Pharmacologic Treatment of Equine Self-Mutilation Syndrome

The effects of drugs that either stimulate or inhibit central opioid, dopamine, norepinephrine, and serotonin neurotransmitter systems were examined in horses demonstrating signs of equine self-mutilation syndrome (ESMS), a condition similar to Tourette’s syndrome in humans. Eight flankbiting horses with ESMS were recruited for the study. A series of drugs selected for their activity on the aforementioned neurotransmitter systems were administered to the horses in a saline-controlled behavioral study. Specific behaviors associated with the syndrome were videotaped for 4 hours following administration of drug or saline. Behaviors recorded hourly during each phase of the study were compared with those of a composite saline control baseline to determine whether there were significant differences among the treatments. Acepromazine, a dopamine blocker, produced a significant reduction in the primary ESMS behaviors of self-mutilative attempts and hemiballismus. Detomidine, an α-2 antagonist, also produced a significant (P < .05) reduction in these behaviors, as did the Vol. 2, No. 2, 2004 • Intern J Appl Res Vet Med 90 TS. Opioids may have dual modulatory effects on tic expression, although low doses of opioid antagonists often reduce tic frequency. Dopamine pathways are also involved in the pathogenesis of TS, as evidenced by the palliative effects of both dopamine antagonists (e.g., haloperidol) and agonists (e.g., apomorphine and pergolide). Additionally,norepinephrine appears to be to be involved in the pathobiology of TS because norepinephrine levels in cerebrospinal fluid are 55% higher in patients with TS than in healthy controls, and clonidine suppresses TS tics. Finally, the fact that selective serotonin reuptake inhibitors are drugs of choice in TS suggests the involvement of serotonergic mechanisms. The only evidence of a role for any of these neurotransmitter systems in ESMS is from a clinical survey in which two horses with ESMS reportedly responded to therapy with a dopamine antagonist and a case report describing the suppression of ESMS tics with nalmefene, an opioid antagonist. The present study was conducted to investigate neurobiologic similarities between ESMS and TS. The effects of drugs that either stimulate or inhibit central opioid, dopamine, noradrenergic, and serotonin pathways on ESMS were studied. Individual drugs were selected based on familiarity with their use in horses and their principal mechanism of action. MATERIALS AND METHODS Horses Eight horses identified as flank-biters were admitted to the Large Animal Hospital of Tufts University School of Veterinary Medicine for clinical evaluation and possible enrollment in a behavioral study approved by the Institutional Animal Care and Use Committee (Table 1). A signed owner consent was obtained for each horse participating in the study. Each horse was given a thorough physical evaluation, including a general medical examination, gastric endoscopy, retinal examination, reproductive assessment (testicular palpation and measurement or ovarian and uterine examination by rectal palpation), and dermatologic examination (by direct visual inspection and microscopic examination of skin scrapings) to eliminate possible medical causes for the abnormal behavior. Additionally, a blood sample was collected by venipuncture from each horse for a complete blood count and chemistry profile analysis. Finally, at least 48 hours after admission to the clinic, horses were observed and then videotaped for 4 hours to confirm that each was exhibiting behavior characteristic of ESMS. Two horses (M and MD) had to be blanketed at all times to prevent them from injuring themselves during their intense self-mutilation episodes. One horse (B), a high-intensity cribber, was always equipped with a cribbing collar. ESMS was diagnosed based on the observation of self-mutilative attempts or actual flank-biting; hemiballismus; excessive headtossing; bizarre, unsolicited vocalization; preoccupation with the periphery of the stall; and constant sniffing, especially of manure. Sudden explosive bouts of self mutilation or flank-biting associated with any of other characteristic signs (and the absence of any medical explanation for the behavior) was considered diagnostic of ESMS. Treatments Drugs evaluated included detomidine, naltrexone, morphine, acepromazine, cocaine, apomorphine, amphetamine, clomipramine, haloperidol, and buspirone. Dosages, routes of administration, and mechanisms of action Table 1. Identification of Horses Presenting with Equine Self-Mutilation Syndrome Horse ID Sex Breed Age (yr) PST Stallion Arabian 7 M Stallion Arabian 15 DR Gelding Quarter horse 13 RV Stallion Arabian 12