Prevalence of and potential impact on fertility of pars pituitary intermedia dysfunction in a Thoroughbred broodmare population in England

Prevalence of pars pituitary intermedia dysfunction (PPID) has been reported in general equine population as 21.2% in horses and ponies aged > 15 years. 1 Aim was to establish the prevalence of PPID in a Thoroughbred broodmare population using seasonally adjusted cutoff values for basal plasma adrenocorticotrophic hormone (ACTH) concentrations and analyze the potential impact on fertility. Venous blood samples were collected from 79 Thoroughbred broodmares  15 years old (average 18 years) on first January 2019. Samples were analyzed for plasma ACTH concentrations using Immulite 1000 assay. Mares with plasma ACTH concentrations above the seasonally adjusted reference range for nonautumn months (> 29.7 pg/ml) were allocated into a followup group and underwent repeat sampling in 2019 autumn. Prevalence of PPID in January 2019 was 16.4% (13/79), with mean plasma ACTH concentrations of 38.0 pg/ml (30.2 - 61.4) in affected mares. On repeat sampling, 23% (3/13) mares remained positive (> 47 pg/ml; autumn reference range), whereas 46% (6/13) had a reduction in basal plasma ACTH concentration. Furthermore, 7% (1/13) had small increases in plasma ACTH that was still within normal limits for autumn, whereas 23% (3/13) were lost to followup. Mean pregnancy rate per cycle for 13 mares in the followup group was 52.4%. Five mares were not bred in the 2019 breeding season. Two pregnancies were lost between 15 and 28 days. One mare was lost to followup. Initial prevalence of PPID within this population was comparable to previous studies; however, followup data in autumn had a decrease. Prevalence of PPID in this study population was lower than general equine population, with elevated ACTH concentrations having no impact on fertility. Affected mares had consistent estrous cycles and were able to conceive and maintain pregnancy to 28 days at a pregnancy rate comparable to previous reports for aged mares. Furthermore, these results also highlighted the requirement for repeat testing and need for further investigation into use of single basal plasma ACTH concentrations as a conclusive test for PPID.

Keywords: Equine, PPID, ACTH, fertility, prevalence

References

1.Ireland JL, McGowan CM: Epidemiology of pituitary pars intermedia dysfunction: A systemic literature review of clinical presentation, disease prevalence and risk factors. Vet J 2018;235:22-33.

This manuscript was originally published in the journal Clinical Theriogenology Vol 12(3) Sept 2020.  Clinical Theriogenology is the official journal of the Society for Theriogenology (SFT) and the American College of Theriogenologists (ACT).  This content has been reproduced on the IVIS website with the explicit permission of the SFT/ACT.