Imagine this clinical scenario: you are presented with an 18-month-old female spayed Labrador Retriever with a one-day history of watery unformed stool with increased frequency. The patient does not have a previous history of diarrhoea. Physical examination is normal aside from faecal staining around the anus, and her appetite and behaviour are unchanged to mildly decreased. Faecal flotation and smear tests are negative for parasites and there is no apparent blood in the faecal sample.
The owner would like a treatment that will return the dog’s stools to normal consistency in the shortest amount of time and asked about prescribing metronidazole as this has worked previously for her other dogs with similar presentations. You know that not all cases of acute gastroenteritis need to be treated with antimicrobials and that most will resolve with time, but the owner wants a treatment today. You consider the rising concern about veterinary use of antimicrobials and wonder if prescribing a probiotic/synbiotic would be as efficacious at treating the acute diarrhoea as metronidazole, so you explore the evidence.
Five studies were critically appraised. All were blinded randomised controlled trials, which provide a high level of evidence; however, only one study compared both treatments directly.
The study by Shmalberg et al. (2019) directly compared treatment with metronidazole, a probiotic product and a placebo. The authors found no significant reduction in time to resolution of acute diarrhoea when the dog received a probiotic or metronidazole. Average time to resolution of diarrhoea was very similar between the metronidazole-treated and placebo groups but one day shorter for the probiotic-treated dogs, which may be clinically important for a client.
All [studies] were blinded randomised controlled trials, which provide a high level of evidence; however, only one study compared both treatments directly
Langlois et al. (2020) reported that metronidazole treatment had a significant impact on time to resolution of diarrhoea compared to a placebo, shortening the course of diarrhoea by 1.5 days. However, this study used an unvalidated faecal scoring system and a small sample size, and participants’ diets were not controlled.
Three studies explored probiotic treatment versus placebo. Herstad et al. (2010) found that probiotic treatment was a significant factor for time to diarrhoea resolution versus placebo; however, no significant improvement was found in the probiotic-treated group when the dogs presented with both vomiting and diarrhoea. Limitations were that faecal consistency was based on client opinion, clients were not provided with a standard faecal scoring system and parasitic or infectious causes of diarrhoea were not eliminated.
The study by Nixon et al. (2019) found that probiotic treatment was significant in decreasing the time to resolution of diarrhoea as measured by the passage of three consecutive normal stools. However, while the data is significantly different, the result may not be clinically different (only 0.5 days) for the owner. Additionally, no diagnostic testing was performed to rule out infectious causes of diarrhoea.
Kelley et al. (2009) also found that treatment with probiotics significantly reduced the time of resolution of diarrhoea by 2.5 days
Kelley et al. (2009) also found that treatment with probiotics significantly reduced the time of resolution of diarrhoea by 2.5 days, which may be significant to many owners; however, this was only found for certain dog breeds. The population of dogs used in this study may not be representative of the general population of dogs. But the study does remove the confounding differences in environment and owner compliance as seen in the other studies.
The five studies provided weak evidence for the efficacy of metronidazole compared to placebo for the treatment of acute uncomplicated diarrhoea. Further, previous studies (Pilla et al., 2020; Igarashi et al., 2014) found that oral metronidazole treatment for cases of acute diarrhoea led to changes in the gastrointestinal microbiome composition, which persisted for up to one month post-antibacterial treatment in some dogs. This was particularly evident in the bacteria responsible for bile acid metabolism. In contrast, probiotic use generally appears to be safe in veterinary species.
When evaluating the use of probiotics/synbiotic products as a treatment to reduce the duration of acute uncomplicated diarrhoea, the evidence remains weak. Shmalberg et al. (2019) showed no significant difference between probiotics and metronidazole in the timing of resolution of diarrhoea. Furthermore, while Herstad et al. (2010) and Nixon et al. (2019) showed earlier time to clinical resolution of diarrhoea, they did not show a significant advantage when compared to placebo. Only Kelley et al. (2009) showed a significant reduction in days to resolution of diarrhoea, but only in certain dog breeds.
The use of oral metronidazole will not decrease time to resolution of clinical signs in cases of acute uncomplicated diarrhoea in dogs when compared to probiotic administration
Given the clinical scenario of an otherwise healthy dog presenting with acute and uncomplicated diarrhoea, prescribing probiotic/synbiotic products for owners expecting medication administration would be a reasonable choice. This is because the risk associated with these products is low and no long-term effects have been reported or studied.
Based on the evidence evaluated, the use of oral metronidazole will not decrease time to resolution of clinical signs in cases of acute uncomplicated diarrhoea in dogs when compared to probiotic administration. Thus it should not be a first-line treatment in such cases. Further research is needed directly comparing the use of metronidazole and a probiotic and the duration of clinical signs in cases of acute uncomplicated diarrhoea.