Your horse's hormone levels change to prepare his coat and body for winter. One of these changes is a rise in ACTH, the hormone that signals cortisol release from the adrenal gland. This is a perfectly normal event, but it can spell trouble for some horses.
The ACTH rise is actually probably a secondary event, related to changes in hormone sensitivity whose ultimate effect is to cause the horse to grow a winter coat. In normal, young horses, the magnitude of the ACTH rise is relatively small and it may still stay within the normal range. However, as the horse gets older the rise tends to be higher and for horses with full blown or early Cushings disease, it can be huge.
ACTH in the blood is measured in pg/mL (picograms per milliliter). Laboratories vary in their normal range, but a common upper limit of normal is 35 pg/mL. A young horse may rise from under 10 to the upper normal range, a horse in their late teens but apparently healthy will go above the normal range while horses with uncontrolled or poorly controlled Cushings disease can reach 1000 pg/mL.
Early Cushings disease should be suspected in horses in their teens that experience an unexplained late summer or fall laminitis. These horses often appear normal, but experience the first laminitis of their life during the fall ACTH rise. This may happen for several years before the horse develops the classical abnormal coat of Cushing's disease, or tests with an abnormally high ACTH at other times.
Horses with insulin resistance may also be at increased risk of laminitis at this time, especially as they get into their teens. One effect of increased cortisol is increased insulin resistance. Again, for normal horses this isn?t an issue. However, if the horse already is insulin resistant it can be enough to push them over the edge into laminitis.
Horses that should have ACTH checked in the fall are those with:
A history of unexplained laminitis in the past.
Known Cushing?s disease.
Insulin resistance and in their teens.
If the horse tests high, have a discussion with your veterinarian over possibly treating the horse with pergolide during this high-risk time of the year. For most horses, the ACTH returns to baseline levels by January, although some will remain elevated until spring. The prolonged elevations are most likely to occur in aged horses with Cushing?s disease. In most cases, pergolide can be tapered off starting late December.
Because the rise occurs to variable degrees in all horses, this isn't a good time of year to establish a new diagnosis of Cushing?s, particularly with borderline or only mildly abnormal results. In those cases, your veterinarian may decide it is wise to treat with pergolide anyway.