Feline heart disease includes both congenital and adult onset (often termed acquired) disease. However, very little information regarding the actual inheritance of heart disease is known. There are important issues in terminology that should be understood before getting into any depth with these diseases. The term congenital refers to defects present at the time of birth, generally developmental defects. In the cat, these might include a ventricular septal defect or mitral valve dysplasia. Inherited defects are those that an animal is genetically programmed to develop. These defects may be present at birth (congenital) or may not become apparent until the animal is an adult (adult onset). Not all congenital defects are inherited and vice versa.

There is very little information available regarding the inherited nature of congenital heart defects. The most commonly reported feline congenital defect is the ventricular septal defect. These cases appear to be fairly sporadic in nature, rather than linked to a specific breed of cats.¹

The most commonly reported inherited feline heart disease is hypertrophic cardiomyopathy.This is also the most common heart disease diagnosed in the cat. ¹

Hypertrophic cardiomyopathy (HCM) is an adult onset, primary heart muscle disease. Occasionally other disease processes, like hyperthyroidism and systemic hypertension can lead to the development of hypertrophy (or thickening) of the walls of the left side of the heart. This can result in findings on an ultrasound or at autopsy that look similar to HCM. However, hypertrophic cardiomyopathy is a primary heart muscle disease that is not caused by other systemic diseases. These should be ruled out before your veterinarian makes a diagnosis of HCM.

The onset of HCM results in the development of thickening of the left ventricular free wall and interventricular septum. Eventually, this may lead to increased pressure in the chamber above the ventricle, the atria. Increased atrial pressure can result in the development of fluid in the lungs called pulmonary edema (congestive heart failure) which may make it difficult for the cat to breath properly. Some cats develop a blood clot in the heart that can break free and paralyze the back legs of the cat. Interestingly, not all cats progress in the same way. Some cats seem to have a fairly mild form of their disease that they tolerate for their entire life, while other have a rapid rate of progression that results in the development of heart failure and sometimes sudden death.

The diagnosis of HCM in affected cats is first suspected when a heart murmur or gallop is heard with a stethoscope by a veterinarian. Murmurs and gallops are fairly nonspecific sounds that suggest that cat may have heart disease. Further evaluation with a cardiac ultrasound (echocardiogram, echo) is needed to confirm the diagnosis.

Many owners who have heard of the inheritable nature of HCM in the cat have interest in screening their cats prior to breeding. It should be remembered that HCM is an "adult-onset" disease. This means that it may develop at some point in adulthood. Some cats develop a severe form of disease as a very young adult (8-12 months). This may be particularly true in some breeds of cats, perhaps Ragdolls. While other cats do not develop the disease until they are very mature adults (8-12 years). Therefore, it is very important to evaluate a cat every year to accurately screen. There are two possible methods for screening. The first is auscultation by a veterinarian. This is simply having a veterinarian listen for a heart murmur or gallop. If an abnormal sound is heard, the cat should have an echo performed. Although this is a quick, inexpensive method, it is likely to miss some affected cats. The second method is to have an echocardiogram performed on all cats, every year. This is more expensive, but is also more thorough. Echocardiography is a technique that is highly user dependent. Therefore, an individual that is not properly trained in echocardiography may over- or under- diagnose the disease. We generally recommend that veterinary cardiologists perform the screening whenever possible, to avoid misdiagnosis.

Hypertrophic cardiomyopathy has been shown to be an inherited disease in the Maine Coon and it is suspected to be inherited in the Ragdoll, Rex, American and British shorthair. Therefore, once a diagnosis of HCM had been obtained, affected cats should be removed from the breeding program. However, the mode of inheritance is not understood in these breeds. In the Maine Coon it is inherited as an autosomal dominant trait with variable pentrance.² This means that all affected cats will eventually show the trait, or disease (no silent carriers). However, since it has "variable penetrance" not all cats will have the same severity and some may have only very mild disease that can be identified by echocardiography.

Understanding the inheritance and potential development of a blood screening test for this disease in the different breeds is possible, but will require time and dedication. It is not likely that the disease will be the same in all breeds.

References

1.  Buchanan JW. Prevalence of Cardiovascular Disorders. Textbook of canine and feline cardiology. WB Saunders, Philadelphia, 1999. 457-470.

2.  Kittleson, M.D.; Meurs, K.M.; Munro, M.J.; et al. Familial hypertrophic cardiomyopathy in Maine Coon Cats: An animal model of human disease. Circulation 1999, 99: 102-107.