Feeding the Cushing’s Horse
The classical and most obvious sign that a horse or pony suffers from Cushings is that the animal has a long, curly coat that is not shed as the seasons change. However, the absence of this obvious clinical sign does not mean an animal is free of the disease. Equine Cushings Disease (ECD) can occur amongst all breeds and types of equid although it most commonly occurs amongst aged ponies. It has been diagnosed at post-mortem examination in what were apparently normal animals. Apart from changes in hair coat other signs to watch out for include a pot belly, weight loss, muscle wastage and the appearance of abnormal fat deposits such as around the eye orbit. The long hair coat is often associated with localised sweating and cushingoid animals may be observed to drink and urinate more (polydypsia and polyuria respectively) than contemporaries. Immune function in cushingoid animals is usually compromised so typically, recovery from infection or trauma is affected and animals take longer to shake off these various insults. ECD is not a life-threatening disease per se and animals can survive for a very long time if carefully managed but unfortunately there is no cure. An important part of this management programme is the regulation of feeding and considerable care has to be taken over the selection of feed ingredients and dietary provision. An unfortunate complicating factor is that animals with ECD often suffer from laminitis as well.
At this point it is worth explaining the situation in humans who may suffer from Cushing ’s syndrome. It occurs when the body tissues are exposed to high levels of cortisol for too long. One of the most important functions of cortisol in both man and animals is that it helps the body to respond to stress. However it has other roles which are highly relevant to bodily function and its overproduction can disrupt homeostatic mechanisms. These roles include regulation of protein, fat and carbohydrate metabolism; maintaining cardiovascular function and blood pressure; reducing the immune systems inflammatory response and lastly but most importantly for the horse or pony is that it opposes the action of insulin which of course is necessary for the control of blood glucose. Normal function relies on a small part of the brain, the hypothalamus, sending corticotrophin-releasing hormone (CRH) to part of the pituitary gland (pars intermedia). CRH causes the pituitary to secrete adrenocorticotropin hormone (ACTH) which causes the adrenal glands that lie near the kidneys, to produce cortisol. In healthy individuals, high levels of cortisol would feedback to the hypothalamus where certain cells would produce the neurotransmitter dopamine that would tell the pituitary gland to stop producing ACTH and cortisol production from the adrenal glands would then subside. It is the overproduction of cortisol that produces Cushing’s syndrome in man and ECD in equids. In man, overproduction of cortisol can be as a result of a number of quite different situations. For example treatment with steroids to overcome inflammatory diseases, individual overproduction of cortisol, non-cancerous tumours of the pituitary that secrete excess ACTH (responsible for ~70% of cases-called Cushing’s disease), cancerous tumours outwith the pituitary and adrenal tumours. In contrast, in equids, ECD is as a result of enlargement of the pars intermedia or, of the development of a benign growth there. Thus it is no surprise that ECD is often referred to as Pituitary Pars Intermedia dysfunction (PPID)!
Apart from the necessity for the regulation of the feeding of these animals it is worth giving brief mention to the other aspects of the overall management programme. It should be clear from the foregoing that ECD animals are significantly compromised in terms of their health and thus it is imperative that there is early veterinary involvement in their care. I know it is stating the obvious but ECD animals should enjoy effective parasite control, have pristine teeth and perfect occlusion (regular visits to the dentist!!) and their hoof care should be of the highest order. A long, curly coat in the spring and summer is an embarrassment to the animal so coat condition needs careful monitoring. Finally it is essential to maintain a weight and condition that are appropriate to the type of animal. This will necessitate regular weighing or weight estimation using a weigh tape together with condition/fat scoring.
In terms of feeding ECD animals we must remember two important points referred to earlier and these are that laminitis is common amongst these animals and that blood glucose control is shot to pieces. In very simple terms cortisol release should stimulate glucose production, which in turn would stimulate insulin production. However, in the case of ECD animals it is ineffective in regulating blood glucose. This is because ECD animals become insulin resistant and are thus more prone to laminitis. A dexamethasone suppression test relies on injecting the horse with a steroid and this should normally result in a reduction of circulating cortisol; failure to do so is indicative of ECD.
Selection of feed ingredients: It should be clear that an animal suspected or confirmed as having ECD should be fed dietary ingredients that do not result in an elevation of blood glucose (a glycaemic response). This therefore rules out all feeds that contain starch, particularly those starch-rich feeds that have been cooked (steam flaked, micronized or extruded) because they are readily digested/hydrolysed in the small intestine with a rapid release of glucose. Equally, molassed products such as some straw/alfalfa chops are unsuitable for ECD animals. Sugar beet pulp also contains enough sugar to rule out its use in ECD diets. A major problem facing owners of ECD animals is what to do with them during the grass growing season? Grass may contain a lot of readily digestible water soluble carbohydrates (up to 25%) such as glucose, fructose and sucrose that can elevate blood glucose. Depending on the animal’s status it might be necessary to prevent access to grass altogether, use a grazing muzzle or, provide limited access. The latter is less effective since animals compensate time limitations by eating more quickly rather like at a breakfast buffet! Naturally it is essential to provide a source of forage and in this respect straw can be used or hay that is has been well-soaked with warm water (16C with agitation for 16 hours resulted in a water soluble carbohydrate [WSC] loss of 49%). It has been estimated that WSC content of forage for horses with ECD should be less than 10% and from the foregoing it will be apparent that this is not easily achieved. In both cases micronutrients will be in short supply so a daily vitamin/trace element should be fed. If animals are sweating a lot then electrolytes should be provided. Straw is of course low in protein. To help to try to retain muscle mass it is a good idea to provide quality protein and alfalfa would be a good source. Furthermore, it is relatively low in soluble carbohydrates and yet a good source of electrolytes. Additionally, low volume balancers are ideal for providing the nutrients often lacking in diets based on poor quality forages.
Dietary provision: The glycaemic response of a horse depends on a number of different factors but the size and nature of a meal has quite an impact on this response. Thus it is necessary to limit meal-size if any type of concentrate is to be fed. Clearly low soluble carbohydrate feeds should be used and if a feed contains as much as 10% starch (analogous to the 10% WSC of forage) and is fed to a 400kg pony then only 800g should be fed in order not to exceed a rate of 0.2g starch/ kg body weight. However, the problem with concentrate feed is that it can be consumed very quickly compared to a forage that contains 10%WSC. As a result, a glycaemic response is more likely. Regular small meals should be the order of the day. Of course, most owners are not in a position to feed many small meals a day so they are confronted with a serious moral issue: do I get rid of the animal or feed fewer meals and compromise the animal’s well-being? Perhaps an alternative strategy would be to feed dehydrated alfalfa that only contains ~5%WSC and ~3% starch (both as-fed). The advantage of so-doing is that alfalfa contains ~16% protein on an as-fed basis and this would help to counteract the occurrence of muscle wastage often seen in ECD animals. A problem often encountered with these animals is how to maintain body weight and this drives many owners to want to feed concentrates (potentially a risky business) in spite of the fact that dehydrated alfalfa contains ~10MJ Digestible Energy/kg. Thus, I would suggest top-dressing the alfalfa with oil to increase its energy density in an effort to overcome any problems.
In summary there are medical treatments for this disease but they are outwith the scope of this article. Progression of the disease is not halted but the animal’s quality of life can be improved provided the condition is not too advanced. Unfortunately, treatment is quite expensive and cannot be stopped and started at will so it involves quite a big commitment by the animal’s owner.