Trailering stress creates false positive results in diagnostic testing for Pituitary Pars Intermedia Dysfunction in Horses

Abstract

Pituitary Pars Intermedia Dysfunction (PPID) occurs in over 20% of aged horses. The best diagnostic for PPID compares plasma adrenocorticotropic hormone (ACTH) before and after thyrotropic-releasing hormone (TRH) stimulation. Stress affects the pituitary–adrenal axis and may confound PPID diagnosis. This study examined if trailering stress could cause false-positive PPID diagnosis in horses, and if so, what rest time after trailering is needed before TRH-stimulation. Ten horses were hauled 40 min every 4 weeks. Blood was collected before and 0, 15, 30, 60, and 120-min post-trailering, followed by TRH-stimulation. Data were analyzed using a mixed model with repeated measures. Post-trailering ACTH was higher (P = 0.002) than pre-trailering ACTH and falsely positive for PPID in some horses up to 30 min post-trailering, but ACTH after TRH-stimulation was not affected. Trailering stress caused multiple false-positive results, so veterinarians should wait 30 min after trailering to test horses for PPID.