The jury’s still out on metronidazole, an antibiotic that’s long been the extralabel go-to option for veterinarians faced with dogs suffering from acute diarrhea.

The evidence supporting metronidazole’s efficacy is empiric rather than science based. It’s unclear precisely how it stops diarrhea in dogs, but prescribing it has become a bit of a case of, “Well, it couldn’t hurt.” Especially when veterinarians are faced with a desperate dog owner whose 70-pound retriever has been experiencing unexplained, nonstop bouts of diarrhea for three days straight.

But it might hurt.

The proliferation of antimicrobial-resistant micro-organisms—pathogens that develop the ability to defeat the drugs designed to kill them—is one of the most serious global problems facing both human and animal healthcare. And with leading veterinary organizations calling for increased awareness of antimicrobial stewardship (the AVMA released their report, “Antimicrobial-Resistant Pathogens Affecting Animal Health in the US,” last October) some veterinarians are beginning to question the wisdom of automatically prescribing an antibiotic that doesn’t have the benefit of proven efficacy.

Particularly since research shows that when it’s prescribed for dogs with acute diarrhea, it may only reduce the length of illness by a day or so.

That was the conclusion of a 2019 study by researchers at Michigan State University (MSU).

NEWStat reached out to corresponding author Daniel Langlois, DVM, DACVIM, assistant professor of small-animal clinical sciences at MSU’s College of Veterinary Medicine, to ask about the findings.

Langlois said two things really jumped out at him: “First, nearly all dogs [in the study] got better within a few days, regardless of treatment. Second, there did appear to be a treatment benefit to using metronidazole in the dogs in our study population.”

That presents a bit of a quandary.

Calling the rise in antibiotic-resistant strains of pathogens “a very valid concern,” Langlois isn’t a big fan of continuing to treat acute nonspecific diarrhea in dogs with metronidazole, even if his findings indicate some benefit. “Just because you can, doesn’t mean that you should,” he said.

But he did point out that the metronidazole only improved the outcome by 1.5 days over the dogs who received the placebo. The real question is, how significant are those 1.5 days?

“As a veterinarian, a difference of 1.5 days doesn’t seem like a big deal to me,” he said. “But the owner of a dog with severe acute diarrhea who’s having accidents on the carpet and nice furniture would probably have a different perspective.”

Brennen McKenzie, VMD, MA, director of veterinary medicine at AAHA-accredited Adobe Animal Hospital in Los Altos, California, and past-president and director of outreach and development for the Evidence-Based Veterinary Medicine Association, concedes that the MSU study does show some benefit of metronidazole over the placebo in shortening the duration of acute idiopathic diarrhea. But he said those findings conflict with the findings of a similar 2019 study at the University of Florida. That study tested metronidazole against a placebo (and a probiotic), but no benefit was found for using metronidazole compared to either the placebo or the probiotic.

McKenzie said the reason for the different findings is unclear: “The dogs in the MSU study had much shorter durations of diarrhea than those in the Florida study.” And there may well have been differences in the populations, such as the causes of diarrhea or the health status of the dogs. “With such mixed evidence, the best we can say is that metronidazole might have some benefit, but this is uncertain.”

He agrees with Langlois that a difference of 1.5 days would be clinically significant to owners, though he also points out that acute idiopathic diarrhea is almost always self-limiting. “Whether the convenience of a shorter course of diarrhea justifies the potential risks of using metronidazole (e.g., long-term changes in gastrointestinal flora, possible induction of antibiotic resistance) is a judgment call.”

For McKenzie, his choice comes back to the short-lived nature of the ailment: “Given that acute diarrhea is almost always self-limiting, parasite screening and dietary therapy without a drug treatment seem to be the most reasonable therapy.” But he also understands that this course of treatment is more difficult for veterinarians to justify to clients than simply giving them a pill. “Resisting the pressure to prescribe—especially when the potential harms seem indirect and uncertain—is naturally difficult for clinicians.” 

So, it kind of boils down to this: Is the antimicrobial-resistance risk worth the extra 1.5 days you save? That desperate owner of the 70-pound retriever with the runaway bowels would likely say yes. Especially if they just put in a new carpet.

And if you’re the veterinarian to whom they’re desperately turning for help, you might understandably agree.

But at what cost?

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