A normal upper respiratory tract (URT) contains commensal organisms that help fend off pathological microbes and fungi. Overgrowth of multiple pathogens is possible when a horse undergoes stress, transport, immunosuppression or cancer. Systemic use of antimicrobials creates more opportunity for development of microbial resistance. With this in mind, a topical treatment for URT infection could be invaluable to return respiratory health to normal, particularly as an adjunctive, complementary treatment to systemic therapy.
In an effort to identify a potential treatment against pathogens that would not injure respiratory epithelium or deeper tissues, a study at Colorado State University evaluated serial dilutions of potentiated chlorhexidine (CHD-EDTA-Tris) and EDTA-Tris alone in vitro against specific bacteria and fungi. Once it was identified which solution worked the best, eight healthy horses were enrolled in the study [Stewart, H.L.; Engiles, J.B.; Richardson, D.W.; Levine, D.G. The clinical and histopathologic effects of potentiated chlorhexidine in the upper respiratory tract of horses. Veterinary Surgery Apr 2021; doi: 10.1111/vsu.13649].
Endoscopy of the pharynx, larynx and both guttural pouches was done prior to treatment and at 24 and 48 hours following the first treatment. Two treatments were done—at the first evaluation and 24 hours later—consisting of topical spray of 30 ml of 0.0005% potentiated CHD split into equal amounts to target the pharynx, larynx and guttural pouches.
All horses tolerated the procedures and treatments without complication. The two treatments did not elicit gross evidence of mucosal inflammation, mucus accumulation or tissue thickening. Histological examination of URT tissue also did not evidence damaging pathology following treatment in most of the horses and tissue samples. While this dilute CHD solution was fairly effective with 90% kill of S. equi ssp. equi, it had less bacterial kill on S. aureus and S. equi ssp. zooepidemicus.
At higher concentrations, i.e., 0.005%, CHD has 100% bacterial and fungal kill in vitro. Components of the solution are not only available but are inexpensive.
The authors said in summary: “A 0.0005% CHD solution was safe for repeated short-term application in the equine upper respiratory tract based on clinical, endoscopic and histologic evaluations.”