Metronidazole: Required Treatment for Hemorrhagic Diarrhea?


A recent study suggests that amoxicillin with clavulanic acid is just as effective as combination therapy with metronidazole.

Amoxicillin, often with clavulanic acid, is a frequently prescribed antibiotic for bacterial diarrhea in canine patients due to its broad-spectrum coverage. Metronidazole is also administered commonly; however, potential risks associated with its use in small animals include toxicity, bacterial translocation, and development of drug-resistant Escherichia coli.

In a recent study published in Journal of Small Animal Practice, investigators from the United Kingdom compared the efficacy of amoxicillin—clavulanic acid therapy with and without the addition of metronidazole for treatment of acute hemorrhagic diarrhea.


Enrolled dogs included client-owned pets hospitalized for acute, profuse, bloody diarrhea lasting fewer than 3 days. Cases were excluded if they had concurrent renal or liver disease or had received antibiotics within the previous 3 days. On admission, dogs were assigned randomly to 1 of 2 treatment categories, to which attending clinicians and nurses were blinded.

All dogs received the following:

  • Intravenous (IV) fluid therapy
  • 8.75 mg/kg amoxicillin—clavulanic acid every 8 hours
  • Buprenorphine as needed for analgesia
  • Daily omeprazole for gastroprotection
  • Daily maropitant as needed for nausea or vomiting

Dogs in the treated group also received 10 mg/kg IV metronidazole every 12 hours, while those in the placebo group received an equivalent volume of saline.

Treatment efficacy was assessed by 2 means. First, duration of hospitalization was measured from the time of admission to when the patient was approved for discharge. Second, a clinical scoring system was used to evaluate patient progress. In this method, a blinded observer retrospectively examined patient hospitalization charts to assign daily scores for appetite, vomiting, stool frequency, stool consistency, and administered rate of IV fluid therapy.


Thirty-four dogs that were hospitalized for acute hemorrhagic diarrhea completed the trial over a 24-month period. Participating dogs ranged from 4 months to over 13 years in age, and a wide range of weights and breeds were represented. Seventy percent of dogs were male. Dehydration was recorded in all dogs, but none were pyrexic.

Twenty of 34 dogs were assigned randomly to the treatment group receiving metronidazole in addition to amoxicillin—clavulanic acid and supportive care. No side effects from metronidazole were noted in any dogs.


  • Immune-Stimulating Properties of Antibiotics
  • Multidrug-resistant Infections in Veterinary Clinics

Duration of hospitalization was similar for dogs in both treatment groups. Those receiving the standard supportive treatment and amoxicillin—clavulanic acid had a mean hospital stay of 26.3 hours, while those administered standard treatment plus metronidazole were hospitalized for a mean of 29.6 hours. Likewise, daily clinical scores were similar in the 2 groups, and both groups generally showed significant clinical improvement after 24 hours of hospitalization. A single dog in each treatment group had recurring clinical signs after discharge that necessitated continued treatment for diarrhea.


The authors suggested that metronidazole is not a necessary component of treatment for bacterial gastroenteritis in dogs. They also recommended limited use of metronidazole in general, due to increasing evidence of metronidazole resistance in bacteria isolated from humans and animals, including Clostridium.

Dr. Stilwell received her DVM from Auburn University, followed by a MS in fisheries and aquatic sciences and a PhD in veterinary medical sciences from the University of Florida. She provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting.

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