Objective: To determine whether seasonal variation in equine plasma ACTH concentrations is affected by latitude between 29.7°N and 45.0°N in the US.
Methods: A cohort study was performed between June 2019 and May 2020 in which plasma ACTH was measured monthly by chemiluminescence immunoassay. The study involved 201 apparently healthy horses from 9 locations within the US classified into northern, central, and southern latitude cohorts. Data analysis was performed with 3 major objectives: (1) identify outlier ACTH concentrations and horses; (2) estimate ACTH upper reference limits to define expected ACTH concentrations in healthy horses; and (3) identify factors influencing ACTH concentrations using linear mixed-effects regression models.
Results: Following outlier analysis, 154 horses remained from 3 latitude cohorts: northern (n = 29), central (84), and southern (41). Concentrations of ACTH were highest during late August and September and early October for all latitude cohorts. In horses over 15 years of age, ACTH was higher than in younger horses year-round, with a greater magnitude of increase during the seasonal elevation in ACTH.
Conclusions: A seasonal elevation in ACTH occurred in horses throughout the contiguous US, with lower ACTH in horses in the southern compared to central cohort. Further investigation is required to determine the influence of latitude on horses that reside closer to the equator and the role of the changing photoperiod coinciding with the start of seasonal elevation in equine ACTH.
Clinical relevance: Season and age should be considered when interpreting baseline ACTH concentrations in horses throughout the contiguous US.
Keywords: ACTH; adrenocorticotropic hormone; equine Cushing disease; horse; pituitary pars intermedia dysfunction.