Definition

Probiotics are living microorganisms that confer a health benefit to the host in a safe and efficacious manner when administered in adequate amounts. Suggested mechanisms of action are numerous and include modulation of intestinal immune function, promotion of epithelial homeostasis with enhancement of barrier function, and neuroregulatory effects that may reduce the visceral sensitivity to stress. Additionally, probiotics may block the effects of pathogenic bacteria by reducing their ability to bind to the mucosa, by changing the microenvironment and producing antibacterial substances. Finally, they may have nutritional benefits in assisting in the breakdown of otherwise indigestible foods.

Prebiotics are non-digestible carbohydrates that modulate the intestinal microbiota and promote the growth of beneficial microorganisms in the intestine. Synbiotics are preparations that contain probiotics and prebiotics that act in a synergistic fashion.

Gut Microbiota

In dogs, cats, and humans intestinal microorganisms outnumber the mammalian cells in the host body by a factor 10. Consequently, it is easy to envision that the gut microbiota play an essential role in maintaining GI and general health. GI microorganisms are able to ferment otherwise indigestible dietary fibers, synthesize vitamins and amino acids, prevent pathogen colonization, assist in maturation and regulation of the immune system, modulate GI hormone release, and influence the gut-brain axis.

Digestive diseases have been associated with major changes in the gut microbiota. These may be a consequence of the primary disease, or be involved in the pathogenesis of the disease. In dogs, it has been shown that acute and chronic intestinal inflammation are characterized by a loss of microbial diversity, and by the preponderance of different microorganisms than those seen in healthy animals. Often, the gut microbiome only returns to normal weeks to months after clinical recovery. Understandably, the gut microbiota can also be affected in a severe and prolonged manner following administration of antimicrobial substances such as amoxicillin and metronidazole. Importantly, changes in the gut microbiome may have profound repercussions on physiologic processes, and consequently on the general health of the animal beyond the GI tract.

Probiotics in Canine and Feline Medicine

Probiotics are defined by the type and number of live non-pathogenic bacteria and/or yeast they consist of, by their ability to survive the very acidic gastric environment, proliferate and colonize the intestine, and adhere to the intestinal mucosa. Single strain and multistrain products are available, and commonly used bacteria include Bifidobacterium spp., Lactobacillus spp., and Enterococcus faecium. Recommended doses for a small dog range from 100 million to 225 billion colony forming units (CFU) per day depending on the product. Some animals may experience flatulence and excessive borborygmus when receiving excessive quantities of probiotic bacteria, which are best addressed by empirically reducing the dose.

Overall, there are only very few published studies evaluating the effects of probiotics on the health of dogs and cats. Additionally, their experimental design does not often allow definitive statements on the benefit of probiotics or lack thereof. The following paragraphs summarize a few studies that pertain to probiotics designed for use in small animals and their effects on the digestive tract.

In a recent study, genetic material from the bacterial strains of a commercially available probiotic could be recovered from the feces of healthy dogs and cats that had received it. However, it did not have an apparent impact on the fecal microbiota. Also, administration of a synbiotic to healthy sled dogs increased the fecal concentration of butyrate.

Not All Probiotics Are Equal

Probiotics are categorized as nutraceuticals, and are not subject to strict regulatory scrutiny. Pet owners can purchase probiotics in pet stores and on the internet as well as at their primary care veterinarian. A 2011 study from Canada revealed that most probiotics marketed for use in small animals did not live up to their label. With the exception of 2 products none of them had sufficient amounts of viable organisms when cultured. In addition, the potential benefits of probiotics are often difficult to demonstrate.

New probiotics become available each year, some of them are distributed through veterinarians. It is therefore essential that veterinarians scrutinize them thoroughly and demand evidence of quality control and efficacy from the manufacturers.

Acute Gastroenteritis

In a double-blind, placebo-controlled Swedish study of 36 dogs with acute diarrhea not caused by intestinal parasites, administration of a multi-strain probiotic to dogs with acute (gastro)enteritis resulted in an earlier return to normal fecal consistency (1.3 days vs. 2.2 days). In another well-designed study duration of diarrhea in cats housed in a shelter was shortened by administration of a single-strain probiotic when compared to the placebo group. No such difference could be documented in dogs due to lack of statistical power.

In the conclusion of the Swedish study, the authors state that "probiotic therapy is also a promising option to reduce the indiscriminate use of antimicrobials" administered to dogs with acute diarrhea. Supportive evidence that antibiotics should not be empirically administered to dogs with acute gastroenteritis is available in studies from Germany that have eloquently demonstrated that antibiotics have no beneficial effect on dogs with acute hemorrhagic diarrhea syndrome.

Chronic Enteropathies and Inflammatory Bowel Disease (IBD)

A randomized, placebo-controlled study from Italy recently documented the beneficial effects of a multi- strain probiotic designed for human patients in dogs with idiopathic inflammatory bowel disease (IBD). Control dogs were treated with prednisone and metronidazole while probiotic dogs did not receive any other drugs. Interestingly, clinical improvement in dogs with probiotics alone was comparable with that of dogs treated with prednisone and metronidazole.

Additionally, several parameters of inflammation and integrity of the mucosal barrier improved more significantly in probiotic-treated dogs. In spite of the small size of the treatment groups (10 dogs each) and the fact that dogs in the control group were clinically more severely affected than those in the probiotic group at the outset, these results are encouraging about the role that probiotics play in IBD. However, other studies have evaluated probiotic treatment in dogs with diet-responsive chronic enteropathy (CE) and have been unable to identify any difference between placebo group receiving an elimination diet and dogs receiving the same elimination diet and probiotics. Lack of statistical power due to small dog numbers was a problem.

There are no published controlled studies evaluating the effects of probiotics in feline CE. One study documented improvement of the fecal score and of the owners' subjective perception of their cat's health in 53 adult cats with chronic diarrhea in the absence of intestinal parasites.

Open Questions

Small animal veterinarians have to rely on "what makes sense" to decide if they should use products with a single bacterial strain or multiple strains, or very highly concentrated or regular probiotics. The ideal duration of probiotic treatment also remains unknown, although experts have recommended at least 2 weeks in cases of acute gastroenteritis and 4–6 weeks or longer in chronic enteropathies. It is notable that probiotics do not appear to exert any long lasting effects after discontinuation of treatment. Finally, several experts have reservations about the use of probiotics in dogs with severely compromised intestinal mucosal barrier (e.g., dogs with hemorrhagic diarrhea) fearing the risk that probiotic bacteria may translocate to the portal venous system and the liver. In this situation, they prefer to delay the initiation of probiotic treatment until resolution of the bloody diarrhea.

Outlook

While evidence of the benefits of probiotics in dogs with GI diseases is slowly emerging, there is still a need for numerous studies to provide clearer evidence of their role in a wider range of diseases. This need is even more urgent for cats with GI diseases.

Good quality scientific investigations are required to identify which GI diseases would benefit from probiotic treatment in dogs and cats, and better delineate the preferred type of probiotic and the optimal treatment duration for each indication.

References

1.  Herstad HK, Nesheim BB, L'Abee-Lund T, Larsen S, Skancke E. Effects of a probiotic intervention in acute canine gastroenteritis - a controlled clinical trial. J Small Anim Pract. 2010;51:34–38.

2.  Rossi G, Pengo G, Caldin M, et al. Comparison of microbiological, histological, and immunomodulatory parameters in response to treatment with either combination therapy with prednisone and metronidazole or probiotic VSL#3 strains in dogs with idiopathic inflammatory bowel disease. PLoS One. 2014;9:e94699.

3.  Unterer S, Strohmeyer K, Kruse BD, et al. Treatment of aseptic dogs with hemorrhagic gastroenteritis with amoxicillin/clavulanic acid: a prospective blinded study. J Vet Intern Med. 2011;25:973–979.

4.  Weese JS, Martin H. Assessment of commercial probiotic bacterial contents and label accuracy. Can Vet J. 2011;52:43–46.