Veterinarians commonly use a broad-spectrum antimicrobial agent called potentiated sulfonamide to manage horses’ respiratory infections. While effective, its downsides are that it must be administered twice daily and, as with any antimicrobial drug, it has the potential to disrupt favorable bacteria levels in the colon, leading to diarrhea. With this in mind, researchers explored a new formulation of trimethoprim sulfadiazine (TMS, a type of potentiated sulfa) that the small intestine absorbs more readily than other drugs in that class–so that, theoretically, less reaches the colon–to determine its safety and efficacy when given at a reduced dose (24 mg/kg every 12 hours, as opposed to the currently recommended 30 mg/kg).

Scott McClure, DVM, PhD, Dipl. ACVS, associate professor of equine surgery at Iowa State University’s School of Veterinary Medicine, in Ames, presented the study findings at the 2014 American Association of Equine Practitioners Convention, held Dec. 6-10 in Salt Lake City, Utah.

In their safety study, they looked at effects of the standard 24 mg/kg dose, along with doses three and five times that amount. They also administered these doses for 30 days rather than the normally prescribed 10-day course to check for development of diarrhea or other adverse effects. The objective of this study was to see if a dose up to 5 times the recommended one administered at 3 times normal administration period resulted in any negative effects, McClure said. He noted that the team did not see any significant problems.

Once the safety was establish, McClure and colleagues treated horses with lower respiratory disease caused b