As horses age more gracefully in this era of excellent medical and surgical care, it is common to see the older horse affected by pituitary pars intermedia dysfunction (PPID). Equine practitioners commonly treat confirmed cases of PPID with pergolide (Prascend) at a typical dose of 0.5 mg/day. Dosage is adjusted relative to each individual response and follow-up ACTH lab data.
Some horse owners report unwanted side effects of medicating their horses with pergolide. These side effects include decreased appetite, anorexia, mild central nervous symptoms (such as lethargy and ataxia), and rare cases (one to 10 of 10,000 patients) of diarrhea and colic. The higher the dose administered, the greater the likelihood of side effects. There are reports of sweating, dyspnea, dry mucous membranes, dropping blood pressure and fainting, an irregular heart rhythm and agitation when a horse is initially given a high dose.
Because pergolide is used commonly in equine practice, there is value in knowing what to expect when an owner inadvertently overdoses a horse and how to manage such a case.
A case report described expected problems from an overdose in a pony mare as well as management [Schwarz, B.; Ihry, P. Accidental overdose of pergolide (Prascend) followed by loss of appetite, tachycardia and behavioral abnormalities in a 26-year-old pony mare. Journal of Equine Veterinary Science June 2020; https://doi.org/10.1016/j.jevs.2020.103181].
Pergolide is absorbed rapidly within two to three hours of ingestion. The pony accidentally received 110 times its prescribed dose (0.5 mg) of pergolide—55 x 1 mg tablets—approximately four hours prior to a veterinary emergency exam. The pony presented with tachycardia with a regular rhythm. Her appetite was diminished, and she suffered from some anxiety and hyper-reactivity to bright light and fast movements due to dopamine effects on the central nervous system. No other abnormal clinical signs were observed.
Besides nasogastric intubation with mineral oil and activated charcoal to limit further systemic absorption of pergolide, the pony was treated with several other medications, each with a short duration of action: a) 2.2 mg/kg IM azaperone as a dopamine antagonist, followed by four more reduced doses every six hours IM; and b) oral verapamil (0.22 mg/kg) to prevent arrhythmias, given every four hours for two days, then every six hours for five more days.
Prevention of ventricular arrhythmia can also be treated with magnesium sulfate or lidocaine infusions. Metoclopramide is an alternative useful anti-dopaminergic drug, but it is best used in conjunction with ECG monitoring and blood pressure measurements.
The pony’s heart rate became normal within 24 hours. No side effects were present from either the overdose of pergolide or the treatment medications. Medical therapy was discontinued after two days for azaperone and five days for verapamil.
Safety studies with pergolide indicate that up to a tenfold overdose is not associated with major side effects.
