Many infertile mares of all ages in a Japanese study had high plasma concentrations of adrenocorticotropic hormone.
High levels of adrenocorticotropic hormone, or ACTH, are linked to equine Cushing’s disease, an endocrine disorder more formally known as pituitary pars intermedia dysfunction (PPID).
In recent years, measurement of plasma ACTH concentrations has been recommended in efforts to diagnosis PPID in horses, although several tests may be required because of seasonal fluctuations.
Takeru Tsuchiya and his colleagues, in a study published in the Journal of Equine Veterinary Internal Medicine, set out to explore the relationship between endogenous plasma ATCH concentrations and reproductive performance in thoroughbred broodmares.
Sixty-seven broodmares with infertility of unknown cause were enrolled in the study. Physical examinations found all horses to be healthy, with no obvious abnormalities in appearance, including hair coat and hooves.
The animals were screened in autumn for PPID using baseline plasma ACTH concentrations, and the relationship of their levels with subsequent reproductive performance was analyzed.
The mares were divided into three groups based on their ACTH concentrations, being classed as either low, intermediate or high, using values recommended by the Equine Endocrinology Group.
The scientists compared the results with those of other blood tests, clinical examinations, subsequent conception, and the frequency of normal delivery.
Conception rates were found to be 77.3% in the low group, falling to just 56.5% in the high group.
“The horses in the high ACTH group did not have significant differences in the number of matings, but horses in the intermediate and high ACTH groups had lower rates of conception,” they wrote.
“The reproductive performance of the high ACTH group was significantly lower than the other groups,” the study team said.
The authors noted that PPID is an age-related problem reported to occur in 15% to 30% of horses over the age of 15. Early diagnosis and treatment may minimize complications.
Little research has been performed describing the relationship between the condition and reproductive performance, conception rates, and delivery rates, they noted.
High ACTH concentrations are common in older horses, and it increases with age, with few reports of high levels occurring in horses aged under 10.
“The horses in our three study groups were not significantly different in average age. Based on our results, almost two-thirds of this population of subfertile broodmares that have high or intermediate ACTH concentrations might have PPID based on high endogenous plasma ACTH concentrations, which represents a much higher percentage than reported in the general equine population.
“The numbers of horses that had suspected PPID were higher than previously reported because we only tested mares that had reproductive disorders (infertility), which is considered a clinical sign of PPID.
“Some individuals incidentally may have had high ACTH concentrations in the high and intermediate groups. Younger horses also might be affected by PPID.
“We showed that many infertile mares of all ages had high plasma ACTH concentrations. Furthermore, high ACTH concentrations may indirectly affect reproductive performance, such as conception and delivery, because of abnormal secretion of various reproductive hormones, including ACTH and cortisol.
“This result suggests that early diagnosis of infertile mares using endogenous plasma ACTH concentration may allow improved reproductive performance.”
The researchers also found that the number of injured vaginal and external uterine ostia (opening) was significantly higher in the high ACTH group than in the low ACTH group.
The authors emphasized that the number of animals used in the study was relatively small and of a single breed. External conditions such as feeding management were not the same. Other factors, including other reproductive hormones, should be investigated in addition to the effects of high ACTH or PPID on reproductive performance.
“It will be necessary to carry out additional studies utilizing large farms with larger numbers of animals, consistent feeding practices, and well-managed breeding programs to further evaluate our findings.”
The study team comprised Takeru Tsuchiya and Fumio Sato, with the veterinary school at Gifu University; and Ryusuke Noda, Hiroki Ikeda and Masaya Maeda, with the Equine Medical Center at the Hidaka Horse Breeders Association.
Relationship between endogenous plasma adrenocorticotropic hormone concentration and reproductive performance in Thoroughbred broodmares
Takeru Tsuchiya, Ryusuke Noda, Hiroki Ikeda, Masaya Maeda, Fumio Sato
Journal of Veterinary Internal Medicine, 24 May 2021, https://doi.org/10.1111/jvim.16145