Because nature plays no favorites when it comes to doling out heart disease, cats — just like humans — are at potential risk for serious health problems directly or indirectly associated with this vital organ. In some cases, the signs of it are obvious to the trained eye. In others, a serious feline heart disorder can remain hidden for years before finally expressing itself in a sudden, perhaps fatal, attack.
Whereas the human heart is about the size of a clenched fist, its feline counterpart is, of course, much smaller — two inches or so in diameter. Despite its relatively tiny size, however, the feline heart is virtually identical in its complex structure to the human heart and has the same life-enabling function — to pump blood into the lungs, where carbon dioxide and other waste products are removed, and to subsequently pump refreshed, oxygen-rich blood throughout the body.
Congential diseases are rare
Most feline cardiac diseases are acquired during the course of an animal’s adult life; congenital disorders are diagnosed in only about one or two percent of kittens. The most common congenital disorders, says Suzanne Cunningham, DVM, an assistant professor of cardiology at Tufts, are either (1) a ventricular septal defect, which is a hole between the heart’s two lower chambers; or (2) mitral valve dysplasia, a malformation of the valve that separates the upper and lower chambers on the organ’s left side.
“These conditions will usually be picked up at a kitten’s first veterinary exam, when it is only one to two months of age,” says Dr. Cunningham. “In many cases, the kitten will have a loud heart murmur that will be evident to the veterinarian.”
When heard through a stethoscope, a heart murmur is a clearly audible, rhythmic, swishing sound caused by the turbulent rush of blood through the chambers of an animal’s heart; about one-third of cats will have a murmur at some point during their lives. “A murmur can occur as the result of either an acquired or a congenital heart disease,” notes Dr. Cunningham, “and it can be a totally benign phenomenon or a sign of significant cardiac disease.”
The prognosis for a kitten that is diagnosed with either a septal defect or mitral valve dysplasia, she says, varies from “grave” to “excellent,” depending on the severity of the disorder. “Some young cats with a large hole in the heart may be treated surgically,” Dr. Cunningham points. “And those with mitral valve dysplasia might benefit from medical treatment using beta-blockers or heart failure drugs. Many kittens with only mild defects can go on to lead normal lives. But if they have severe defects, they may go into heart failure, in which case the prognosis is not good.”
Among acquired feline cardiac disorders, some are rarely encountered by veterinary cardiologists. These include, for example, myocarditis (an inflammation of the heart muscle); myocardial infarction (the death of heart muscle tissue resulting from a blood clot); and a variety of valve-related diseases.
On the other hand, says Dr. Cunningham, heartworm disease is diagnosed in about 10 percent of cats these days, and should be considered as a significant threat to feline health, especially in geographic areas whose hot, damp climates encourage the proliferation of the mosquitos that spread the disease.
The most commonly diagnosed acquired cardiac conditions, however, are diseases of the heart muscle termed cardiomyopathies, which are rarely seen in kittens but are commonly diagnosed in mature cats. Cardiomyopathies are characterized by structural abnormalities in the muscle enclosing one or both ventricles, with the affected chamber — depending on the specific type of cardiomyopathy — taking on a thickened, dilated, or scarred appearance.
The left ventricle is almost always affected; right-ventricular involvement may also occur, but is rare. The most frequently diagnosed cardiomyopathy, says Dr. Cunningham, is hypertrophic cardiomyopathy (HCM), a thickening of the ventricular muscle tissue that accounts for the vast majority of all feline cardiomyopathies. (Other forms of the disease, which are far less common than HCM, include restrictive cardiomyopathy, which is caused by the abnormal buildup of scar tissue on the inner lining of a ventricle; dilated cardiomyopathy, which results from a thin-walled, poorly contracting left ventricle; and arrhythmogenic right ventricular cardiomyopathy, in which damaged heart muscle is gradually replaced by fat and scar tissue.)
Among the common consequences of HCM is a compromise of the heart’s blood-collecting and blood-pumping mechanics, a dysfunction that can progress to congestive heart failure — a collection of fluid in the chest cavity (pleural effusion) or in the lungs (pulmonary edema) that can cause acute respiratory distress.
Other potential consequences of this disease include the development of blood clots, which arise in the left atrium of the heart and can travel to the limbs and other sites, resulting in paralysis and even death. HCM most frequently affects male cats, although it is often diagnosed in females as well. And while genetic predisposition seems to put some breeds at elevated risk — Ragdolls and Maine Coons, for example — cats of all breeds are susceptible.
Efforts to diagnose suspected HCM will initially involve blood tests and a variety of measures that could include X-rays, an electrocardiogram and — probably most revealing — an echocardiogram. Treatment for the HCM and other heart conditions depends on the type and the severity of the disease. Depending on the diagnosis, medications may be prescribed that, for example, slow the patient’s heart rate, decrease the organ’s need for oxygen, reduce demands on the ventricles, help the heart muscle to relax, or inhibit the formation of blood clots.
Causes are being pinpointed
Considerable progress has been made over the years in identifying the precise causes of feline heart disease. Several decades ago, for example, researchers discovered that cats can acquire dilated cardiomyopathy from a dietary deficiency of an important amino acid called taurine. Consequently, pet food manufacturers began adding taurine to their products, and the incidence of the disorder immediately began to decline dramatically. Today it is rarely diagnosed.
Currently, says Dr. Cunningham, research at Tufts and elsewhere is focusing on blood clot formation and prevention; novel heart failure medications; dietary measures that may lessen a cat’s susceptibility to cardiomyopathy and other life-threatening feline heart ailments; and the identification of genetic mutations that might increase a cat’s susceptibility to cardiomyopathy. — Tom Ewing