It’s no secret that “good” microbes play a vital role in maintaining horses’ proper digestive health—a necessity to keep them happy, healthy, and performing at their best. But less is known about how the medications veterinarians use to treat equine health problems, such as antimicrobials, impact good microbes’ populations. Previous studies on the matter have yielded conflicting results; however, a group of researchers from the University of Guelph, in Ontario, Canada, recently decided to take another look.

“Traditionally, studies investigating microbial communities have used culture-based methods,” explained researcher Marcio Costa, DVM, DVSc, now based at the Universidade Estadual de Londrina, in Brazil. “However, only a minority of species can grow with those methods, as many bacteria require very specific conditions and nutrients to grow. The method we used—DNA sequencing—has been considered a landmark on the characterization of microbial communities, as it detects the bacterial DNA and classifies them by comparison with reference data banks.”

The team randomly assigned 24 healthy mares to one of four groups that received the following treatments:

• Intramuscular (IM) procaine penicillin;
• IM ceftiofur sodium;
• Oral trimethoprim sulfadiazine (TMS); or
• No treatment to serve as a control.

The horses received treatment for five consecutive days. The team collected fecal samples before treatment on Day 1, at the end of treatment on Day 5, and on Days 14 and 30