As the number of horses with Cushing’s continues to rise, we need to keep building on our understanding of the disease, as well as how to recognize and treat it.
Cushing’s syndrome has become one of the most common diagnoses in equine practice. There are several reasons why. Perhaps a major one is that we keep our horses going for a greater number of years than we did in the past, and Cushing’s tends to more commonly affect elderly horses. Veterinarians and horse owners are also much better informed about the condition and are knowledgeable about symptoms. In addition, our ability to test for the Cushing’s has improved, and the main drug prescribed for it has been approved for use in horses, leading to easy conventional treatment.
What is Cushing’s syndrome?
A more correct term for Cushing’s among researchers is PPID (Pituitary Par Intermedia Dysfunction). This is because Cushing’s in humans and dogs affects a different part of the pituitary gland than it does in equines. PPID is a more correct functional term for the disease in horses, and should be adapted to describe this condition.
Pituitary adenomas (tumors) of the pars intermedia part of the pituitary gland have been considered an almost normal part of aging in horses. However, texts show conflicting reports about how common the true adenoma tumor is. About half the sources feel an actual adenoma is less common, and that hyperplasia (abnormal and overactive, but not tumorous growth of cells) occurs instead. Both adenomas and hyperplasia cause similar sets of symptoms, but hyperplasia, being a functional disturbance rather than a tumor, is easier to treat. Many horses respond well to treatment; in these cases, the condition is probably more functional than cancerous. Older horses that do not respond well to treatment may have actual tumors.
A holistic perspective of PPID
Looking at this condition from a holistic perspective, the primary issue is basically that the pituitary gland is aging and becoming less functional. The AAEP description of PPID supports this thinking by defining it as a condition that arises because the hypothalamus is no longer properly regulating the pituitary gland. Since the pituitary acts as the “grandmother” or master gland for the entire hormonal system, the entire system becomes dysfunctional as it loses function (see Fig. 1). Damage to any part of the system can affect other hormonal pathways.
PPID has a large group of possible symptoms, depending on which system is most affected in an individual horse. Since natural medicine is generally tailored to the individual, it becomes possible to support each horse. Younger horses with a diagnosis of Cushing’s disease have most likely been stressed, often by a show or competitive career. There is also evidence that the inflammation caused by imbalances in the hormones contributes to tissue oxidation and breakdown.
Laminitis remains one of the more frustrating complications of Cushing’s disease. Chronic cases can take a significant amount of time and energy to treat, and still yield unsatisfactory results. Natural medicine provides a different and more diverse toolbox of treatments for the spectrum of symptoms collected under the title of Cushing’s disease.
The clinical signs most commonly associated with Cushing’s syndrome in horses are hirsutism (long hair that does not shed out in the summer), difficult-to-treat laminitis, and weight problems (over- or underweight). Many other symptoms appear in certain individuals, with some horses exhibiting very few clinical signs, and others exhibiting many.
Laminitis with no outward reason for its occurrence is one of the most serious symptoms of Cushing’s. Some of the most refractory cases of Cushing’s-based laminitis occur in the winter, an uncommon season for typical cases. Even the more common summer laminitis, which appears to be caused by overeating grass, can be quite difficult to treat, especially when the horse has other clinical signs of Cushing’s.
Many horses diagnosed with Cushing’s laminitis are overweight and very easy keepers, sometimes unable to eat more than a small amount of hay each day to control their weight. In some cases, horses that were previously easy keepers suddenly start requiring more food to maintain their body weight. The overweight horses generally have fat pads in specific places, along with cresty necks. The fat pads are generally behind the shoulder blades, on each side of the tail, and along the lumbar area. In addition, the fat on the horses’s body is often visibly lumpy. Some horses that are beginning to lose weight nevertheless maintain their fat pads.
Research is showing that horses with insulin resistance and a high body condition score may also have pituitary lesions, and are more prone to internal lipomas, which can be fatal. The immune system is often weak, so many of the symptoms are related to infections or parasites taking over the body.
The most important diagnostic tool for identifying Cushing’s as the primary problem is the history and clinical signs discussed above. A thorough physical exam may reveal some of the less obvious signs, such as poor teeth or reproductive problems. Supporting lab work can be inconclusive, but can still be helpful and should be performed if possible.
Part of the problem with lab work in the equine Cushing’s patient is that single blood samples are taken whenever the practitioner is at the farm, so there is little standardization in the timing of samples. Many parameters have daily variations and may change due to stress or other factors, including the amount of exercise a horse has had before the blood was drawn.
For example, elevated blood cortisol can indicate high levels of stress in the body. But is the high cortisol coming from Cushing’s, or has the Cushing’s come from the chronic stress of something like laminitis? Cortisol as a single sample appears to be an inaccurate test for Cushing’s syndrome. High cortisol levels suppress the immune system and are the reason for higher levels of infection in these horses. Cortisol is the stress hormone, so past (horse showing, overwork, abuse) and present stressors all contribute.
Now that we have an overall understanding of how to recognize and test for Cushing’s, we can take a look at how to support and treat these horses with integrative options. Be sure to follow along next issue as we look at managing this common condition through nutrition and integrative therapies.
Signs of Cushing’s syndrome
- Hirsutism (long hair)
- Hair not shed out in summer
- Refractory laminitis
- Winter laminitis
- Weight problems (over- or underweight)
- Sluggish thyroid gland
- Insulin resistance
- Thyroid dysfunction
- Muscle soreness
- Polyuria/polydipsia (drinking and peeing excessively)
- Collagen breakdown (back sags, tendons and ligaments may stretch)
- Poor hair coat
- Frequent infections of the skin or other organs
- Poor teeth
- Multiple dental abnormalities
- Lowered immunity to intestinal parasites
- Decreased intestinal wall integrity
- Muscle wasting
Testing for Cushing’s syndrome
- To test for Cushing’s, insulin levels, ACTH and glucose are usually checked. ACTH remains a useful test, though there is a normal rise in levels during the fall, so tests must be interpreted carefully at that time of year. A single-sample thyroid test does not give a true picture of thyroid function, as there is a significant variation in thyroid levels in even normal horses.
- A low dose dexamethasone suppression test (LDDS) can be useful, but it is falling out of favor. I would personally avoid using it since dexamethasone is a steroid and sensitive horses can get laminitis from steroids. I have seen a number of serious cases of laminitis in Cushing’s horses following steroid injections.
- The TRH response test is the latest to be added. It involves taking a blood sample, giving a dose of thyroid stimulating hormone, and taking a second blood sample 15 or so minutes later. Normal horses will only have a small rise in ACTH, while horses with Cushing’s will show a much more significant rise on the second test. Though this is a new test and we need to evaluate it over time, it makes sense to stimulate the system to see how it is responding.
- Leptin testing is also being done. Leptins are hormones produced by the fat cells. This test appears to help separate out horses that are just stressed from those with significant insulin resistance. It is not a specific Cushing’s test. Leptin does undergo a daily variation, which means if you want to compare results, it’s best to have the vet collect blood samples at the same time of day on follow-up visits.
Fasting for the purpose of testing, whether for insulin, glucose, leptin or anything else, is stressful to a horse. Personally, I do not think it is wise to fast a horse, unless he is basically healthy and you want to see a baseline. A horse that is showing clinical signs does not need the additional stress for the sake of a blood sample, in my opinion.
In addition, have your veterinarian collect a complete blood count (CBC) to examine your horse’s immune system status, and a chemistry screen to check organ function.
Insulin resistance and Cushing’s
Cushing’s syndrome in horses has many of the same characteristics as insulin resistance. Many Cushing’s horses have elevated insulin levels in their blood. The reason insulin is elevated is because it is not able to get into the cells. Normally, when a sugar or carbohydrate is eaten, blood sugar levels increase, the pancreas secretes insulin, glucose is carried into the cells by the insulin, and blood sugar goes back to normal. In insulin resistance, the cell walls are too stiff to let the insulin do its job properly. So instead of providing energy for the cells, the glucose gets stored as fat.