The Veterinary Journal 188 (2011) 73–76

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The Veterinary Journal

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Heritability of premature mitral valve disease in Cavalier King Charles spaniels

Tom Lewis a,*, Simon Swift b, John A. Woolliams c, Sarah Blott a

a Kennel Club Genetics Centre at the Animal Health Trust, Lanwades Park, Kentford, Newmarket, Suffolk CB8 7UU, UKb Northwest Surgeons, Ashville Point, Sutton Weaver, Cheshire WA7 3FW, UKc Roslin Institute, Royal (Dick) School of Veterinary Studies, University of Edinburgh, Roslin, Midlothian EH25 9PS, UK

a r t i c l e i n f o

Article history:Accepted 19 February 2010

Keywords:HeritabilityDogCavalier King Charles spanielsMitral valve diseaseHeart murmur

1090-0233/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.tvjl.2010.02.016

* Corresponding author. Tel.: +44 1638 751000x12E-mail address: tom.lewis@aht.org.uk (T. Lewis).

a b s t r a c t

Mixed model analysis of 1252 records of cardiac auscultation of 4- to 5-year-old Cavalier King Charlesspaniels (CKCS) from 1991 to 2008 in conjunction with the Kennel Club pedigree records of all dogs reg-istered from the mid 1980s to September 2007 was used to estimate variance parameters of prematuremitral valve disease (MVD). Data were limited to dogs P4 and <5 years of age to ensure diagnostic dis-tinction between early and late onset MVD. Cardiac murmurs were detected in 108/1252 (8.6%) dogs.Heritability estimates of 0.67 (standard error, SE 0.071) for the grade of murmur and 0.33 (SE 0.072)for the presence/absence of murmur were calculated. The variance due to clinician was 0.02 (SE 0.012)for grade and 0.03 (SE 0.017) for presence/absence of murmur. These results indicate that the presenceand severity of MVD, as assessed by cardiac auscultation, in 4- to 5-year-old CKCS is highly heritableand that selection against the disease should be successful.

� 2010 Elsevier Ltd. All rights reserved.

Introduction

Mitral valve disease (MVD) is the most common cardiac diseasein dogs (Borgarelli et al., 2008). Myxomatous thickening and coap-tation of the mitral valve leaflets results in mitral regurgitationfrom the left ventricle to the left atrium during systole, causingan audible ‘murmur’ during cardiac auscultation. MVD typicallyprogresses slowly in severity over many years, often leading tocongestive heart failure, although variable pathology of the diseasecan result in sudden death.

Many studies have reported an increased prevalence and rate ofprogression of MVD in the Cavalier King Charles spaniel (CKCS)compared to other breeds (Darke, 1987; Haggstrom et al., 1992;Malik et al., 1992; Beardow and Buchanan, 1993; Pedersen et al.,1999; Wood et al., 2000; Serfass et al., 2006). This has lead to par-ticular concerns regarding the high prevalence of a ‘premature’form of disease in the breed. Evidence of a genetic predispositionfor MVD in the CKCS was provided by Swenson et al. (1996),who reported that dogs with a high disease status (based on theage and severity of cardiac murmurs) produced more offspringwith murmurs and with murmurs of higher intensity than dogswith a low disease status. Swenson et al. (1996) concluded thatthe distribution of murmurs by age was suggestive of a polygenicdisease and that MVD was a multifactorial, polygenic thresholdtrait; this has also been suggested in other breeds (Olsen et al.,

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75.

1999). However, to the best of our knowledge, no estimate of theheritability for MVD has been published.

The severity of heart murmur is usually graded out of six byauscultation according to its intensity and the presence of a precor-dial thrill (Gompf, 1988). There is evidence that the intensity of themurmur detected by auscultation is dependent on the degree ofmitral regurgitation and is prognostic for the progression of heartfailure (Haggstrom et al., 1995; Haggstrom, 2008). Thus, the gradeof murmur would appear to be an adequate diagnostic indicator ofthe severity of mitral valve disease (Gompf, 1988).

The aim of the present study was to describe the genetic attri-butes of premature rather than age-related MVD in the CKCS; itwas therefore necessary to make a distinction between early andlate onset MVD as two separate diseases. There is evidence thatthe presence and intensity of murmurs determined by auscultationis an adequate indicator of the degree of mitral valve prolapse3 years previously (Pedersen et al., 1999). Thus, dogs with an audi-ble murmur at 4-years of age can be considered to be affected withseverely premature prolapse. The present study applied mixedmodel quantitative techniques to the analysis of the presenceand grade of cardiac murmur indicative of MVD in 4-year-old(P4 and <5 years old) CKCS to estimate the heritability of bothvariables.

Materials and methods

Data consisted of the results of cardiac auscultation of CKCS undertaken as partof the breed club heart monitoring scheme, in which owners presented the dog to acardiologist from the Veterinary Cardiovascular Society auscultation panel (all withfurther qualifications and training in auscultation) either at ‘open sessions’

74 T. Lewis et al. / The Veterinary Journal 188 (2011) 73–76

organised by local breed clubs or by private appointment. The results were collatedonto a central breed database and data were extracted directly from the databasefor this analysis. The data recorded the presence or absence of an audible heartmurmur consistent with mitral regurgitation secondary to degenerative mitralvalve disease and, where present, the grade of intensity. These murmurs are typi-cally left sided, caudal and systolic. The unique Kennel Club registration numberof each dog and the date of cardiac auscultation were also recorded.

There were 1310 records from 1252 dogs (820 female, 432 male) evaluated atP4 and <5 years of age (1460–1824 days). Data were recorded from 1991 to2008 (Table 1); 55 animals (4.4%) had >1 record of cardiac auscultation. The pedi-gree used in the analysis was the Kennel Club pedigree of all CKCS registered fromthe early 1980s to September 2007 and contained 333,287 individual dogs.

The narrow age range of utilised data, defined in the objectives, and the conse-quent small number of animals with repeat records within this age range meantthat repeatability analysis was not feasible. Accordingly, a single record was se-lected for each of the 1252 animals in the data. For the 55 cases with multiple re-cords, the record for which the age at evaluation was closest to the mean age atevaluation for all 1310 records (1618.4 days) was selected. The resultant 1252 datarecords consisted of evaluations carried out by 453 veterinary clinicians. The expec-tation was that the diagnostic clinicians’ propensity to detect and grade murmursconsistent with MVD would be normally distributed.

Statistical analysis of the data had the objective of fitting a mixed linear modelusing ASReml (Gilmour et al., 2006) to estimate genetic and environmental varianceof the presence of an audible left-sided caudal systolic murmur consistent withdegenerative mitral valve disease (murmur: present, 1; absent, 0) and the gradeof severity of such murmurs (grade: no murmur, 0; to most severe murmur, 6) in4- to 5-year-old CKCS. Univariate linear mixed models fitted to murmur and gradewere of the following form:

Y ¼ Xbþ Zaþ Vc þ e

where Y is the vector of observations; X, Z and V are known incidence matrices; b isthe vector of fixed effects; a is the vector of random additive genetic effects, with thedistribution assumed to be multivariate normal (MVN), with parameters (0, r2

AA); cis the vector of veterinary clinician effects with the distribution assumed to be MVN,with parameters (0, r2

C I); and e is the vector of residuals distributed MVN withparameters (0, r2

E I). I is an identity matrix of the appropriate size, A is the additivegenetic relationship matrix and r2

A , r2C and r2

E denote the variances of each of therespective random effects.

Preliminary analysis established that there was no difference between modelswhen a variance component for litter was included, which was tested using a like-lihood ratio test (P > 0.05), so litter effect was not included in any further models.The fixed effects included in the model as factors were sex and year of evaluation.Age in days was fitted as a covariate after being centred and scaled according to themean (1616.8) and standard deviation (SD, 97.2), with both linear and quadraticterms.

The heritability (h2) is defined as the proportion of the phenotypic variance (i.e.the observable variation in the trait in question) that is made up by the additive ge-netic variance:

h2 ¼ r2A=r

2P

Similarly, the ‘clinician effect’ can be described as the proportion of phenotypic var-iance made up from the variance due to the veterinary cardiologist evaluating thepresence and intensity of a murmur:

Table 1Distribution of data in relation to year of evaluation.

Year of evaluation Number of dogs %

1991 79 6.31992 87 7.01993 64 5.11994 78 6.21995 66 5.31996 93 7.41997 105 8.41998 83 6.61999 75 6.02000 73 5.82001 64 5.12002 76 6.12003 63 5.02004 69 5.52005 59 4.72006 46 3.72007 52 4.22008 20 1.6

Clinician effect ¼ r2C=r

2P

Heritability estimates made using binary data (i.e. murmur) are an underesti-mate of the heritability on the assumed continuous underlying liability scale (Gian-ola, 1982), since the binary phenotype is a less precise indicator of disease liabilitythan a phenotype measurable on a more graduated scale (such as hip score from 0to 106). However, estimation of heritability on the continuous liability scale can bemade from the binary scale heritability estimate and the prevalence of the diseasein the data using the following equation (Dempster and Lerner, 1950):

h2c ¼ h2

011� p

i2p

!

where subscripts c and 01 signify heritability estimates on the continuous liabilityand binary scale, respectively; p is the prevalence of cardiac murmurs in the data;and i is the mean liability of individuals with a cardiac murmur at prevalence p, inSD units, from the population mean, assuming normally distributed liability.

Best linear unbiased predictors (BLUP) estimated breeding values (EBV) are thecalculated genetic effects with respect to premature MVD of every animal presentin the pedigree used in the analysis.

Results

Audible murmurs consistent with MVD were recorded in 108/1252 (8.6%) dogs in the data set (mean murmur 0.086). The meangrade of murmur among the 1252 dogs was 0.149 and, of the 108records where a murmur was detected, the mean grade was 1.731.The distribution of grades is shown in Table 2.

The estimated heritability of grade was very high at 0.67 (stan-dard error, SE 0.071). The variance due to veterinary clinician wassmall at 2% of phenotypic variance (0.02 ± 0.012), but significantlygreater than zero (P < 0.001) using a likelihood ratio test. The pre-dominant effect of age was linear, with the quadratic term of agebeing small, with a magnitude less than its SE and not significantlydifferent from zero (P > 0.05). The linear term was small, althoughstatistically different from zero (P < 0.05), indicating an increase inmean grade of 0.12 over the 12 months from 4- to 5-years of age.

The estimated heritability of murmur was 0.33 (SE 0.072). Cal-culation of the heritability on the continuous liability scale accord-ing to Dempster and Lerner (1950) yielded a value of 1.05,exceeding the theoretical maximum. The variance due to veteri-nary clinician was small at 0.03 (SE 0.017), but significantly greaterthan zero (P < 0.001), using a likelihood ratio test. Both the esti-mated linear age and quadratic age effects on murmur were smallin magnitude and smaller than the SE of the estimate, thus indicat-ing failure to detect an age effect on the presence of a murmur con-sistent with MVD over the 12 months from 4- to 5-years of age.

Males had higher grade and murmur than females(0.081 ± 0.030 and 0.030 ± 0.016, respectively). There was a signif-icant effect of year of evaluation (P < 0.001) for both grade andmurmur, with a broad trend of increasing effect over the yearsfrom 1991 to 2008.

Discussion

Calculation of high heritability estimates for both grade andmurmur indicate that there is a substantial genetic basis to boththe presence and severity of cardiac murmurs, consistent with mi-tral regurgitation secondary to degenerative MVD, in CKCS be-tween 4- and 5-years of age. Although many smaller breeds of

Table 2Distribution of data in relation to grade of murmur (zero indicates absence ofmurmur).

Grade Number of dogs %

0 1144 91.41 52 4.22 37 3.03 15 1.24 4 0.3

T. Lewis et al. / The Veterinary Journal 188 (2011) 73–76 75

dog are affected with MVD, the CKCS breed is recognised as exhib-iting a higher prevalence at an earlier age than other breeds (Darke,1987; Haggstrom et al., 1992; Malik et al., 1992; Beardow and Bu-chanan, 1993). Thus, it is reasonable to surmise that the CKCS issusceptible to a ‘premature onset’ form of MVD, while still beingvulnerable to the age-related form of the disease that is character-istic of many of the smaller canine breeds.

In this study, data were restricted to include only observationson 4- to 5-year old dogs to ensure adequate diagnostic distinctionof premature MVD, as opposed to age-associated MVD (the preva-lence of which would have gradually increased in older dogs), andto allow sufficient time for secondary clinical signs indicative ofpremature mitral valve prolapse (i.e. cardiac murmur) to becomemanifest. This narrow age range successfully defined a group ofdogs with a cardiac murmur consistent with MVD and interpretedthis as a premature form of MVD; this study demonstrates that thispremature form is highly heritable.

Further analysis of sources of variation in the progression ofMVD with age would require repeat data and more sophisticatedstatistical techniques, such as the inclusion of random regressionsin the mixed model. This would confer the benefit of enabling dis-tinction between: (1) animals with no genetic predisposition toMVD; (2) animals with genetic predisposition to age-relatedMVD typical of many small canine breeds, and (3) animals withthe putative CKCS breed-specific genetic predisposition to prema-ture MVD, allowing selection for the former.

The heritability of murmur was high (for a binary trait) at 0.33.The heritability of a trait on the binary scale is an underestimate ofthe heritability on the continuous underlying liability scale (Gian-ola, 1982), with a maximum of approximately 0.64 only if it is fullyadditive on the liability scale (Lynch and Walsh, 1998). The esti-mate of the heritability on the continuous liability scale whenapproximated using Dempster and Lerner (1950), which accountsfor the prevalence of detectable murmurs in the data (8.6% in thisstudy), is >1, which is greater than the theoretical maximum valueof heritability (i.e. all phenotypic variation is genetic in origin).Such results occur with ad hoc corrections; nevertheless the esti-mate indicates a very high estimate for heritability of underlyingliability to premature MVD. There are alternative evaluation proce-dures for fitting models to binary data; however, these can sufferfrom biases when used with mixed models, particularly when largenumbers of individuals in the pedigree have no phenotypic record(Goldstein and Rabash, 1996; Rodriguez and Goldman, 2001).

The prevalence of heart murmurs in the presented data (8.6%) islower than that described by Pedersen et al. (1999) (33.3% in 4- to5-year old dogs) but similar to that of Wood et al. (2000) (9.7% in 5-year old dogs). Wood et al. (2000) felt that their reported preva-lence was an underestimate and speculated that owners wouldbe less likely to report results from affected animals or would onlyreport them once. Thus, although the data used in this study areprimarily composed of results of a ‘monitoring scheme’ and there-fore closer to a random sample than a case study database, it isprobably still subject to some biases. Even a slight increase in theprevalence (e.g. from 8.6% in this study to 9.7% reported by Woodet al., 2000), with the same estimate of heritability on the binaryscale, would bring the heritability estimate on the continuous lia-bility scale (using the correction of Dempster and Lerner, 1950)down to <1. Nevertheless, the estimate is still close to 1 and is inaccordance with the large reported heritability estimate for grade,a trait likely to be subject to more environmental variation in cat-egorisation of the intensity of a cardiac murmur.

The high estimates of heritability reported for both murmur andgrade imply that selection against premature MVD diagnosed byleft sided, caudal and systolic murmur in CKCS will be successful.Superficially, with a high heritability, phenotypic records alonemight be assumed to be adequate, rather than developing BLUP

EBV; however, this is not the case. Firstly, whilst observation ofthe phenotype may be ‘premature’ in terms of lifespan, it is ‘lateonset’ genetically, since most breeding animals are selected andbred before 4-years of age, whilst others will have been neutered.In contrast, the EBV will be available from the moment of birth forselection (although newborn littermates will have identical EBV).Secondly, the EBV will further increase the accuracy in predictingbreeding value, and hence the rate of improvement, by utilisingall the available information and updating as further informationbecomes available, e.g. from offspring or siblings. Thirdly, theEBV will provide predictors for those animals where disease statusis not recorded, hence increasing selection opportunities andintensity, which again enhances rate of improvement. Finally, asa welfare problem, not pursuing what is simply achievable is inap-propriate. It may be that the development of a genomic predictorwould help to distinguish littermates and further increase accuracyand might also lead subsequently to scientific benefits throughidentifying the major underlying genes.

Although murmurs detected by auscultation as described arewidely accepted as indicative of MVD, it is acknowledged that aninnocent murmur or a murmur due to a different disease may occa-sionally be misdiagnosed as MVD. Thus, auscultation has imperfectspecificity for the diagnosis of MVD, in spite of efforts to minimisesuch errors (i.e. only including diagnoses from certified cardiolo-gists). However, imperfect specificity leads to underestimation ofheritability (Bishop and Woolliams, 2010), which, given the highestimates presented in this study, means such misdiagnoses arelikely to be rare and of negligible effect. Underestimation of herita-bility would also occur if there are unrecognised factors leading to aloss of sensitivity in diagnosis (Bishop and Woolliams, 2010).

Finally, it is to be expected that there is variation among clini-cians in both the detection of a murmur indicative of MVD andthe assignment of a grade of severity. However, the variation issmall in magnitude compared to the overall phenotypic variation(3% of phenotypic variation for murmur). Even with grade, wherethe differing degrees of severity offer more opportunity for varia-tion between clinicians, this component remains small when com-pared to the total phenotypic variation observed (2%). Thisdemonstrates good quality control among clinicians in the detec-tion and grading of murmurs, possibly due to the extra training re-quired for members of the Veterinary Cardiovascular Societyauscultation panel. Such variation in clinical diagnoses is probablyto be expected for a disease that relies on a single subjectiveassessment and given the natural variation in the accuracy of mak-ing such assessment.

Conclusions

The presence and intensity of left sided, caudal, systolic cardiacmurmurs in 4- to 5-year-old CKCS are highly heritable, indicatingthat variation in severity of MVD (as typically described by thistype of murmur) at this age is substantially genetic in origin. Thedata used in this study were restricted to 4- to 5-year-old dogsin an attempt to ensure, as far as possible, exclusive analysis of‘premature’, as opposed to ‘late age onset’, MVD. The magnitudeof the heritability estimates suggests that selection against prema-ture MVD would be successful and, to that end, the production ofEBVs for premature MVD would be a useful addition. However,care must be taken in any proposed breeding programme to ensurethat breeding away from premature MVD does not result in a con-comitant increase in other notable diseases in the CKCS, such assyringomyelia, or that too high a selection intensity leads to a dras-tic loss of genetic diversity, thereby reducing the general robust-ness of the breed in the UK and potentially leading to anothergenetic disease in the future.

76 T. Lewis et al. / The Veterinary Journal 188 (2011) 73–76

Conflict of interest statement

None of the authors of this paper has a financial or personalrelationship with other people or organisations that could inappro-priately influence or bias the content of this paper.

Acknowledgement

This work was funded by the Kennel Club Charitable Trust.

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