Equine Cushing’s Syndrome
This is a 20-year-old pony that was presented at Greenmount Equine Hospital with a history of weight loss but no decrease in appetite, abnormal hair growth and a few episodes of laminitis. Could you guess what condition this pony suffers from? Could you list 4 common clinical signs of this condition? Can it be diagnosed and is it treatable? What else can be done to keep the horse in good health? Read on to find out all the answers from our veterinary expert – Nikki Walshe.
Equine Cushing’s syndrome is a hormonal disease of horses caused by dysfunction of the pituitary gland. It is one of the most common diseases in horses and ponies 15 years and over, with the average age being about 19 years old. Equine Cushing’s syndrome (ECS) is becoming more widely recognised due to increased public awareness and the fact horses are living longer.
What causes Equine Cushing’s syndrome?
ECS is a neurodegenerative disease where there is damage to the neurons controlling an area of the pituitary gland. The pituitary gland is a pea sized gland that sits at the base of the brain and is the main centre of hormonal activity of the body. A decrease in neuronal control allows a specific area (the intermediate lobe) of the pituitary to become enlarged. This results in the increased production of certain hormones (eg ACTH – adrenocorticotropin hormone). This escalation in production of ACTH causes increased levels of steroids (cortisol) in the blood and a high level of cortisol has a wide range of deleterious effects on the body’s systems.
How does Equine Cushing’s syndrome present?
Clinical signs likely develop from a combination of hormone over-production, coupled with loss of function of the rest of the pituitary. As the affected area of the pituitary grows it compresses the surrounding tissue, inhibiting the normal function. There are number of clinical signs (Box 1) but any one of these should prompt an investigation for disease, particularly in our older horses. Having said that, it has been shown that ECS can start to develop in horses as young as 10 years old. The earliest signs are insidious and include weight loss, muscle wastage and poor performance.
Diagnosis of Equine Cushing’s syndrome
Equine Cushing’s syndrome is common and can be life threatening, so early accurate diagnosis is imperative. However, that is easier said than done.
Cushing’s is a slow, insidious disease, making early detection difficult. Moreover, none of the diagnostic techniques are 100% accurate with stage of disease, season, stress and pain all affecting results. So often we are forced to diagnose horses on the clinical signs of the disease and while this is often accurate, as stated before, not all cases present with all clinical signs.
The diagnostic methods;
- Dexamethasone suppression test
- Thryrotropin releasing hormone test
- A combination of them both
- Resting ACTH and insulin level measurements
The overnight dexamethasone suppression test is the most commonly used diagnostic tool. An intramuscular injection of dexamethasone (a steroid) is given. In a normal horse, an injection of dexamethasone causes a reduction in ACTH production from the intermediate lobe of the pituitary gland and a subsequent reduction of cortisol levels in the blood. However in cushingoid horses there is no reduction in cortisol levels as the abnormal pituitary is not inhibited by steroid blood levels. The injection is given on day one and a blood is taken on day two.
If there are clinical signs of the disease, resting ACTH is a commonly taken with the view to confirm the presumptive diagnosis. Although this can be affected by season and stress, a high level of ACTH is highly suggestive of the disease.
There are suggestions of an increase in susceptibility of ponies with a history of equine metabolic syndrome. Therefore, it would be prudent to always ask your vet to check for high insulin in the blood, or to do an oral glucose test to check if hyperinsulinemia (high insulin in the blood) could be a complicating factor.
Unfortunately there is no cure for Cushing’s!
But there are several treatment methods
There are pharmaceutical therapies such as Pergolide mesylate. This decreases the circulating ACTH, thereby lowering blood cortisol levels, however this will only improve clinical signs and cannot reverse them. Clinical and diagnostic test improvement will not be apparent from months so a re-check by your vet every 6-12 months to adjust dose will be appropriate.
Geriatric health management program
The mainstay of treatment is based around managing the disease and reducing the risk of disease complications that ultimately lead to fatality. Cushingoid horses need aggressive preventative health care.
Reducing circulating insulin levels is key to managing the diet and consequently this condition. Certain foods that are high in starch and simple sugars elevate glucose levels in the blood that lead to increased insulin production. Forage sugar content is very dependent on the conditions at the time it was cut (night time, rainfall, fertiliser…etc) so ideally getting you hay analysed is the best way to assess the sugar content of your horse’s diet. The NSC% (non-structural carbohydrate) is the value that is measured to evaluate sugar content, this should be no more than 12%. However analysis is often impractical or expensive. So below are a few guidelines that can help to reduce the starch levels in the diet.
- Feed low quality hay or soak hay for 30-60 minutes (then remove hay from water)
- Feed Alfalfa hay as it averages the lowest simple sugar content
- Unsweetened beet pulp is an excellent feed source (make sure you soak appropriately)
- Rice bran oil/flaxseed oil can be a good fat source for underweight cushing horses.
- A balancer should be used to ensure the nutritional requirements of the horse are met
NOTE: Stress can worsen this condition so do not restrict food intake, instead allow free-choice low starch hay.
Unrestricted grazing is not recommended. Different weather and different seasons all affect the starch levels of grass.
There is some evidence magnesium and chromium supplementation can help decrease insulin levels and reduce fat deposition.
Insulin regulation can be aided by the addition of omega-3 fatty acids, so flaxseed oil or chia seeds can be beneficial to add to the diet.
Preventative health care
Cushings is one of the most common cause of laminitis in horses. Due to the increased risk associated with the disease, preventive measures are crucial. As discussed above, low starch diets are key. However, regular visits by the farrier can help decrease risk and allow for effective monitoring of the disease. If you have any suspicion of laminitis, veterinary consultation and Xrays can be taken to confirm its presence and in conjunction with the farrier paring adjusted accordingly.
Dental care is crucial, not just in Cushingoid horses but in all old horses. Teeth can become deformed and as a result the horse can’t pick up food or chew it properly, which results in poor nutritional intake. This can often go unnoticed, due to the insidious onset of weight loss. So observe older horses eating, monitor for “quidding” (dropping food out of the mouth). And be sure to schedule routine dental checks. It’s also important to monitor your horse’s body condition score and if rugged, be sure to take off the rug when assessing the body condition of your horse.
Older horses generally have a good immunity to parasites. However, due to the immunosuppression effects of high cortisol levels, Cushingoid horses are much more susceptible to parasitic disease. As a result, it’s important to conduct regular faecal egg counts to monitor their worm burden and administer anthelminthics accordingly.
Immunosuppression can also leave these animals at a great risk of picking up secondary bacterial infections. What may seem like an innocuous thing, such as mud rash or superficial lesions, can develop into more serious problems. Regular veterinary inspection and prompt treatment of all infections is advised.
These horses often have issues regulating their temperature especially with the abnormal hair growth that may develop. As a result, it’s important to:
- Clip excessive hair
- Provide ample water
- Provide shelter
- Blanket when necessary
Equine Cushing’s syndrome is becoming much more common as our horses our living longer (due to improved management techniques and expert veterinary care) and due to an increased awareness of the disease. Even though there is not yet a cure for the disease, well cared for horses can live up to their 30’s and some case 40’s. Most fatalities are due to laminitis or secondary illnesses, so preventive care and prompt veterinary consultation is essential for their survival.
So don’t forget the old soldier in the field – get him checked out so his pasture ornament days will be long and plentiful.
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