Download Endocardiosis in the Toy Dog - Japanese Chin Club of America

Endocardiosis in the Toy Dog
by Dr. Leanne Bertani
Atrioventricular Endocardiosis, or Myxomatous Valvular Disease, is a
chronic degenerative disease affecting the mitral and tricuspid valves of the
heart. It is the most common cardiac disease in small-breed dogs, and has a
hereditary predisposition. The condition is caused by polysaccharide
deposits in the valve leaflets that eventually distort the valves and cause
them to leak. The leaks cause the heart to work less efficiently, and the
increased workload may eventually lead to congestive heart failure.
The incidence of endocardiosis is estimated to be about 35% in all
dogs over the age of 12, and about 5% of dogs in middle age.
Unfortunately, the condition appears to have a greater incidence and an
earlier onset in some dogs, particularly in toy breeds such as the Cavalier
King Charles Spaniel, where 50% may have murmurs due to endocardiosis
by the age of five. Other related breeds, such as the English Toy Spaniel and
the Japanese Chin, are particularly at risk for increasing prevalence of earlyonset endocardiosis if careful breeding practices are not followed. In breeds
with small gene pools, it only takes a few affected “popular sires” to
significantly alter the genetic make-up of the breed. The early-onset form
of endocardiosis may drastically shorten a dog’s life expectancy -- deaths
due to endocardiosis have been reported in dogs as young as four years old.
The incidence of endocardiosis is also high in the miniature poodle,
miniature pinscher, whippet, schnauzer, yorkshire terrier, dachshund, cocker
spaniel and chihuahua, but usually at an older age. Some large breeds, such
as German Shepherds and Afghans, may also be affected, usually in
association with other cardiac conditions. In some studies, males are more
affected than females by a ratio of about 1.5 to 1.
Early-onset Endocardiosis and How to Prevent It
For now, the only way to prevent early-onset endocardiosis is by
selective breeding. The condition has received the most scrutiny by the
Cavalier community, as about 50% of Cavaliers in the United States have
murmurs due to mitral valve insufficiency by the time they are five years
old. It has been demonstrated in several studies that the tendency towards
early-onset endocardiosis is inherited, and that its prevalence can be reduced
by using older sires and dams that are free from disease.
One protocol, developed by a panel of cardiologists and geneticists at
the request of the Cavalier King Charles Spaniel Club, USA, suggests using
only the top 50% for breeding stock. For cavaliers, that would mean
breeding only animals that are at least 5 years old and heart-cleared by a
cardiologist. Knowing that it is not very practical to delay breeding a bitch
for five years, a compromise protocol was suggested. The compromise
protocol is to breed only animals that are at least 2.5 years old, that have
parents that are at least 5 years old, all of which are certified by a
cardiologist as being murmur-free. Some cavalier breeders are presently
working toward improving the longevity of their stock using this
compromise protocol. There has been some difficulty in implementing the
protocol on a wide scale, due to the limited breeding stock that meets the
protocol. It is sometimes difficult to have sires and dams of the breeding
animals certified by cardiologists, particularly when they are in the hands of
different owners, or not even in the same country.
Some breeders are attempting to achieve similar results by loading
their pedigrees with long-lived animals. Breed clubs and individuals have
begun to maintain registries of long-lived dogs; there are registries of
Cavaliers living to the age of 14, and of Japanese Chin living to the age of
12. Breeders are encouraged to keep longevity in mind when considering
matings.
At the very least, fanciers of toy breeds should ask their veterinarians
to pay special attention to the cardiac exams of their breeding animals,
particularly the mitral valve area, and avoid breeding any dog that has onset
of a mitral valve murmur prior to the age of five. In breeds predisposed to
early onset endocardiosis, yearly examination by a board-certified
cardiologist of all breeding stock over the age of 2 ½ is preferable, as is
registration of those exams with the Orthopedic Foundation of America’s
cardiac registry.
Diagnosis
Endocardiosis is sometimes referred to as “Mitral Valve Disease” or
“MVD” by those in afflicted breeds, because the mitral valve is more
commonly involved than the tricuspid valve. While there are other
conditions that affect the mitral valve, a mitral valve regurgitation murmur
in a toy breed dog is most likely due to endocardiosis. Therefore, the most
common method of diagnosis of endocardiosis is by auscultation (listening
with a stethoscope) for regurgitation murmurs of the mitral and tricuspid
valves. If the origin and nature of the murmur is uncertain, sometimes an
echocardiogram is done to verify the murmur’s location.
Unfortunately, a dog may be affected for a couple years before the
regurgitation is significant enough to produce a murmur, so the practice of
using auscultation for diagnosis often delays identification of affected dogs
until after they have already been used in a breeding program. There is
some work being done to attempt to identify affected breeding stock at an
earlier age than is possible by auscultation. Canadian researchers have been
studying the use of color doppler as a means of early identification of
endocardiosis. It is hoped that the disease can be diagnosed by
echocardiogram at an early stage, either by measuring the degree of mitral
valve prolapse or the degree of regurgitation.
Although there are some abnormalities that may occur in the
electrocardiogram (EKG) of a dog with advanced endocardiosis, the EKG is
often normal, particularly in the early stages of the disease, so an EKG is not
a very useful screening method.
The use of DNA identification has also been entertained, but
diagnosis via DNA is not likely close at hand, because the heredity of earlyonset endocardiosis appears to be polygenic (due to more than one gene).
There is one study that suggests that dogs with more severe disease have a
greater number of endothelin receptors in their valve leaflets, but it is not
practical to do mass screenings for that trait at this time.
Treatment
Once diagnosed, it is recommended that the dog have a chest x-ray
every 6-12 months, to monitor for enlargement of the heart. Affected dogs
should be kept trim and should not have a salty diet. Mild exercise is
encouraged, but animals that are in heart failure should avoid sudden,
explosive exercise or any activity that induces shortness of breath.
At this time, medication is generally not recommended until the dog
becomes symptomatic (shortness of breath, coughing) or has cardiac
enlargement. Though there are some cardiologists who feel that earlier
treatment (as soon as a murmur is heard) might increase longevity, it has not
been proven. Studies are ongoing.
The most commonly used medications in symptomatic dogs are the
ACE inhibitors, such as enalapril (Enacard, Vasotec), or Fortekor, which
decrease the workload of the heart. When the dog begins to have some heart
failure, additions to the regimen may include diuretics such as furosemide
(Lasix), or medications to strengthen the heart beat, such as digoxin
(Lanoxin). These drugs should be used with caution if the dog has kidney
disease, and the veterinarian may want to check bloodwork on a regular
basis to monitor for kidney failure.
Although surgical correction of the valve has been done, the
procedure is far from routine at this time.
Complications
When the heart enlarges, it may compress the bronchus to the lung,
causing coughing and wheezing. The enlarged heart may lead to congestive
heart failure, and the back-up of fluid into the lung, interfering with
oxygenation, and causing shortness of breath. The congestion may also
cause abnormalities in kidney or liver function.
Because endocardiosis distorts the valves, it can also be complicated
by endocarditis, a bacterial colonization of the valve that can cause
septicemia (blood poisoning) and further damage to the valve. For this
reason, dogs with valvular disease are often required to take antibiotics prior
to surgical procedures or dental work.
An enlarged heart is susceptible to arrhythmia, irregularities in the
heartbeat due to disruption of the heart’s electrical system. One symptom of
this is syncope (fainting). If this occurs, the veterinarian may have to do an
electrocardiogram and prescribe additional medication to control the
arrhythmia.
Another complication is rupture of the chordae tendonae. These are
the struts that hold the valves in place. Rupture of the tendonae may totally
disrupt the path of blood flow in the heart, sometimes even causing sudden
death.
Prognosis
The prognosis of an individual dog is difficult to predict, because the
disease seems to progress faster in some dogs than others. Generally, the
earlier the onset of a murmur, the lower the life span. But there are many
exceptions to the rule, and some dogs that had mitral regurgitation murmurs
at the age of four have gone on to live to a ripe old age.
The best treatment is prevention. Choose your breeding stock
wisely. With every breeding, keep in mind that many of the future offspring
will likely end up in someone’s home as a beloved family member,
hopefully living a long and happy life.