studies have revealed that the administration of undenatured Type II collagen to animals affected by arthritis helped clinically improve their overall mobility by reducing inflammation and discomfort. This occurs via immune modulation across the gastrointestinal tract.⁹ While this is not a study reflecting the benefits of a probiotic supplement and its use in management of arthritis, it does show that there is some potential benefit in considering the microbiome in the management of patients with arthritis.

Behavioral management is another area where consideration of the microbiome may prove to be beneficial. Studies have shown that the composition of the microbiome can differ in animals affected with behavioral concerns when compared to normal animals. One study revealed that mice supplemented with a single strain probiotic Lactobacillus rhamnosus reduced stress induced corticosterone and anxiety related behaviors.¹⁰ There are not a lot of studies examining the potential effects of probiotics on behavioral concerns in companion animals; however, there is evidence supporting that the microbiome may be a player in the influence of certain behaviors.

While the research in the microbiome field continues to grow, there are various studies supporting the benefits of probiotic supplementation in animals to improve overall health and disease management. Hopefully, as research advances additional studies will be performed to assess the clinical efficacy of probiotics in various disease states commonly encountered in veterinary medicine.

About the author: Dana Hogg, DVM, cVMA
Dr. Dana Hogg graduated in 2015 from North Carolina State University. Growing up with several animals in Wilson, N.C., Dr. Hogg was drawn to the field of veterinary medicine at a young age. She completed her undergraduate degree at NCSU in 2009 and her master's degree in 2011.

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