ACTH suppression and adverse effects of cabergoline in horses with pituitary pars intermedia dysfunction

Summary Background The diagnosis and management of pituitary pars intermedia dysfunction (PPID) in horses includes evaluating abnormal plasma concentrations of adrenocorticotrophic hormone (ACTH). Treatment commonly includes the oral dopamine agonist pergolide mesylate, which suppresses the pathologic overproduction of ACTH. Cabergoline is a commercially available long‐acting injectable dopamine agonist that may provide an alternative means of treating horses with PPID by producing similar effects on plasma ACTH concentrations. Objectives To evaluate the effects of intramuscular cabergoline administration on plasma ACTH concentrations in horses with PPID. Study design Randomised controlled trial. Methods Twelve adult horses with PPID were randomly divided into treatment and control groups. The treatment group was administered a 0.01 mg/kg bwt dose of intramuscular cabergoline twice, 2 weeks apart. Control horses received an intramuscular injection of sterile saline at the same time points. Horses were monitored for adverse effects. Blood samples were collected for endogenous ACTH concentration monitoring at serial time points and compared using a linear mixed regression model. Results Horses in the treatment group had a reduction in plasma ACTH concentration from a mean of 153.35 pg/mL to 24.117 pg/mL following the initial administration of cabergoline. Their ACTH levels remained within the seasonally appropriate range until 12 days after the second cabergoline injection. Horses in the control group never achieved normal ACTH concentrations. The treated horses showed appetite suppression immediately following cabergoline administration, but no significant weight loss when compared to the control horses. Main limitations The study group was small, and data collection overlapped with the natural autumnal rise in ACTH, which complicated interpretation of the results. Conclusions Injectable cabergoline appeared effective in reducing plasma ACTH concentrations in the PPID horses to seasonally appropriate ranges. There appeared to be a transient reduction in appetite following cabergoline injection, but not a significant loss of weight in comparison with control horses.