Cardiomyopathy is a primary myocardial disorder that changes the structure and function of the heart muscle in the absence of other disease sufficient to cause the observed myocardial abnormality. Until Dr Joon Seo’s recently completed research, there were no studies evaluating the prevalence of cardiomyopathy in cats in New Zealand. However, studies of cats in the UK and USA suggest that approximately 15% of clinically healthy cats in the general population have cardiomyopathy that is detectable using echocardiography.
Several types of cardiomyopathy have been described in cats. Of these, hypertrophic cardiomyopathy (HCM) is most common, constituting > 94% of apparently healthy cats diagnosed with cardiomyopathy. The clinical definition of HCM is left ventricular concentric hypertrophy caused by a genetic abnormality or an unknown aetiology. Therefore, HCM is diagnosed by ruling out other diseases such as hyperthyroidism and systemic hypertension, which can mimic HCM. Cats with HCM can remain asymptomatic for years (i.e. have subclinical disease) after diagnosis and may live a normal lifespan. However, the disease may progress in some cats and result in congestive heart failure, aortic thromboembolism, myocardial infarction, and sudden death.
Dr Seo set out to study three separate feline populations in New Zealand, the non-purebred, Sphynx and oriental cat breeds. Each group was prospectively studied and scanned. The association between the heart ultrasound findings and the genetic results in these three populations of cats were statistically assessed. The aim is that the findings of the studies will the improve understanding of the natural disease process of feline cardiomyopathy, and provide a foundation for future research to identify and evaluate novel therapeutics agents to effectively treat feline cardiomyopathy. This could enhance our management of cats with cardiomyopathy and significantly enhance their quality of life and longevity.
In the first study to be published, a variation in the Alström syndrome protein 1 (ALMS1) gene was recently identified as a possible cause of hypertrophic cardiomyopathy (HCM) in Sphynx cats. The primary aims of this study were to describe the prevalence of HCM in Sphynx cats in New Zealand, and to assess the association between HCM and ALMS1 gene.
This was a prospective study with all six registered Sphynx breeders in New Zealand invited to take part. Three breeders declined or were unable to take part, but a total of 55 apparently healthy adult Sphynx cats from registered Sphynx breeders and pet owners in New Zealand were enrolled and screened by a cardiologist. A total of 42 of these cats had a repeat cardiac examination between 1.6 and 2.2 years later. Each cat also had a buccal swab collected to test for the ALMS1 variant, the results of which were unknown to the primary researcher until all data was collected.
Twelve cats (21.8%) were diagnosed with HCM at the initial examination. The diagnosis of HCM was associated with being male, presence of a heart murmur, greater heart murmur intensity, larger left atrial size, reduced left atrial function, hyperdynamic left ventricle function, and longer anterior mitral valve leaflet. Two cats with HCM had systolic anterior motion of the mitral valve. One cat with HCM had congestive heart failure (CHF) at the initial evaluation. One cat was removed from the final HCM group due to resolution of the initial findings, suggesting a transient myocardial thickening.
Following removal of this cat and introduction of 11 new cases of HCM from the second scan, the final prevalence of HCM was 40.0% with a median age of 5.8 years. Three cats with HCM died during the study with congestive heart failure. Necropsy revealed that all three cats suffered a heart attack (myocardial ischemia or infarction).
Previous studies of non-purebred cats have reported the overall prevalence of HCM to be 15% but this can increase up to 29.4% in cats older than 9 years old. Similar to the study in France, these results support the view that Sphynx cats are predisposed to HCM. While only cats in New Zealand were investigated, the global genetic pool of Sphynx cats is small and breeding cats are often shared internationally. As an example, 75% of the founding members of the studied family lines were imported from other countries. Therefore, the prevalence of HCM in Sphynx cats is potentially high across the world
The frequency of ALMS1 variant was 70.9%. This result is similar to the findings in North America and Europe and suggests that the ALMS1 variant is commonly present in Sphynx cats throughout the world. The similarly high frequency of ALMS1 variants in cats in New Zealand, North America, and Europe further illustrates how breeding cats are shared internationally. The ALMS1 variant was not associated with HCM in the present study. This finding is consistent with a recent study of European and American Sphynx cats that also detected no association between the ALMS1 variant and HCM.
There are several guidelines for HCM screening in breeding cats. At minimum, these guidelines recommend the use of 2D, M-Mode, and Doppler imaging to identify cats with LV hypertrophy and outflow tract obstruction. Additionally, subjective assessments are incorporated to assess the mitral valve apparatus and papillary muscle morphologies. However, the effectiveness of these guidelines is currently unknown. For example, most cats in the present study had been screened by cardiac sonographers and boarded cardiologists prior to breeding. Furthermore, all imported breeding cats were determined clear of HCM by echocardiography prior to leaving their respective country.
The high prevalence of HCM in the studied cohort despite having a history of vigorous screening raises concerns with the current approach to breed screening. A possible explanation for the high prevalence of HCM is that cats are bred at young adulthood after screening clear at an early age. However, delaying screening can be difficult as breeders would need to keep intact cats at home for additional years. A possible solution to this is carefully evaluating the family line and focusing on screening the older founding members of the breeding line. Another possible solution could be incorporating echocardiographic variables that precede LV hypertrophy in feline HCM in the screening criteria. However, the accuracy, repeatability, and diagnostic cutoffs of these variables (LAFS%, AMVL length, and LVWT) in purebred cats have not been described. Ideally, determination of a genetic cause of HCM in this breed may allow development of a screening test prior to breeding
In summary, Hypertrophic cardiomyopathy is common in Sphynx cats in NZ similar to the general veterinary consensus. There is no association between the ALMS1 variant and HCM in Sphynx cats. Additionally, there was no association between the ALMS1 variant and previously documented risk factors of HCM (LA FS%, AMVL, and LVWT). Sphynx cats appear to be predisposed to myocardial ischemia and infarction, which might be associated with early death in some cats with HCM.
In the second study just published in the New Zealand Veterinary Journal, the aim was to evaluate the prevalence of subclinical cardiomyopathy and cardiac mortality in a research colony of non-purebred cats, established as a model of the wider cat population inNew Zealand
All apparently healthy, compliant, non-pregnant, non-neonatal cats in the colony at the Centre for Feline Nutrition (Massey University, Palmerston North, NZ) underwent physical examination and echocardiography using a 4.4–6.2-MHz probe by a board-certified veterinary cardiologist. Cardiac phenotype was classified following current guidelines. Hypertrophic cardiomyopathy (HCM) phenotype was defined as an end-diastolic left ventricular wall thickness ≥ 6 mm. Colony mortality data from February 2012 to February 2022 was reviewed to determine cardiac mortality.
A total of 132 cats (65 females and 67 males) were included in the study, with a median age of 4.1 (IQR 3.0–8.0) years. Thirty-two (24%) cats had a heart murmur, and three (2%) cats had an arrhythmia. Echocardiography revealed heart disease in 24 (18.2%) cats, including 23 with an HCM phenotype and one with a restrictive cardiomyopathy phenotype. Of the cats with the HCM phenotype, 3/23 had systemic hypertension or hyperthyroidism or both, and these cats were excluded from the final diagnosis of HCM.
Between 2012 and 2022, 168 colony cats died, with 132 undergoing post-mortem examination. Heart disease was considered the cause of death in 7/132 (5.3%) cats; five had HCM, one a congenital heart defect, and one myocarditis. The overall prevalence of death related to HCM in the colony during this period was 3.8%. Three cats with HCM and the cat with a congenital heart defect died unexpectedly without prior clinical signs, while congestive heart failure was observed prior to death in two cats with HCM and the cat with myocarditis. Additionally, 30/132 (22.7%) cats had cardiac abnormalities but died for non-cardiac reasons.
Subclinical cardiomyopathy, specifically HCM, was common in cats in the colony. Given that the colony originated as a convenience selection of non-purebred cats in New Zealand, the true prevalence of HCM in the wider New Zealand population is likely to fall within the 95% CI. The proportion of deaths of colony cats due to HCM was lower (3.8%) supporting the conclusion that subclinical cardiomyopathy may not progress to clinical disease causing death. Veterinarians therefore should be aware of the high prevalence of subclinical HCM when treating cats.
The final publication on Oriental cats in New Zealand is still in progress. We look forward to bringing you those findings once it is published.