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Equine Cushing's disease is a fairly common term in the horse industry.  That's because 1 out of 5 horses over the age of 15 have the disorder.  But for how common Cushing's is in horses, there remains a lot of misunderstanding about this condition.  In this blog, we'll try to make things a little more clear!

Note: 
While Cushing's disease can contribute to other metabolic disorders (such as insulin resistance), we're going to focus solely on Cushing's disease for the purpose of this post.   We'll tackle insulin resistance in another blog post!

The term Equine Cushing's Disease was coined for the similarity to the syndrome in humans and dogs.  (Cushing's disease in humans was originally described by a neurosurgeon at Johns Hopkins University named Harvey Cushing.)  In humans and dogs, Cushing's is most commonly caused by either 1) an adrenal tumor or 2) a tumor in the anterior lobe of the pituitary gland which causes hyperplasia (excessive growth) of the adrenal gland.   Both of these cause an increase in cortisol (a steroid).  In horses, Cushing's is primarily due to a tumor in the intermediate lobe of the pituitary gland, but there is no associated hyperplasia of the adrenal gland.  That's why the technical name of it is Pituitary Pars Intermedia Dysfunction (PPID).  Whew!  If you thought that was scientific, just wait!

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So what exactly is going on to make this disease such a concern?  Well, as the pituitary tumor grows, it secretes a hormone called ACTH (Adreno-Cortico-Trophic Hormone.  Yes, it's a mouthful!)  This hormone signals the adrenal gland to secrete cortisol, also known as a "stress hormone".  The chronically increased level of steroids in the body wreaks havoc on the balance that the system usually tries to attain.  This is what leads to the common signs of PPID. 

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The most common sign of Cushing's is excessive hair growth and inability to shed out appropriately (known as hirsutism).  This can affect up to 80% of horses with Cushings.  Other things we often see are lethargy, weight loss/muscle wasting, recurrent infections (such as hoof abscesses) and chronic laminitis.  If we see horses with these signs, we may recommend further testing.  However, in some instances, clinical signs are obvious enough, and we will merely recommend to start treatment. 

Testing for Cushing's can be done in a few different ways, but they mainly focus on circulating levels of ACTH.  You might say, "Well, if cortisol levels are increasing, why don't we just test for that?"  Glad you asked!  In the past, horses with PPID have shown cortisol levels that are either increased, normal or decreased.  Cortisol fluctuates throughout the day and from one individual to another, so it is very difficult to make an accurate assessment of any disease process by associating with steroid levels. 

So on to the tests we do use!  The first (which we typically rely on) is to test the resting ACTH levels.  This is simple enough - if the measured level of ACTH is above or below a certain threshhold, the horse is positive or negative, respectively. Caution must be used with this as there is a 3-fold increase in the fall, but this has been measured in normal horses, so we can adjust values for that.  The second test is known as a Dex Suppression Test, whereby a blood sample is taken, then a small amount of dexamethasone (a steroid) is administered.  A second blood sample is taken approximately 20 hours later.  This test should cause a significant drop in the amount of ACTH present.  If it does not, then the horse is positive for Cushings disease.  However, this test can be concerning as we are giving steroids to a horse with possibly high circulating levels of steroids.  Since excessive levels of steroids can rarely cause laminitis, we reserve this test for specific cases.
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If results of the resting ACTH are not significantly elevated, we can do an additional test to rule the disease in or out.  This is called a Thyrotropin Releasing Hormone Stimulation test (TRH Stim for short).  This test is similar to the dex suppression test in that a sample is drawn, the TRH is administered, and a second sample is drawn 10 minutes later.  Again, we will be looking at ACTH levels and how much they rise after administration of the hormone.  This gives us a very reliable test to determine the presence or absence of Cushings disease, without the added risk of giving Dexamethasone to a potentially laminitis-prone animal. 

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If we have successfully diagnosed Cushings disease, or just suspect it based on clinical signs, treatment includes a combination of medical therapy as well as management.  Currently, the only approved drug for Cushing's Disease is Prascend® (pergolide).  This is a daily medication that will need to be given for the remainder of the horse's life.  This drug aims to reduce the amount of circulating ACTH, thereby decreasing cortisol levels.  We often see a quick turnaround with horses once we start them on Prascend - owners will often comment that their horse seems to have "dropped 10 years" a month or so after treatment starts.

For a long time, pergolide was only available as a compounded formulation.  Now that we have an FDA approved form, we don't recommend or even dispense the compounded drug.  This is because studies have shown that compounded pergolide is not stable and does not have a very long shelf life.  The concentration of compounded drugs is also quite questionable and is not always what the label states.  For more on the discussion of Prascend vs. compounded pergolide, you can read our blog from when Prascend was first introduced.  Check out this study published in the Journal of the American Veterinary Medical Association on stability of compounded pergolide. In addition, compounded pergolide is no longer allowed under FDA rules.  

The other aspect of treating Cushing's is management.  Since Cushing's disease will often lead to insulin resistance (again, more on that in another blog!), it is usually important to control the intake of starches and sugars in the diet.  These horses should obtain most of their calories from low-starch hay and fat (since fat is a much safer form of calories than starch/sugar for horses with metabolic conditions).  An appropriate diet should be outlined with your veterinarian for best results.  Appropriate foot care is important, to reduce the incidence of hoof abscesses and laminitis.  Finally, horses with Cushing's disease should always be managed more carefully with regards to wounds, parasite control - their immune system is often not 100% normal due to the effects of cortisol, so they will always be prone to chronic infection, non-healing wounds, or higher parasite burdens.  Your veterinarian can help you come up with a plan to reduce these risks.

While Cushing's disease is common and incurable, it can be successfully managed.  Through owner and veterinary diligence, we can help keep these horses happy and relatively healthy for many years. 

 
 
In our last post, we discussed (mainly) non-surgical types of large intestinal colic.  However, as we all know, sometimes colic goes beyond what we can treat in the field and referral for surgery is necessary.  Typically, these result from the colon moving (or displacing) into a place that it shouldn't be.   That being said, some of these can be treated medically, however not always successfully.  Since we've already gone over the anatomy of the large colon, (See Large Intestinal Colic: Part One) let's jump right into what might have happened if your horse's bowel becomes blocked.
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Right Dorsal Displacement: The left colon is quite mobile and likes to get stuck in places.  In a right dorsal displacement (RDD), the left colon slides around to the right side of the abdomen and gets stuck between the cecum and the body wall.  The pelvic flexure (the U-shaped area at the top of the picture) moves from the back end of the abdomen (near the rectum) to end up near the diaphragm.  This is all kinds of crazy!  The large intestine becomes partially occluded and gas distended.  In practice, we can see this occur in conjunction with a pelvic flexure impaction.  Generally speaking, the blood supply remains intact so the bowel remains healthy.  We can sometimes treat these with fluids and withholding feed, but often times surgery is required.  At surgery, the colon just needs to be decompressed and repositioned into the appropriate anatomic location.  As long as nothing more serious is going on, these horses recover quite well. 


 
 
Previously, we have discussed small intestinal colic and gastric ulcers.  However, when discussing colic, we have to determine if it is small intestinal in origin or large intestinal. 
Below is a brief list of problems associated with the large colon.  Typically, these are non-surgical and can be resolved with some medical management.  That's not ALWAYS the case, but for the most part it is.  (Part 2 will focus on more severe types of large intestinal colic.)


 
 
Colic is a scary word to horse owners and veterinarians.  However, some types of colic are much worse than other types.  While 90% of colic cases resolve with minimal treatment on the farm, some require more intensive care.  One such type of more serious colic  is small intestinal strangulation, which is an obstruction of the blood supply and the lumen (the inside of the tube, where food passes through) of the small intestine. 
GI Anatomy1. Stomach 2. Small intestine 3. Cecum
The horse's gastrointestinal system is quite lengthy - it spans approximately 120 feet, and the majority (70-80 feet) of that is small intestine.  Furthermore, it's mostly floating freely in the abdomen, with minimal attachments to the body wall and surrounding organs.  This conformation gives lots of opportunities for something to go wrong, whether that's a twist, or some other way that the intestine ends up in the wrong spot. 
 
The small intestine (or SI, for short)  is the same in horses as it is in dogs, cats or humans.  It's made up of 3 segments - the duodenum, the jejunum and the ileum.  The duodenum is the first section, just after it leaves the stomach and for about the next 12-16 inches (that's it - so, not much goes wrong with the duodenum!)  The jejunum makes up the bulk of the small intestine, and the ileum comprises the last foot or so before it enters the cecum.  There are minor variations in these structures, but we don't have to worry about them here.  All this SI is suspended within the abdomen by a sheet of connective tissue called "mesentery".  (If you laid a garden hose straight across a towel, then picked the towel up at all 4 corners it would create the same effect.)  The mesentery brings blood supply to the intestines. 


 
 
The equine digestive tract is always a concern for horse owners - and veterinarians!  Colic can be caused by many different things, and one possibility is EGUS - Equine Gastric Ulcer Syndrome.  Gastric ulcers are actually quite common in horses.  There are a variety of reasons for this, but have no fear!  While they are common, they are rarely life-threatening and can be cured with appropriate treatment.
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First, let's start with an anatomy overview.  The equine stomach is divided into two sections - the non-glandular (or squamous) section, and the glandular area.  The line dividing these areas is known as the "margo plicatus".  Stomach acid is secreted by the glandular area.  We typically see ulcers along the margo plicatus on the squamous side of the stomach, but we can also see ulcers throughout the non-glandular and glandular regions.  Oh yeah, and just to be clear, gastric ulcers are erosions in the normal lining of the stomach.  Pretty simple so far, right?


 
 
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The eye is a very delicate organ, and horses specifically have eyes which are in a very precarious position.  In order to allow for a nearly 360 degree view around them, horse eyes are placed well on the outsides of their heads.  This anatomic location combined with a horse's normal "flight" response makes eye injuries quite common.  One of the most common things we see are corneal ulcers. 


 
 
Every so often it happens – your horse just doesn’t seem right and you need to call the vet.  When you do, it’s important to have as much information as possible so your vet can get a good picture of what’s going on.  Here are five of the most common questions we ask horse owners – know these, and you’re one step ahead of the game!


 
 
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With the warm fall we've been enjoying, it may be hard to remember that winter is just around the corner.  Soon enough, however, we'll be battling snow and frozen ground as we take care of our equine companions.  It's important to put some thought and time into preparing your horse and your property before winter sneaks up on us completely.  We've outlined a few things to keep in mind to help this transition time be as seamless as possible. 


 
 
Recently, mosquitoes infected with EEE (Eastern Equine Encephalomyelitis virus) were found in Voluntown, CT.  Not only that, but there have been 2 confirmed cases of EEE positive horses in Massachusetts.  While it would be nice to hide from it, the fact is that it is in our backyard.  But what do we really know about it? 
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USDA map Oct. 2012
EEE is mainly found in the US east of the Mississippi river, and throughout areas of Central and South America.  The virus persists in “reservoirs” – wild animals that carry the disease such as bats, rodents, and birds.  A vector (such as a mosquito) becomes infected when it feeds on one of these animals.  Most often, EEE is maintained through a transmission cycle between birds and mosquitoes.  The mosquito then carries the virus for life and can transmit it through its saliva.  If a horse happens to be the source of the next blood meal, then they can become infected and quickly begin to show signs. 

Once infected, it takes approximately 5-15 days for signs to show up.  These signs can be very mild such as a fever and depression; or severe including blindness, stumbling, seizures or death.  Any neurologic signs could be indicative of EEE, however other diseases such as rabies must be considered as well, and a diagnosis should be confirmed via testing.  Unfortunately, a diagnosis is often obtained post-mortem.  If EEE is suspected, the state veterinarian must be contacted and the disease needs to be reported.


 
 
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When it comes to the equine eye, we often discuss common diseases such as corneal ulcers, conjunctivitis or uveitis.  We do treat these conditions quite a bit, however there are some  serious uncommon diseases that we do have to think about when considering an inflamed eye.  One of these diseases is glaucoma, which can be set off by any of the above listed issues, or by another underlying problem.


 

Twin Pines Equine Veterinary Services, LLP. All rights reserved.
Twin Pines Equine Veterinary Services, LLP