Hypertrophic Cardiomyopathy (Discovered in the Maine Coon)

Hypertrophic Cardiomyopathy (HCM) is the most common cardiac disease in cats worldwide. The disorder is characterized by an increase in heart wall thickness of the left ventricle and the interventricular septum. This causes turbulence in blood flow and increased venous pressure in the left atrium and lungs, which may or may not coincide with a cardiac murmur upon auscultation. The clinical signs most commonly noted are respiratory signs associated with congestive heart failure such as tachypnea, exercise intolerance and panting, difficulty breathing, and (rarely) coughing. Thromboembolism may also occur, and affected cats have an increased risk of sudden cardiac death. Clinical and echocardiographic signs typically appear after the cat has reached breeding age, as the most common age of diagnosis for HCM is five to seven years. In the Maine Coon, a point mutation (A31P) in the MYBPC3 gene has been found to be causative for the disease with a mode of inheritance that is best described as autosomal recessive (Boeykens, et al. 2024). Research indicates cats with two copies of the mutation (homozygotes) have significantly increased risk of developing HCM while no increased risk has been found in cats with only one copy of the mutation (heterozygotes). HCM has also been diagnosed in Maine Coons negative for A31P, thus indicating that this variant is not the sole cause of HCM in Maine Coons.

Cats that are at risk for developing hypertrophic cardiomyopathy should be examined at least once a year by a veterinarian, preferably a cardiologist, as HCM requires an echocardiogram (heart ultrasound) for definitive diagnosis and to understand the severity of the condition for the individual cat. As there is no curative treatment for the condition, therapy is directed at addressing disease sequelae. Clinical signs vary among affected cats and treatment should be optimized for the individual cat. Treatment options may include beta-blockers and medications for prevention of thrombi (clots). Should the disease progress to heart failure, medications for treatment of cardiac failure are indicated.

There are many responsibilities to consider when breeding cats. Regardless of test results it is important that your cat is in good general health and that you are in a position to care for the kittens if new responsible owners are not found. For first time or novice breeders, advice can be found at most cat registry websites.

This genetic variant is considered to most closely to follow an autosomal recessive mode of inheritance meaning that usually two copies of the variant are needed for disease signs to occur. Due to the high frequency of the HCM variant within the Maine Coon breed, cats with one copy of the disease variant may be considered for breeding with cats tested clear for the HCM variant in order to maintain genetic diversity within the breed. Approximately half of the kittens in a litter from this breeding will have no copies of the HCM variant and half will have one copy of the HCM variant. Kittens in a litter expected to contain carriers should be tested before breeding. Carrier to carrier matings are not advised as the resulting litter may contain kittens with two copies of the HCM variant. Please note: It is possible that disease signs similar to the ones caused by this HCM variant could develop due to a different genetic or clinical cause.