1. Giardia Infection - In the past, metronidazole was commonly used to treat infections caused by the protozoan parasites Giardia and Trichomonas, which are known to cause diarrhea in both cats and dogs. Over time, however, both of these organisms have developed metronidazole resistance, so that this medication by itself is no longer a sufficient treatment for these protozoal infections.
2. Inflamatory Bowel Disease (IBD) - Because metronidazole has a positive effect on Crohn’s disease in humans, it is often used for diarrhea in dogs with IBD and other chronic diseases of the intestines, usually in combination with the steroid prednisone. However, researchers established in 2010 that metronidazole doesn’t actually add any benefit in this scenario: prednisone plus metronidazole is no more effective for IBD than prednisone by itself.

3. Acute Diarrhea - Because of its historical effectiveness against such diarrhea-inducing agents as Giardia and C. diff, metronidazole has been increasingly used by veterinarians to manage diarrhea due to other causes. But there is little evidence that it actually helps acute nonspecific diarrhea (diarrhea with an undetermined cause)—a category that represents the majority of diarrhea cases in both cats and dogs. In the veterinary world, there is some evidence that metronidazole reduces the time it takes for acute diarrhea to resolve. However, a study in dogs found that this reduction amounted to only a couple of days. And as the authors pointed out, most cases of diarrhea in dogs resolve in a few days “regardless of treatment.” Another study that looked at treating acute diarrhea in dogs found no significant difference between metronidazole and a placebo. The authors concluded that the use of metronidazole for such cases “should be discouraged until evidence-based data demonstrate a difference in treatment outcome.”

A Gut Health Test can tell you a lot about what’s going on in your dog’s digestive system and even how to correct certain problems. By identifying all the different kinds of bacteria in your dog’s gut, microbiome testing can determine whether those different bacterial populations are present in balanced amounts when compared to the gut bacteria of healthy dogs. Those correct proportions are the key to a thriving, well-balanced microbiome.

Bacteria belonging to the genus Escherichia (a group that includes E. coli), for example, are beneficial members of the community when they make up less than 5% of a dog’s gut microbiome. But an overgrowth of Escherichia can cause diarrhea and other digestive issues. Bacteriophages (viruses that infect and kill certain strains of bacteria) that specifically target pathogenic strains of E. coli (such as PreforPro) can help to bring the digestive tract into better balance. The journal Antibiotics recently reported that such bacteriophages provide an effective alternative to antibiotic use in medical practice.

The Fusobacterium genus is also present in most healthy dogs. When these bacteria are missing, digestion can suffer, but high levels of Fusobacterium are also associated with digestive issues, particularly diarrhea, so moderation is key. Bacteria in the Fusobacterium family do best in protein-rich environments, so if your dog’s Gut Health Test shows a deficiency in this kind of bacteria, increasing the protein content of their diet may help. If the test shows that Fusobacterium levels are high, the addition of a small amount of prebiotic fiber (like inulin or psyllium husk) to the diet can help bring these levels down by supporting bacteria in the gut that make molecules that help fight inflammation.

A Gut Health Test can even offer clues about the underlying reason for a dog’s diarrhea. Low levels of Coprococcus and Oscillospira bacteria in the gut, for example, are often observed in dogs with food allergies. Deficiencies in Faecalibacterium have been associated with IBD in dogs.