Understanding Boxer Cardiomyopathy

Used with permission from the Purina Pro Club Boxer Update, Nestle Purina PetCare Company

Breeders and owners of boxers often know firsthand how devastating a heart condition called cardiomyopathy can be. Dogs affected by the disease may succumb and die without warning, even during physical activity. 

Boxer cardiomyopathy (BCM) is a defect that shows no symptoms until an affected animal collapses. Some experts estimate that nearly 50 percent of the breed has this condition. While a few boxers do recover after they collapse, most do not. For this unfortunate majority, the first sign of BCM proves fatal.

Researchers have been working on two fronts to combat BCM: first, to determine whether it is an inherited condition, and second, to stop the disease without removing an excessive number of dogs from the breeding pool. Research led by Kathryn Meurs, D.V.M., Ph.D., associate professor of cardiovascular medicine at The Ohio State University, is aimed at finding the gene(s) causing BCM and learning whether factors such as diet and exercise contribute to its prevalence.

What is BCM?
To understand BCM, it is important to understand how the heart works. The heart is like a house with four rooms, or chambers. In the upper half of the heart are two chambers: the right and left atriums. The lower portion contains the right and left ventricles. These four chambers work together to pump blood through the body.

Blood enters the heart at the right atrium, where it is stored briefly before being pumped down into the right ventricle, which in turn pumps blood into the lungs. In the lungs, the blood receives oxygen before flowing back to the heart, this time to the left atrium. The blood stays in the left atrium for a few seconds before being pumped down into the left ventricle. From there, the blood is pumped throughout the rest of the body.

Valves regulate the flow of blood to help the heart work efficiently. These valves, located between the atrium and ventricle on each side of the heart, serve as doors between the two areas. When either atrium fills with blood, the valve between the atrium and the ventricle closes to prevent the blood from entering the ventricle prematurely. When the valve opens, the blood passes down to the ventricle; once the blood reaches the ventricle, the valve closes again to prevent blood from flowing back up into the atrium.

This movement of blood results from electrical impulses that are transmitted from the brain to the heart. The impulses not only direct the heart to beat in the first place, but also enable the heart to maintain a steady, regular rhythm. In BCM, however, the electrical impulses are disturbed, and the heart beats erratically, which is an arrhythmia.

Although there are several kinds of arrhythmias, the most common in BCM is called premature ventricular contraction (PVC). Instead of beating in a steady, regular rhythm, the ventricle contracts earlier than it should.2 The result is a heartbeat that does not have a corresponding pulse, unlike the normally functioning heart, where there is a pulse for every beat. Some experts characterize the malfunction as a stutter.

A boxer with only occasional PVCs probably will not show any symptoms of heart disease. But if multiple, successive PVCs occur — what experts call a “run” of PVCs — the heart cannot produce a normal, effective contraction, which results in a stoppage of blood flow to the brain and other vital organs, causing a boxer to collapse. A prolonged run of PVCs can lead to complete cardiac arrest and death, unless the heart resumes a normal pattern of contractions.

Sometimes a lack of adequate contractions causes a boxer’s heart to swell to compensate for the inefficient pumping function. This results in congestive heart failure. Symptoms include coughing, labored breathing, lethargy and abdominal swelling.

Dealing with BCM
Clearly, the key to helping a boxer with BCM is to diagnose the disease before symptoms develop. This can be challenging because some standard tests designed to detect heart irregularities are ineffective at unmasking BCM.

For example, an electrocardiogram (ECG), a three-minute test conducted when a dog is resting, is designed to measure electrical activity in the heart. While an ECG can be very useful in detecting most arrhythmias, it is not helpful at finding PVCs in boxers. For many boxers, the arrhythmia is intermittent and may not occur during a standard ECG. Thus, cases of BCM may be missed completely if diagnostic testing is confined to a resting ECG. 5

The same is true with an echocardiogram, an ultrasound of the heart. Although cardiac ultrasound can help determine whether the heart is contracting properly and detect changes in the heart muscle itself (e.g., thickening or thinning), it is not effective at detecting arrhythmias such as PCVs unless they occur frequently. Consequently, as with the resting ECG, a boxer with BCM is likely to have a normal echocardiogram.

For these reasons, the best way to evaluate a boxer for BCM is to use a portable ECG called a Holter monitor. The device, which is the approximate size of a Sony Walkman®, is taped along with several wires and electrodes onto the boxer’s body.6 The boxer then spends 24 hours performing normal activities while the Holter monitor records the heart’s activity. This longer testing period usually is sufficient to detect both single PCVs and the PCV runs that indicate that a boxer is at a relatively high risk for collapsing or even dying from BCM.

If the Holter monitor test indicates that the Boxer is at high risk for collapse or death from BCM — or if the dog already has suffered one or more episodes of collapse — treatment is likely to focus on prescribing medications that help the heart to resume a normal rhythm. Some veterinarians have found that Betapace® is successful in treating boxers that have aggressive PCVs. However, the drug should be prescribed only after a full cardiac workup, and a dog needs to be monitored closely while on Betapace® therapy.

On the Research Front
Future prospects for early BCM screening will improve significantly when scientists understand how the disease is transmitted between generations and can identify the specific gene or genes that cause BCM. Meurs and her research team at The Ohio State University have received a grant from the American Kennel Club (AKC) Canine Health Foundation to decode the inheritance patterns and perform molecular genetic evaluations of boxers.

So far they have determined that the ventricular arrhythmias that lead to sudden death in boxers are inherited as an autosomal dominant trait. That means that affected individuals occur in every generation, that the disease affects approximately as many males and females, and that two affected parents can produce an unaffected female dog.  In addition, Meurs has compiled data on three large families of Boxers with ventricular arrhythmias and is performing genetic evaluations of the dogs in those families. This information will provide important clues in the hunt for the specific gene or genes that cause BCM.

With a second grant from the AKC Canine Health Foundation, Meurs is evaluating the progress of boxers that have ventricular arrhythmias such as PVCs but do not show symptoms. Over the next three years, she’ll follow the progress of 150 boxers that are more than 4.5 years of age and that have had more than 50 PCVs during a 24-hour period while on a Holter monitor.

She’ll evaluate the dogs every year through an owner questionnaire, physical examination, Holter monitor examination, electrocardiogram and echocardiogram. At the end of the evaluation period, she’ll perform a statistical analysis to see if there is any relationship between the likelihood of collapse or heart failure related to factors such as diet, PCV occurrence, heart rate, and exercise. With this study, veterinarians may be able to evaluate more precisely which dogs with PCVs are at risk for collapse and death from BCM.

Ultimately, eradicating BCM is up to breeders. Determining an appropriate breeding protocol can be difficult because so many boxers experience at least some of the PCVs that characterize the disease. Removal of all boxers with PCVs from breeding programs could reduce the breeding pool to dangerously low levels that would leave the remaining individuals more vulnerable to other genetic diseases.

Steps to Avoid Boxer Cardiomyopathy
Kathryn Meurs, D.V.M., Ph.D., associate professor of cardiovascular medicine at The Ohio State University and a leading expert of boxer cardiomyopathy, recommends that breeders adopt the following practices to try and avoid the disease in their breedline:

  • screening of adult boxers by Holter monitoring;
  • removing symptomatic boxers (e.g., a boxer that collapses) from a breeding program;
  • removing asymptomatic boxers from a breeding program if they have
  • produced offspring that either exhibit symptoms or died suddenly; and
  • using sound reasoning when a dog with just a few PCVs is under consideration for breeding. If the dog has other quality traits and is from a line that is known for its longevity, breeding may still be possible. 

Important Update: Vet Cardiologist Discovers Mutant Gene For Cardiomyopathy