Zuclopenthixol decanoate toxicity in an 8-month-old colt causing
extrapyramidal neurological signs.
Abstract
Zuclopenthixol 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 with reported use
in horses to provide 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. Initial suspicion was of fluphenazine decanoate toxicity,
therefore treatment as reported in the literature for this syndrome was
commenced. The limited available literature regarding treatment options
includes use of intravenous formulations of diphenhydramine
hydrochloride and benztropine mesylate, which were unavailable without
an import licence in the UK at the time of publication. The closest
alternative option, oral diphenhydramine (Nytol) 1mg/kg was administered
alongside intravenous atropine (0.01mg/kg) and pergolide (2mcg/kg/q 24
hours). Improvements in the colt’s demeanour and neurological status was
noted within 24 hours. Ongoing supportive treatment was continued and
slowly tapered in line with clinical improvement. 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 of the use of zuclopenthixol
decanoate was raised, and this 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 translocation of wildlife to relieve
stress and aid acclimatisation. To our knowledge there is no available
literature describing administration of zuclopenthixol decanoate in
equine patients.