Current Concepts in Antimicrobial Therapy for Horses

Antibiotic therapy for horses has always been challenging because there are many drugs that are not practical to administer to horses; they may not absorb some oral medications; and many drugs are very expensive because of the horse’s size. Horses are also prone to adverse reactions that limit the use of some drugs, for example, oral lincosamides may disrupt the intestinal bacteria and cause enteritis, aminoglycosides can injure the kidneys, and fluoroquinolones should not be administered to young horses because of a risk of injury to the developing articular cartilage. Despite these drawbacks, it is essential that horses with serious infections receive appropriate therapy to prevent a chronic or lifethreatening condition. Drug-resistant bacterial infections are an emerging problem and the use of highly active drugs has become more important than ever before. Yet, the rational use of these drugs is essential to prevent the development of further resistance. Foals, especially, need highly active drugs because they may be immunocompromised at the time of treatment. Drug treatment for foals produces additional challenges because of differences in drug disposition in foals vs. adults. An indepth discussion of these differences would be the subject of a separate paper and can be reviewed elsewhere. But, certain differences in oral absorption, volumes of distribution, metabolism, and clearance between foals and adults must be considered when selecting antibacterial dosage regimens for foals. In order to assist veterinarians in prescribing effective antibiotics to their equine patients, there have been several advances that have provided veterinary medicine with effective drugs and the tools (specifically, pharmacokinetic and pharmacodynamic information) to guide dosing. The most appropriate drug selection has been facilitated by new approaches to bacterial identification and susceptibility testing. This article will review some of the concepts that guide antibiotic therapy for equine patients and provide important strategies for effective dosing.