Equine Pituitary Pars Intermedia Dysfunction (Cushing’s Disease)

Pituitary Pars Intermedia Dysfunction (AKA Cushing’s Disease; PPID) in horses is caused by an abnormal pituitary gland. Overgrowth of the gland results in production and release of excessive stress hormone circulating in the body.

Signs of PPID

Classic clinical signs are long shaggy hair, laminitis, chronic founder, pot-bellied appearance, excessive drinking, excessive urination and a big cresty neck. However, recent improvements in diagnostic testing have allowed us to diagnose this disease much earlier. Now we associate this disease with much more subtle clinical signs such as: Failure to completely shed out the hair coat in the spring (long hairs may linger on the neck) recurrent foot abscesses (particularly in the spring and fall), muscle loss, poor topline, and lameness due to suspensory ligament desmitis.

Diagnosis

In severely affected patients, diagnosis may be made based on clinical signs alone. In the past, blood testing for PPID was difficult to perform due to cumbersome protocols and results were often inconclusive. However, since 2015 we have been using a new test known as the TRH Response test. The protocol for this new test is easy and fast. Samples can be collected over a ten-minute period during a regular appointment and then mailed to a lab for testing. Ideally, this test should not be run in the fall from mid-September to mid-November as the shortening day length can affect hormone release and thus the result of the test. Most importantly, the TRH Response test has been crucial in allowing us to diagnosis this disease in horses as young as 10 years old! Now we can begin treatment YEARS before the patient progresses to a point of chronic founder and life threatening complications.

Treatment

At this time there is NO CURE for PPID. Fortunately, there is a very effective treatment. Prascend (pergolide) is the only FDA-approved product to treat PPID in horses. Prascend is a tablet given daily that acts on the pituitary gland to reduce the excessive hormone secretion that causes clinical signs. This medication is lifelong and will require adjustments over time. Once treatment begins, patients should be retested in 60 days to determine whether the dosage is adequate or whether an adjustment is needed. During that time, you should notice that hair length decreases and abnormal fat deposits are reduced. Once regulated, a test should be conducted once yearly or anytime that the clinical signs appear. Since this disease cannot be cured, it WILL worsen over time. However, with proper treatment and management, many horses will have an excellent quality of life and continue their normal level of use.

Management of PPID

There are several steps in dietary and daily management that are crucial in overall management of PPID.

Pasture Management

Horses with PPID are exquisitely sensitive to sugar (simple carbohydrates) and more specifically fructans. Fructans are the horse equivalent of high fructose corn syrup. Fructan content in pasture grasses varies by type of grass, length of grass, weather conditions and time of day.

Type of Grass: Cool season grasses have periods of particularly high sugar content. These include orchard grass, fescue and Kentucky bluegrass – all of which are very common in West Virginia. Sugar content in these grasses rises seasonally when we have cool nights and warm days (Think, 35-45 degree nights and 65-70 degree days of spring and fall). This is the main reason we see more episodes of laminitis during the spring and fall. In our practice, the exact timing of this critical risk period ranges from March to May and August to November, but depends on county of residence and elevation. For your own location, consider when you see frost while still having nice weather through the day.

Length of Grass: New grass has soft tender structure and more sugar compared to older, taller grass. As the grass grows it develops more lignin (fiber) which provides strength and structure. Therefore, as the grass gets taller it has proportionately less sugar in each bite (Think, tender sweet baby vegetables compared to large older vegetables that are tough and chewy). Access to pasture should be avoided in the spring when you see the young bright green grass starting to emerge. Pasture access should be restricted until the grass has reached at least 8 to 10 inches in height and the fiber content is higher.

Time of Day: Research conducted at Virginia-Maryland College of Veterinary Medicine found that fructan content in the grass changes within the same day. Fructan content is highest in the grass during the afternoon after a cool night. Based on this, we recommend avoiding grazing times in the afternoons, or turn out at night and restrict daytime grazing during the grazing season.

Fertilizer: Generally, good pasture management is great for increasing yield. Unfortunately, this is exactly what you don’t need for PPID horses. Reconsider your pasture management, you may find that you actually need to decrease treatments to reduce grazing quality.

Muzzles vs Drylot vs Timed Turnout

We encourage as much turnout as possible for mental well-being, socialization and exercise. Ultimately you will need to choose a method that works with your property and lifestyle.

Muzzles: Must be left on anytime the horse is grazing. Monitor the face for rubs and the bottom of the muzzle for being worn out and grazing hole enlargement. Muzzles often hard to keep on if there are multiple horses in the pasture as they will play and rip them off.

Drylot (or reduced pasture size): This is a more expensive but more permanent option. In some cases, just increasing the number of horses on the property to “overgraze” the pasture will be helpful. In other cases, you will need to cross fence and reduce access to a smaller area. This is convenient because it does not require a muzzle or daily maintenance but may be difficult if you have “hard keepers” and “easy keepers” in the same pasture. In our area, this seems to be the most successful long term management plan. Reducing the pasture requires an investment of fencing but requires less day to day management in the long term.

Timed Turnout: This works well in boarding stables where turnout is often limited anyway. However, it is important to note horses can eat an entire day’s worth of calories in short period of time. Therefore, if using this method, you must use a drylot or still use a muzzle to restrict intake during the turnout.

Dietary Management

Hay: Ideally, hay should be tested prior to feeding to ensure a low starch count (NSC). Hay testing can be done through Equi-Analytical. Information can be found at https://equi-analytical.com/.

However, keep in mind that hay quality may vary within the same farm and even the same field. If tested hay is not available, generally first-cutting hay will be a more appropriate choice than second-cutting hay. Just as the pasture length described above, second-cutting hay is more sugar-dense. First-cutting hay must grow longer and go to seed before it is cut, which lowers the sugar content in the stem.

Overweight Horses: Feed first-cutting hay at a rate of 2-3 % of bodyweight or 20-30 lbs per day. Use a scale to weigh the flakes of hay. Hay should be fed in a slow feeder. Slow feeders will mimic more natural feeding and help keep the horse occupied as you reduce their overall calorie intake. When introducing the slow feeder, offer the full ration as normal and then extra ration in the slow feeder. Slowly increase the portion offered in the slow feeder and decrease the portion that is free fed until eventually the entire ration is offered in the slow feeder. Gradual transition to the slow feeder will prevent stress and hunger panic from the horse as it becomes accustomed the new feeding method. There are many slow feeders available on the market and several versions that can be hand made. We recommend NibbleNets. As an alternative, you may place the hay in several piles around the lot instead of one large pile. However, this generally produces more waste than a NibbleNet or other homemade slow feeder. Caution: some horses will cause abnormal wear on their incisor teeth from chewing at nylon or metal grates on the slow feeders. This should be monitored.

Concentrate Feed (grain): Avoid feeding any grain with excessive starch such as sweet feeds or corn. Feed only a ration balancer pellet. Ration balancers offer all the benefits of much needed protein and minerals without any excess sugar. Search for a product with a minimum 28% protein content. Ration balancer pellets are readily available from many brands at many feed stores. However, many brands will refer to them by different names such as “grazing pellet, hay stretcher pellet or protein supplement” which certainly makes it difficult to choose. Locally available balancer pellets include: Enrich (PurinaMills), Empower Topline Balance (Nutrena), Essential K (Tribute), and 30% Ration Balancer (Triple Crown). Note that with any of these protein supplements, you will feed much less than with a senior feed or sweet feed. Generally, these protein supplements are designed to feed at a rate of 1-2 lbs per 1,000 lbs of bodyweight.

Weight Issues

Overweight Horses: Many horses with PPID are overweight. Once the PPID is appropriately managed, patients that remain overweight should be tested for concurrent insulin resistance (IR) or insulin dysregulation (ID). IR or ID can occur as a result of PPID or as an independent problem. If insulin abnormalities are present, additional medications, such as metformin and levothyroxine, can be prescribed for continuous or seasonal therapy to help manage the patient.

Underweight/Poorly Muscled Horses: In recent years, we have grown to appreciate more and more that not all PPID horses are obese. Many PPID horses will have chronic muscle loss, failure to gain weight and weak toplines.

For weight gain, add a fat supplement to increase calorie intake. Omega-3 fatty acids are an excellent source of dietary fat for horses. In horses, flaxseed, rice bran and flaxseed oil or rice bran oil are the most commonly recommended sources. These are generally very palatable and easy to find in feed stores. Omega-3 fatty acids offer several benefits such as anti-inflammatory effects that benefit horses with arthritis, inhalant allergies and skin allergies. Note that fats may get rancid in storage, so ensure that you have a fresh source. In addition, fat is not always palatable to every horse, particularly when given in high amounts, and it may cause some loose stool. Search for a product that has a minimum 22% fat. Locally available fat supplements include: Essential Omega Blend liquid (Triple Crown), Amplify pellets (Amplify), or K Finish (Tribute). Feed as a directed on the product label. Generally, since these products are very calorie dense, you will not need to feed a very high volume.

Supplements: There are tons of supplements marketed for both PPID and IR (or ID) horses. Most of these supplements contain chromium and/or magnesium. Research has not proven any benefit from these supplements. Therefore, we do NOT recommend any particular supplements.

Farriery/Hoof Care

Horses with PPID are at great risk of developing laminitis and chronic founder. Hooves should be regularly trimmed to maintain appropriate angles and a shortened toe. Special care or special shoes may be recommended for patients with discomfort from chronic laminitis. Monitor patients closely for foot abscesses. Recurring foot abscesses are a sign of micro-episodes of laminitis. Foot abscesses, particularly recurring foot abscesses, in a horse with PPID are an indicator that the patient’s PPID is not well regulated.

Monitoring

Horses with PPID should be continually monitored for signs that they are not appropriately regulated (i.e.: recurrent foot abscesses, dental disease, laminitis, failure to shed their hair coat in the spring, abnormal sweating, etc.). Annual blood tests should be conducted during the spring.

Long-Term Management

Most horses with PPID will continue to live comfortably and have long, successful careers. Many owners comment that their horses seem to be more active, more energetic and generally “feel” better once they are regulated. This may be due to the general decrease in “stress” hormone and/or decrease in foot pain related to micro-episodes of laminitis.