Zuclopenthixol decanoate toxicity in an 8‐month‐old colt causing extrapyramidal neurological signs

Abstract

SummaryZuclopenthixol decanoate is a dopamine antagonist licensed for human use to manage schizophrenia and paranoid psychosis. It is in the same drug class (thioxanthene) as fluphenazine decanoate, a drug whose use in horses is reported anecdotally for providing long‐acting sedation. This case study describes severe extrapyramidal signs seen in a Thoroughbred colt after zuclopenthixol decanoate administration. The colt was presented to a referral hospital for an unusual manifestation of colic signs. On admission, the colt began to show clear extrapyramidal neurological signs. The initial suspicion was of fluphenazine decanoate toxicity, which in the limited published literature is treated with intravenous formulations of diphenhydramine hydrochloride and benztropine mesylate. At the time of writing both were unavailable in the United Kingdom without an import license. The closest alternative option, oral diphenhydramine was administered alongside intravenous atropine and pergolide. Improvements in the colt's demeanour and neurological status was noted within 24 h. The colt was hospitalised for a total of 26 days and at time of discharge was clinically normal. Follow‐up communication 3 months after discharge confirmed that the colt had remained neurologically normal. Crucially, toxicology testing submitted to Cornell University was negative for common toxins, including fluphenazine decanoate. Specific secondary testing was performed after suspicion the administration of zuclopenthixol decanoate was raised, which was positive for this compound. Routine drug testing would, therefore, not identify the use of this behaviour modifying drug. Clinical data regarding the use of zuclopenthixol in animals is limited to its use in the translocation of wildlife to relieve stress and aid acclimatisation. To our knowledge, there is no available literature describing the administration of zuclopenthixol decanoate in equine patients.