Horse of a different color : Peculiarities of equine pharmacology
Oklahoma State University, Stillwater, OK, USA
Horses are different. Practitioners expect such differences, given the unique anatomy and physiology of the many species encountered within veterinary medicine. Nonetheless, the appropriate use of therapeutic agents can be particularly challenging in the horse. Small animal practitioners have an advantage over other veterinary clinicians when novel therapeutics are investigated for veterinary use, as the pharmacology of investigational drugs is often described in dogs before they are used in people. However, if the dog is the prototypical species of basic pharmacological research, then the horse is just the opposite. The many idiosyncrasies of equine anatomy and physiology can make the behavior of drugs in this species highly unpredictable. Drugs that are well absorbed after oral administration, safe, and efficacious in other species may be poorly absorbed, toxic, or ineffective in horses. This chapter will investigate the sources of some of these pharmacological peculiarities and address strategies for using drugs safely and effectively in horses despite these inherent challenges.
Idiosyncrasies related to route of administration
Administration of drugs per os is the most natural route in companion animals. Administering foreign material to the gastrointestinal tract (GIT) is safer than parenteral administration, since the immunological and physical defenses in the GIT are designed to cope with all manner of foreign substances. Particulates, excipients, bacteria, fungi, endotoxins, and other toxins can be devastating if inadvertently administered parenterally, but most of these contaminants will have little impact if inadvertently delivered to the GIT. The cost of a drug formulated for oral administration is often less than its injectable counterpart, because of the stringent requirements needed to prepare safe injectable formulations. In addition, owner and patient compliance is usually better when oral drug administration is employed, especially when the duration of therapy encompasses days, weeks, or even for the remaining life of the patient. For all of these reasons, the oral route of administration is generally preferred in the nonhospitalized equine patient. In counterpoint to all of the advantages of oral drug administration, there are certainly disadvantages that the equine practitioner will recognize. First, getting the drug into the stomach of the horse can be challenging. Some small capsules and powders can be administered as a topdressing to highly palatable feed. For horses that don't avoid top-dressed drugs and drugs that are palatable, this method of administration probably represents the most effective and convenient method of oral administration. Capsules, tablets, and powders may be hidden in pieces of carrot or apple with space carved or drilled by drug placement. Capsules and powders can also be mixed with applesauce, molasses, or syrup and mixed with feed, bran, or beet pulp. These methods can all be effective but require that the feed bucket be observed after administration to confirm that the entire dose was ingested. For unpalatable drugs and horses that avoid top-dressings, a powder can be mixed with applesauce, syrup, or molasses and placed in a syringe. A catheter-tipped syringe can be used for thinner mixtures, or the tip of the syringe can be removed and surfaces sanded smooth for the administration of thicker mixtures. Tablets can be crushed with a mortar and pestle, coffee grinder, or hammer to make a powder for administration. Some tablets and capsules can also be dissolved in a small volume of water. However, not all drugs are soluble and stable in water, so knowledge of the drug's aqueous stability