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Heart Disease in Horses

Reprinted from Morris Animal Foundation


When we think of companion animals, it’s easy to focus on dogs and cats and forget the many horses that provide companionship and love to their owners. And, while many diseases are unique to horses, one problem they share with dogs and cats (and people) is heart disease.


To help learn more about horse heart health, we reached out to experts at the University of California, Davis. Dr. Josh Stern, a Foundation-funded, board-certified cardiologist and Dr. Jessica Morgan, a board-certified specialist in equine sports medicine and rehabilitation, sat down with us to answer some questions about horse heart disease, diagnosis and treatment.


What are the most common heart diseases in horses?

Dr. Stern - We see all types of heart disease in horses. I think most problems are either due to valve degeneration, affecting mainly the mitral and aortic valve, or related to atrial fibrillation, a type of arrhythmia.


Dr. Morgan – When I examine a horse, the most common murmurs I hear are systolic murmurs (which are heard during the heartbeats) associated with mitral valve degeneration or diastolic murmurs (which are heard when the heart rests between beats) associated with aortic valve degeneration.


Are horses that get heart disease typically middle-aged or elderly horses?

Dr. Morgan – There are a couple of groups. There are young performance horses, like Standardbreds that experience atrial fibrillation at the peak of their career, from 3 or 4 years of age to 10 or 12 years.


When we’re talking about heart-valve related disease, we see it more commonly in horses that are in their 20s to 30s. Of course, congenital heart problems are seen in foals.


To start, what are the most common clinical signs in horses with heart disease noted by owners?

Dr. Morgan – The signs I hear most commonly from owners point to two kinds of problems – either fulminant heart disease, where owners report a cough and ventral edema or swelling of the abdomen. Alternatively, owners have poor performance complaints, or their horses are seen for routine care and their vet notices a murmur or irregular heartbeat – these are less fulminant.


Are there breeds more commonly affected by heart disease?

Dr. Stern – Much like dogs and cats, there are breed-related predispositions of some issues; for example, Standardbreds have a high incidence of atrial fibrillation (a type of irregular heartbeat) that’s been shown to be heritable, and ventricular septal defects (a hole in the wall that separates the two lower chambers of the heart) are seen in Arabian foals.


Do horses get heart muscle diseases?

Dr. Stern - Cardiomyopathies, or diseases of the heart muscle, outside of toxin-related, are rare in horses compared to dogs and cats.


Let’s circle back and talk a little more about atrial fibrillation. What are common signs owners and veterinarians might note in a horse that has atrial fibrillation?

Dr. Stern - Atrial fibrillation in the horse is quite different than what we think about in small animal species. In dogs and cats, atrial fibrillation is almost always associated with significant structural heart disease, while in horses, it is primarily the arrhythmia that is the issue.


Atrial fibrillation has at least moderate heritability across a lot of horse breeds and we know it will limit performance in many instances. It’s not that it is associated with horrible underlying disease, but it does limit performance.


The challenge is that converting a horse back to a normal rhythm isn’t easy, and the way that we do that is not entirely benign. For example, during the process of using drugs to convert the heartbeat back to normal, they can go into colic. Another method uses electro-cardioversion by catheters which is a major procedure that’s limited in its availability.


There are some horses that get atrial fibrillation in conjunction with structural heart disease, and they follow the more traditional path we see in small animals. They have structural heart disease and arrhythmia and those two aren’t helping each other. In those cases, we try to convert them to a normal rhythm, just like you would a dog.


Are there any statistics on how many horses have atrial fibrillation?

Dr. Morgan – That’s a tough question because although there are some publications that focus on racehorses, the incidence in the broader population is hard to estimate since the only papers out there look at horses who are brought to referral hospitals. It’s a pretty common problem, and since many horses don’t show signs of disease, there are lots of horses that likely go undiagnosed and live perfectly normal lives.


I’ve known lots of successful pleasure horses and hunter-jumpers that can do their job in atrial fibrillation without issue. Once you get into more athletic pursuits, and an individual that has to race at speed or endurance, upper-level eventing, any of those disciplines, they can’t perform their job up to expectations in atrial fibrillation. There is a middle ground where some horses can and some horses can’t perform and then you get into issues of safety. Draft horses, in particular, tend to be less exercise tolerant in atrial fibrillation.


How is heart disease diagnosed in the horse?

Dr. Stern - We can do most of the same diagnostic tests that are done in dogs and cats, but it’s accessibility that can be a challenge. It’s easier to pop your cat in its carrier and drop it off at the cardiologist than getting a horse to a clinic. In addition, not all veterinary cardiologists in practice offer services in equine cardiology.


Dr. Morgan - You can do cardiac ultrasound just fine. We do EKGs and exercising EKGs a lot, you can do 24 EKG or Holter monitoring which we also do fairly frequently.


Dr. Stern - We do chest radiographs. They’re more abstract than small animal radiographs, but chest radiographs are certainly possible. Other cardiac diagnostics include routine cardiac biomarker testing like measuring troponin 1, a protein in the blood that is indicative of heart muscle damage.


Are the same treatment options available for horses that are used in dogs and cats, or are there specific differences?

Dr. Stern - There are a lot of special considerations in horses when it comes to treatment, and many of them have to take into account that while some horses are companion pets that just hang out in a pasture, a lot of horses are expected to return to work or be safe in certain environments, such as if horses are around children in a barn.


These are risks we have to consider, so risk mitigation strategies become something we think about in horses that we don’t typically think about in small animals.


In addition, return to performance is a big consideration and something we need to think about in horses that’s somewhat unique relative to dogs and cats.


There also are challenges in pharmacology in horses that render some of the drugs, that normally would be quite useful in dogs and cats, less useful. Some drugs in horses have faster clearance rates or lower efficacy. With others, there are challenges in administering the dose you need because it’s a massive number of pills!


Can you give an example of a drug or drugs commonly used to treat heart disease in small animals that you can’t or don’t use in a horse?

Dr. Morgan - Pimobendan and ACE inhibitors.


Wow, those are really common drugs we use in small animals. What drugs are used for horses?

Dr. Stern - Lasix, a diuretic also used to treat heart problems in small animals, is a mainstay of heart failure therapy in horses.


Dr. Morgan - Lasix is a very effective rescue drug for horses in heart failure.


It also is used to treat exercise-induced pulmonary hemorrhage in horses without heart disease. In those horses, it does affect potassium levels, so there is some discussion that it may be a risk factor for developing atrial fibrillation by causing electrolyte disturbances.


We couldn’t help horses without experts.

We are thankful for the many experts, like Drs. Stern and Morgan, who make sure that our companion animals are heart healthy. But they couldn’t do their work without support and health research.


Morris Animal Foundation has been a leader in funding research focused on all aspects of equine health since 1959. We’ve invested in studies looking at all aspects of equine health, from infectious diseases such as Potomac Horse Fever and salmonellosis to equine herpesvirus. We’ve funded studies on better ways to transport horses and understanding the complexities of genetic diseases.


But we fund heart health, too. Our current grant portfolio includes a recently funded study on the genetics of atrial fibrillation in horses. The study team hopes to pinpoint genetic markers that could be used to develop a diagnostic test for this important disease, which could save the lives of countless horses.


Learn more about our current and past horse health studies and find out what you can do to help improve the health of horses around the world!

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