As more horses age, the care and treatment of diseases in older horses becomes increasingly important. Pituitary Pars Intermedia Dysfunction (PPID), or Cushing's disease, is the most common hormonal disease in older horses. About 20-25% of horses over 15 years old are diagnosed with PPID. The condition disrupts the production of hormones causing various health problems. Because more older horses are diagnosed with PPID, increased attention is being given to this disease, leading to more research and awareness among horse owners.
What is PPID and how does it occur?
Located near the brain, the hypothalamus and pituitary gland are crucial for hormone production. For the production of certain hormones, other components are required that control the production. Through nerves located in the hypothalamus, the component dopamine is produced 1. Dopamine has multiple functions of which one is regulating the production of the hormone ACTH and inhibiting its production in the pars intermedia of the pituitary 2. In elderly horses, the amount of nerves might decrease therefore less dopamine will be produced 1. Insufficient dopamine levels prevent the inhibition of ACTH production, leading to uninhibited ACTH production by the pituitary gland 3. Horses with PPID typically have an enlarged pituitary gland 1. The hormone ACTH stimulates the adrenal cortex to produce cortisol. Cortisol is also known as the stress hormone and causes glucose to be released into the body, raising blood sugar levels 4. During stressful situations where the horse needs to respond quickly or flee this is useful. But with uninhibited production of ACTH, blood sugar levels are constantly elevated which has a negative effect on the horse's health.
Symptoms and health issues
The development of PPID is not linked to either gender or horse breed. Any horse over 15 years of age has a chance of developing PPID 1. As horses age, PPID becomes more common, leading to increased awareness among owners regarding the symptoms and signs.
Several symptoms can indicate a horse that has developed PPID. Some symptoms include abnormal growth of the coat (e.g. curly hair), behavioural changes, weight loss, reduction in muscularity, (often recurring) infections and thus impaired immune system function, excessive sweating, insulin resistance and laminitis 5.
Curly and abnormal coat growth is one of the most common and specific symptoms in horses with PPID. A curly coat is therefore also seen by horse owners as an obvious symptom. However, it is usually a symptom which occurs in horses with more advanced PPID. In the early stages of PPID, it occurs that horses do not have a curly coat, but instead have difficulty shedding their winter coat. This is also known as an ageing ailment rather than a symptom of PPID so horse owners do not register this as an early symptom of PPID.
Horses with PPID are also at risk of developing laminitis due to elevated blood sugar levels and hormonal disruption 1. Laminitis is a painful condition in which inflammation develops between the hoof wall and the hoof bone. Severe laminitis can progress to the point where the hoof bone becomes loose and tilts within the hoof. Therefore it is important to check horses with laminitis over the age of 15 for PPID.
Horse owners should contact their veterinarian if their horse shows symptoms of PPID to ensure the horse is tested and diagnosed. The earlier PPID is diagnosed, the sooner they can be treated.
Diagnosis
Diagnosis of PPID is determined by symptoms and a blood test, which checks if the concentration of ACTH is elevated in the horse's blood. This occurs because horses with PPID produce excessive ACTH, which can be found in blood samples. Based on the blood test and symptoms, a horse can be diagnosed and treated for PPID.
Because horses with PPID have an increased risk of developing laminitis, blood glucose and insulin concentrations can be tested to see if blood glucose levels are elevated which is what puts the horse at risk for laminitis 6.
Treatment
Through multiple studies, more information about PPID is available. Unfortunately, no current treatment can cure PPID in horses. However, treatment with medication can reduce symptoms and, if diagnosed at an early stage, prevent the horse from developing further symptoms. The medication contains the substance pergolide, which serves as a replacement for dopamine 7. As dopamine production decreases in horses with PPID, this substance inhibits hormone production instead of dopamine, decreasing excessive production of ACTH 1.
Nutrition and supplements
To promote the horse's health and support treatment, it is crucial to modify the management of a horse with PPID. For example, the right nutrition plays a role in managing horses with PPID.
To create a suitable diet for a horse, first assess the horse's Body Condition Score (BCS), which provides insights into its physical condition and what is necessary to maintain optimal health 8. As mentioned earlier, losing weight is one of the symptoms of PPID and generally occurs regularly in older horses. Therefore, it is also important to regularly monitor the BCS of older horses, especially those with PPID, to prevent the horse from losing too much weight too quickly.
In horses with PPID and insulin resistance, fat accumulates in different areas of the body, which can make it look like the horse has a higher BCS while, for example, the ribs are still visible. This may prevent owners from accurately assessing the BCS, ultimately resulting in improper feed adjustments. If the horse is insulin resistant, which causes fat accumulation, it is advisable to ensure that the feed contains as little sugar and starch as possible 9. Make sure a meal does not contain more than 1 gram of non-structural carbohydrates per kilogram of body weight. Once you know the horse's BCS, you can adjust the feed accordingly and use different feeds to provide the horse with a balanced diet.
Roughage is the basis of the diet and, by fermenting fibre, provides the horse with energy. Fibre is a structural carbohydrate and therefore a suitable source of energy for horses with PPID. There are different types of roughage such as hay, silage grass etc. The amount of nutrients that roughage contains varies by species and some species such as fresh spring grass, for example, contain a lot of non-structural carbohydrates. Therefore, try to feed roughage to horses with PPID that is low in sugars and starch. If you are in doubt about the amount of sugars and starch in roughage, have the nutrient profile of the roughage analysed 9.
If the roughage does not contain enough energy for the horse, the diet can be supplemented with Linseed oil. This oil contains an ideal balance of omega-3 and 6 fatty acids and does not add extra sugar and starch to the diet. Due to its fatty acids, Linseed oil provides the horse with a long-lasting source of energy and supports the maintenance of body weight.
As roughage often does not contain enough vitamins and minerals to meet the horse's requirements, it is recommended to supplement the ration with a balancer or vitamin and mineral supplement. But before purchasing a balancer or supplement, check how much sugar and starch the product contains and avoid products with high amounts of sugar and starch.
One of the common symptoms of PPID is losing muscle mass. Maintaining muscle requires protein from the feed combined with exercise. However, training or exercise is not possible for every older horse with PPID. Nevertheless, it is advisable to ensure that the horse exercises lightly to train and support muscle development. Of course, training does not have to be intensive but find a way of exercise and training that is suitable for your horse. Additionally, roughage often lacks sufficient protein to meet the body's needs. If so, the diet can be supplemented with a supplement containing proteins and amino acids to support the horse's muscles.
Horses with PPID who suffer from excessive sweating may benefit from supplementing electrolytes that are lost when the horse sweats. 9. Especially when the horse exercises and sweats a lot, it is advised to replenish the losses using a supplement.
Because horses with PPID are more susceptible to infections and this has an effect on the functioning of the immune system, it is recommended to add supplements to the diet that support the immune system 9.
In short, it is important to take action as soon as possible when a horse experiences symptoms of PPID so that treatment can be started. In addition to treatment through medication, good nutrition also provides the necessary support. Therefore, if in doubt, consult a nutritionist who can provide you with advice and help in formulating a suitable diet.
References
Kirkwood, C.N., Hughes, K.J., Stewart, A.J. (2022) Pituitary Pars Intermedia Dysfunction (PPID) in Horses. Veterinary Sciences, 9(556):1-22.
Kirkwood, C.N., Hughes, K.J., Stewart, A.J. (2022) Prospective Case Series of Clinical Signs and Adrenocorticotrophin (ACTH) Concentrations in Seven Horses Transitioning to Pituitary Pars Intermedia Dysfunction (PPID). Veterinary Sciences, 9(572):1-21.
McGowan, T.W., Pinchbeck, G.P., McGowan, C.M. (2012) Prevalence, risk factors and clinical signs predictive for equine pituitary pars intermedia dysfunction in aged horses. Equine Veterinary Journal, 45(1):74-79.
Spelta, C.W. (2015) Equine Pituitary Pars Intermedia Dysfunction: Current Perspectives on Diagnosis and Management. Veterinary Medicine: Research and Reports, 6:293-300.
Stewart, A.J., Ireland, J.L., Durham, A.E., McGowan, C.M. (2023) Diagnosis of equine pituitary pars intermedia dysfunction. The Veterinary Journal, 300-302:1-6.
Bertin, F.R., De Laat, M.A. (2017) The diagnosis of equine insulin dysregulation. Equine Veterinary Journal, 49(5):570-576.
Miller, A.B., Loynachan, A.T., Bush, H.M., Hart, K.A., Barker, V.D., Campana - Emard, A.G., Grubbs, S.T., Adams, A.A. (2021) Effects of pituitary pars intermedia dysfunction and Prascend (pergolide tablets) treatment on endocrine and immune function in horses. Domestic Animal Endocrinology, 74:1-21.
Hart, K., Durham, A., Frank, N., McGowan, C., Schott, H., Stewart, A. (2021) Recommendations for the Diagnosis and Treatment of Pituitary Pars Intermedia Dysfunction (PPID). Equine Endocrinology Group, 1-13.
Galinelli, N.C., Bailey, S.R., Bamford, N.J., Harris, P.A. (2023) Nutritional considerations for the management of equine pituitary pars intermedia dysfunction. Equine Veterinary Education, 35:33-44.
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