It’s an unfamiliar look for a horse: a long coat all year, abnormal sweat patches, and unusual body fat distribution. Yet a proportion of aged horses may be exhibiting these signs as part of the changes that occur with a disease called Pituitary Pars Intermedia Dysfunction (PPID). However, of greatest concern is the increased risk of laminitis. (Find out more about laminitis here).
PPID, formerly termed Equine Cushing’s Disease, results from a change in hormone regulation in the pituitary gland, which is situated just below the brain. In healthy equids, the release of some hormones from this gland is controlled through another; dopamine. However, in those with PPID, there is a reduction in the amount of dopamine released. This, in turn, removes the restriction of the hormones from the pituitary gland. One such hormone under this control is adrenocorticotropic hormone, ACTH.
The concentration of ACTH circulating in the blood is often used as a test for the disease. ACTH exceeding a certain level is used to diagnose PPID. Whereas, results below another threshold are normal. However, there is an intermediary “grey zone” and some important caveats.
The circulating concentration of this hormone is seasonal, peaking in the Autumn. It is also known to vary with stress, exercise and nutritional state (fasted or fed). When the results lie in the grey zone (between the maximum normal and the minimum disease level), a modified test can be used. This involves measuring the ACTH level before and after administering thyrotropin-releasing hormone (TRH). TRH stimulates the release of hormones from the pituitary gland for all horses. However, the subsequent increase in the circulating ACTH is much greater for those with PPID, even in the early stages of the disease, than healthy animals.
Impact of diet
A recently published study investigated the impact of different factors on the level of ACTH. Sixteen horses formed two age groups; seniors (around 20 years old) and adults (around 9 years old). They were all fed grass hay and four complementary feed options, each in turn. The base diet was a low starch and sugar pellet fortified with vitamins and minerals. Horses were fed either the base feed alone or in reduced amounts with one of three options; feeds rich in sugar, starch or fibre. All diets provided the same amount of total energy. The effect of each of these on the ACTH concentrations at specific time points was measured.
As expected, the ACTH in the blood from the senior horses was higher than for the adults. Like previous studies, the level of ACTH was raised in October (Autumn) compared to March, May and August, regardless of age.
However, the results revealed that diet can also influence the ACTH levels in horses. The senior horses had much higher levels of circulating ACTH when fed the starch rich diet compared to the adult group at the same time of the year.
This study is the first to report that diet, and more specifically, a starch rich food, can influence ACTH levels. This is an important consideration when ACTH is used as a diagnostic test for PPID, potentially leading to incorrect disease diagnosis in some horses. The threshold values for diagnosis of the disease currently vary dependent on the season, but these new findings suggest that diet should also be considered.
Jacob S.I., Geor R.J, Weber P.S.D., Harris P.A., McCue M.E. 2018 Effect of dietary carbohydrates and time of year on adrenocorticotropic hormone (ACTH) and cortisol concentrations in adult and aged horses . Domestic Animal Endocrinology, 63, 15-22
The study was a collaboration between Michigan State University, Massey University, University of Minnesota and WALTHAM.