Hypertrophic Cardiomyopathy (HCM) is the most common cardiac disease in cats worldwide. The disorder is characterized by an increase in 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 with the disease 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 causative for the disease with a mode of inheritance that is autosomal dominant with incomplete penetrance. Cats with two copies of the mutation (homozygotes) have earlier onset and increased severity of the disease compared to those with only one copy of the mutation (heterozygotes). The relative risk for developing HCM is around 1.8 in Maine Coons heterozygous for the mutation and 18 in Maine Coons homozygous for the mutation compared to Maine Coons negative for the mutation. HCM has also been found in about 5.4% of Maine Coons negative for A31P, thus indicating that this mutation 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 guided to 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 disease is autosomal dominant with incomplete penetrance meaning that one copy of the variant is needed for disease signs to occur. However, the relative risk of disease development in cats with two copies of the variant is ten times that of a cat with one copy of the variant. 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. However, it is important to note that cats with one copy of the variant will have a slight increased risk of disease development in comparison to cats with zero copies of the variant. Please note: It is possible that disease signs similar to the ones caused by the HCM variant could develop due to a different genetic or clinical cause.