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Metabolic diseases

- October 30th, 2020 - Ask the Vet

By Daniel H. Grove, DVM

In the horse, two metabolic diseases are very common. The first is pars pituitary dysfunction (PPID), a.k.a. Cushing’s Disease, and the other is Insulin Resistance. For the purposes of this article I will refer to Cushing’s disease as PPID. Many developments in the knowledge and testing of these diseases has changed in recent years. I am going to try to explain each easily, discuss the symptoms, discuss how we diagnose them and, finally, how we treat them.


PPID is a disease in which the pituitary gland, which is just below the brain, is not functioning properly. It is over-secreting a few different hormones, with adrenocorticotropic hormone (ACTH) being the most talked about one. The excessive level of this hormone stimulates the tiny adrenal glands, located next to the kidneys, to over produce Cortisol, the stress hormone. This causes a state in the body of constantly trying to be prepared to deal with stress. This disease is most commonly seen in horses in their late teens and older. The signs associated with PPID are listed below:

  1. Long hair coat that does not shed properly in the summer
  2. Muscle wasting that can lead to a pot-bellied appearance
  3. Lack of healing
  4. High blood sugar levels
  5. Polyuria/polydipsia or excessive urination/excessive drinking
  6. Abnormal sweating
  7. Laminitis

All of these symptoms above can be seen in a PPID horse, but not all of them have to be seen for it to be a PPID horse.

How is it diagnosed? Well, diagnosis needs to be made not only based off of bloodwork, but with the clinical signs evident also. The blood tests commonly used today are either a simple ACTH level or a thyrotropin releasing hormone (TRH) stimulation test. Your veterinarian will help you decide what test is best for your situation. The ACTH test is less sensitive to finding the borderline cases but is less expensive. The TRH test is more sensitive but costs more money.

Treatment is straightforward. The drug of choice is pergolide. It is important to use the commercially prepared version which is Prascend. Compounded versions have been tried and tested, and their reliability is very questionable. It may take some trial and error to find the right dose for your horse. Once you see resolution of the clinical signs, you are on the right track, as far as the dose goes.

Insulin Resistance (IR)

IR is different. With IR, the body is having a problem with taking up blood sugar therefore it increases the insulin levels. This is like in humans when you are prediabetic. This disease can show up anytime in life, but you are more likely to see it in young horses than PPID. The clinical signs are as follows:

  1. Obese-A body condition score of 6 or higher
  2. Thick Cresty neck
  3. Abnormal fat deposits around the tail head
  4. A very easy keeper
  5. Laminitis
  6. High blood sugar levels

Diagnosis is off of clinical signs combined with blood tests. Two common ways to test for it are insulin and leptin testing and oral sugar tests.

Treatment is a little more labor intensive, as dietary restriction is key. Your goal is to keep the soluble starches and sugars below 10 percent of the diet. This is very challenging, but it can be done. Not letting your horse graze on lush pasture, especially in the spring, is important. Medications that are commonly used are levothyroxine and metformin. Levothyroxine increases metabolism, helping to get rid of excess fat and helps insulin work better. Metformin Helps insulin work better, decreases glucose production in the liver and decreases sugar absorption from the intestines.

A lot of work has been done on these two diseases, and much more still needs to be worked out. As you can see from our symptom lists, there are some similarities, but also some differences. If you notice some of these clinical signs, please discuss them with your veterinarian. The consequences of not doing so can be catastrophic.


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