Antibiotic Metronidazole and Hindgut Health in Horses

Gastrointestinal disorders such as colic and colitis have been linked to alterations in the intestinal microbiome. Whether microbial changes cause colitis or microbial changes are a result of colitis remains unclear. When the intestinal microbiome is in disarray, certain consequences may occur:

• Overgrowth of disease-causing bacteria;
• Bacterial production of toxins;
• Inflammation of the intestinal lining; and
• Alterations in the metabolism of carbohydrates and production of volatile fatty acids that horses use for energy.

“A healthy microbiome often equates to a healthy horse. This is why high-quality supplements that stabilize the pH of the hindgut are so important for some horses. For example, EquiShure controls the pH of the cecum and colon, which helps maintain a stable microbiome,” explained David Nash, director of nutrition technology at Kentucky Equine Research.

The antibiotic metronidazole is sometimes used to treat horses with diarrhea, especially those cases of clostridial origin. Other antibiotics reportedly cause diarrhea in horses. Antibiotics used before surgeries to prevent postsurgical infections, for example, can result in antibiotic-associated diarrhea despite their advantages in staving off incisional infections.

A group of veterinary researchers wondered if metronidazole would cause microbial imbalance, even though it is sometimes used to treat diarrhea. To answer this question, researchers recruited five healthy horses and administered a standard dose of metronidazole (15 mg/kg) by mouth twice daily for seven days. Cecal and fecal samples were collected and analyzed during the study to determine the bacterial composition.^*

On the third day of the study, all horses developed inappetence, an early indicator of colitis. The study was therefore suspended; no more metronidazole was administered but the researchers continued to collect cecal and fecal samples.

Analyses of the cecal and fecal samples revealed that metronidazole, like other antibiotics, resulted in altered bacterial communities compared to pre-administration analyses. Changes in pH were also appreciated, as cecal samples were more acidic than normal following administration of metronidazole early in the study period.

“This study highlights the fact that any antibiotic can cause dysbiosis, including metronidazole, even when it is intended to treat diarrhea,” said Nash. “Antibiotic-associated diarrhea in horses reportedly develops three to four days after initiation of therapy. EquiShure may therefore benefit horses when started at the same time as the antibiotic, or even earlier if the owner knows antibiotic therapy is planned, such as prior to a scheduled procedure.”

^*Arnold, C.E., A. Isaiah, R. Pilla, J. Lidbury, J.S. Cloverdale, T.R. Callaway, S.D. Lawhon, J. Steiner, and J.S. Suchodolski. 2020. The cecal and fecal microbiomes and metabolomes of horses before and after metronidazole administration. PLoS One 15(5):e0232905.