Is pasture ever safe for horses with PPID?

If a horse's PPID is well controlled by medication and management, he may be able to enjoy some grazing. But his risk of developing laminitis depends on several factors.
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Question: Does pergolide protect a Cushing’s horse from developing founder? We have an older pony who was diagnosed and started on the drug last fall. He seems to be responding well. We’ve typically pulled him off of pasture in the spring to prevent founder, but someone mentioned that now that his Cushing’s is under control we might be able to let him have some spring grass. Is this true?

This is a great question. Before I answer, I want to commend you for correctly tying your horse’s risk of laminitis to both to the seasonality of the grass and to your pony’s endocrine disease status. An accurate risk assessment will consider all of these factors.

The endocrine system is confusing, I think, because it is essentially invisible in the body yet at the same time influences all of its functions. Equine Cushing’s disease, more correctly called PPID (pituitary pars intermedia dysfunction) is complicated. A quick review: PPID is caused by an enlarged and overactive pituitary gland producing excess levels of a slew of hormones including adrenocorticotrophic hormone (ACTH). These elevated ACTH levels, in turn, lead to overproduction of the steroid hormone cortisol. The cascade of events continues as excess hormones cause a slew of systemic signs including a long haircoat that is slow to shed, muscle weakness, lowered immune function and susceptibility to infection.

Horses with PPID are also prone to slow-onset chronic laminitis that can go undetected until hoof structures have been irreversibly damaged. Further complicating the issue is the fact that not all PPID horses and ponies are affected by the same circumstances. For instance, if a horse’s PPID is well-controlled and he is not obese and has no other health problem, then yes, grass in the high-risk spring and fall periods would be as safe/unsafe as it would be for a horse without PPID. You probably noticed that I put some strong limits in that statement, which I will address more specifically:

1. “Excellent control” of PPID. This means that while on Prascend (pergolide)---the “gold standard” treatment for equine PPID---your horse’s health has been restored and annual (or more frequent) blood tests show that his ACTH levels have drastically decreased or returned to normal. Some horses will have resolution of signs or improved laboratory testing, but not both. This isn’t considered excellent control. The medication’s side effects or the owner’s budget can prevent us from reaching this point---and in some cases, we cannot get a horse to this point even with an unlimited budget.

2. He has only PPID. Many horses and most ponies have a second endocrine disease, called Insulin Dysregulation (ID). Horses and ponies with both diseases have an increased risk of laminitis on grass. If your pony has not been tested for ID, or his ID has not been stabilized with diet, exercise and medications, then I don’t recommend allowing him any time on grass, no matter the season.

3. He is not obese. This is a tough one. Many owners have become so used to seeing overweight horses and ponies that they do not always recognize obesity in their own animals. The problem is that adipose (fat) tissue is an endocrine organ itself so it can contribute to PPID, which of course is an endocrine disease. An article in this issue includes an explanation of the Body Condition Score system (page 00). Your pony will be safest at a 5 on the 9-point scale. Have your veterinarian check your eye after you have tried scoring him yourself. Then use a weight tape to monitor change and check back in with your veterinarian regularly. Note that I have included both horses and ponies in this discussion, but ponies more often have the stubbornly persistent endocrine disease---and more often are going to have a persistent risk of laminitis. Donkeys and mules are also at risk and benefit from these precautions.

Melinda Freckleton, DVM

FIRESTAR Veterinary Services, LLC

Catlett, Virginia

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