Effects of Behavior-Modifying Drug Investigated (AAEP 2011)

"If only he'd stand still and keep quiet!" Many situations faced by horse owners and trainers would be far easier to manage if a temperamental horse would do this, and it might be tempting to initiate long-term sedation when confinement, stall rest, and tractability are necessary. But one veterinarian explained that sometimes the drawbacks of a commonly used human drug preclude its use in horses.

At the 2011 American Association of Equine Practitioners convention, held Nov. 18-22 in San Antonio, Texas , John Baird, BVSc, PhD of the University of Guelph described the side effects that might occur following use of a long-acting antipsychotic drug, fluphenazine decanoate,

This drug is used to treat humans with schizophrenia and is considered a performance enhancer by racing jurisdictions and show organizations. It's been known to cause a variety of adverse side effects, with one of the most notable adverse effects, Baird said, being dystonia, a condition characterized by abnormal involuntary muscle movements. Fluphenazine binds to dopamine receptors located in the extra-pyramidal system (part of the nervous system that controls movements), resulting in a blockage of the brain's dopamine pathways. This results in sustained muscle contractions which presents as peculiar postures and twisting of body parts. The drug also can elicit repetitive patterned movements, particularly of the neck, back, face, and tongue. Affected horses might also display akathisia--restlessness and a need to move, particularly the legs, coupled with anxiety or agitation.

Baird stressed that fluphenazine is highly potent and able to cross the blood-brain barrier to accumulate within the brain. When given to a pregnant mare, it is also able to enter the fetal circulation and milk. Horses that experience adverse side effects display the abnormal postures along with severe depression almost to the point of somnolence (drowsiness), as well as repetitive motions that at times could be accompanied by dangerous behavior. Many of these odd behaviors can be confused with other serious medical conditions, such as colic, rabies, equine herpesvirus myeloencephalopathy, West Nile virus encephalitis, or moldy corn poisoning. Baird remarked that there is no association between the severity of adverse signs and the plasma concentration of fluphenazine.

Videos of case studies helped the audience visualize many abnormal side effects, such as rhythmic head tossing, pawing, sweating, or circling. Some of the cases he presented had received multiple doses of fluphenazine, while others had received only a single dose. While side effects might become apparent within 18-24 hours of injection, it's possible that they won't appear until weeks later.

To counteract these adverse physical and behavioral changes, veterinarians administer the antihistamine diphenhydrazine and/or the Parkinson's drug benztropine mesylate. In severe cases, horses might need a barbiturate (pentobarbital) so they don't injure themselves.

Fluphenazine is a forbidden substance for horses competing in United States Equestrian Federation events, and is banned in Fédération Equestrian International competitions as well. There is at least a 90-day withdrawal time (meaning that a horse might still test positive up to 90 days after administration). Besides such restrictions, veterinarians and owners should be aware of the listed potential adverse effects when considering use of fluphenazine for behavior control.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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