Genetic analyses of the radiographic appearance of the distal sesamoid bones in Hanoverian Warmblood horses

Kathrin F. Stock Institute for Animal Breeding and Genetics, University of Veterinary Medicine Hannover (Foundation), Bünteweg 17p, D-30559, Hannover, Germany.

Search for other papers by Kathrin F. Stock in
Current site
Google Scholar
PubMed
Close
 Dr med vet
and
Ottmar Distl Institute for Animal Breeding and Genetics, University of Veterinary Medicine Hannover (Foundation), Bünteweg 17p, D-30559, Hannover, Germany.

Search for other papers by Ottmar Distl in
Current site
Google Scholar
PubMed
Close
 Prof Dr, Dr habil

Click on author name to view affiliation information

Abstract

Objective—To evaluate whether additive genetic correlations existed between certain aspects of the radiographic appearance of the distal sesamoid (navicular) bones (RNB) or between RNB and other types of radiographic changes in the limbs of Hanoverian Warmblood horses.

Animals—5,157 horses.

Procedures—Quasi-linear and binary traits were defined by the appearance of canales sesamoidales (CSs) and the structure and contour of the forelimb navicular bones (NBs). Prevalences of osseous fragments in the metacarphophalangeal and metatarsophalangeal (fetlock) and tarsocrural joints and deforming arthropathy in tarsal joints were analyzed as binary traits. Genetic parameters were estimated by use of multivariate linear models.

Results—Heritability estimates for the RNB traits ranged from 0.10 to 0.34. Additive genetic correlations among those traits were usually close to unity. Extensive radiographic changes in the NBs, including changes in CSs and alterations in structure and contour, had correlations with less distinct radiographic changes. Negative additive genetic correlations were observed between small numbers of short and conical CSs in the central portion of the distal border of the NB and osseous fragments and arthropathy, and between most types of radiographic findings in the NBs and osseous fragments in tarsal joints.

Conclusions and Clinical Relevance—The genetic bases for different types of RNB were not identical. The detection of correlations between normal RNB and findings of short and conical CSs versus deformed CSs and structural and contour changes warrants further study. Genetically justified distinction between physiologic and pathologic NB changes will increase the efficiency of selecting against NBs with radiographically apparent alterations.

Abstract

Objective—To evaluate whether additive genetic correlations existed between certain aspects of the radiographic appearance of the distal sesamoid (navicular) bones (RNB) or between RNB and other types of radiographic changes in the limbs of Hanoverian Warmblood horses.

Animals—5,157 horses.

Procedures—Quasi-linear and binary traits were defined by the appearance of canales sesamoidales (CSs) and the structure and contour of the forelimb navicular bones (NBs). Prevalences of osseous fragments in the metacarphophalangeal and metatarsophalangeal (fetlock) and tarsocrural joints and deforming arthropathy in tarsal joints were analyzed as binary traits. Genetic parameters were estimated by use of multivariate linear models.

Results—Heritability estimates for the RNB traits ranged from 0.10 to 0.34. Additive genetic correlations among those traits were usually close to unity. Extensive radiographic changes in the NBs, including changes in CSs and alterations in structure and contour, had correlations with less distinct radiographic changes. Negative additive genetic correlations were observed between small numbers of short and conical CSs in the central portion of the distal border of the NB and osseous fragments and arthropathy, and between most types of radiographic findings in the NBs and osseous fragments in tarsal joints.

Conclusions and Clinical Relevance—The genetic bases for different types of RNB were not identical. The detection of correlations between normal RNB and findings of short and conical CSs versus deformed CSs and structural and contour changes warrants further study. Genetically justified distinction between physiologic and pathologic NB changes will increase the efficiency of selecting against NBs with radiographically apparent alterations.

The distal sesamoid bone (navicular bone) is located in the palmar or plantar aspect of the equine distal interphalangeal joint. It serves as a deflection plate for the deep digital flexor tendon and is involved in distribution of load and traction forces in the distal portion of the limb. The podotrochlea in horses is composed of the navicular bone and the synovial (bursa podotrochlearis and distal interphalangeal joint), tendinous (distal part of the deep digital flexor tendon), and ligamentous structures of the distal portion of the limb.

Lameness plays an important role in equine medicine, and many problems originate from the distal portion of the forelimbs. The podotrochlea is often determined to be the origin of recurrent or persistent forelimb lameness in riding horses. Special radiographic views have been developed to reveal bony alterations in the navicular bones, and diagnosis of so-called podotrochleosis syndrome (ie, navicular disease) usually involves clinical examination and radiographic evaluation. Changes in the number, location, and shape of the CSs and in the contour and structure of the navicular bone as observed radiographically may serve as diagnostic criteria, but varying degrees of importance have been ascribed to individual radiographic findings.1–3 Furthermore, it is not clear whether radiographic changes in the forelimb navicular bones represent separate related or unrelated traits or different appearances of a single trait, from a genetic perspective.

Considerable disagreement exists regarding the interpretation and prognostic value of various aspects of the RNB; no clear relationship has been established between the type or extent of radiographic alterations and signs of pain or lameness.a–g Although associations between certain radiographic findings and development of clinical signs of navicular disease remain speculative,g,h observation of marked radiographic alterations is commonly considered to signify a higher risk for development of clinical lameness, and deviations from what is considered to be a normal radiographic appearance of the navicular bone are likely to affect the sales value of affected horses.4

Genetic determination of radiographic features thought to be associated with podotrochleosis has been reported.1,5–9,i However, in those studies, genetic analyses were conducted on the basis of radiographic examination alone, without consideration of clinical findings. Therefore, the importance of those findings with respect to clinically apparent navicular bone disease remains undetermined. Before breeding strategies can be developed to decrease the prevalence of radiographic changes in the navicular bone, genetic correlations between specific radiographic findings must be determined. It is also possible that genetic correlations exist between the RNB and other radiographic findings in equine limbs.

Material and Methods

Horses—Five thousand one hundred fifty-seven Hanoverian Warmblood horses designated by the Society of Hanoverian Warmblood Breeders (Verband hannoversches Warmblutzüchter eV) to be sold at riding-horse auctions in Verden on the Aller, Germany, from 1997 to 2004 were included. Information was available for 141 stallions, 2,847 geldings, and 2,169 mares. Data for the horses were provided by the Society, and radiographic data were obtained from the horses' official veterinary records.

All horses were examined clinically and radiographically by the same veterinarian. Routine radiographic examination of the horses included dorsoproximo-palmarodistal projections (ie, upright pedal view10) of the navicular region of the forefeet, lateromedial (90°) projections of all 4 feet, and lateromedial (90°) and dorsolateral-plantaromedialoblique (45°) projections of the tarsocrural joints. Radiographs were independently evaluated by 2 experienced radiologists. Deviations from normal radiographic appearances of bony structures were recorded.

Definition of traits—Navicular bones were evaluated on the basis of their appearance in the upright pedal projections. Where applicable, lateromedial projections of the front feet or additional views that had been taken for clarification of equivocal findings were also considered. Variables recorded were the number, location, and shape of the CSs and the structure and contour of the forelimb navicular bones. Categorization of the RNB was as follows: 0 = no abnormal findings; I = few (1 to 4) short and conical CSs in the central portion of the distal border; II = several (≥ 5) short and conical CSs in the central portion of the distal border; III = few (1 to 4) elongated or deformed (ie, enlarged) CSs in the central portion of the distal border; IV = several (≥ 5) enlarged CSs; V = markedly deformed (enlarged, elongated, or branched) CSs in the central, medial, or lateral portion of the distal border; VI = alterations in the contour of the navicular bone (ie, new bone formation at the proximal or distal border or at the medial or lateral extremities of the bone); and VII = alterations in structure of the navicular bone (ie, irregular pattern of spongiosum and increased [sclerosis] or decreased [osteolysis] bone opacity). Categories were defined and the order of categories chosen in accordance with current official guidelines for interpretation of radiographic findings in horses.11 Horses were assigned to one of the RNB categories according to the most distinct finding in either forelimb navicular bone.

The categories of radiographic appearance were used to define 2 quasi-linear traits (RNB0–7 and RNB0–3) and 5 binary traits (RNB0/1a–e) describing the type and distribution of radiographic findings. Each category corresponded to an individual trait level of the most specific trait (ie, quasi-linear trait RNB0–7). For definition of the second quasi-linear trait (ie, RNB0–3), categories III to VII were combined. All-or-none traits were defined such that horses were considered affected if they were allocated to RNB category I (RNB0/1a), category II (RNB0/1b), categories III to VII (RNB0/1c), or categories II to VII (RNB0/1d) or if they had any radiographic changes involving the distal border of the navicular bones (RNB0/1e).

Lateromedial projections of the distal portion of the limbs were used to detect horses with OFMs. Lateromedial and oblique projections of the tarsocrural joints were used to detect horses with OFTs or with DAT. Definitions of OFMs, OFTs, and DAT have been described elsewhere.12,13 Parameter values of 1 were assigned to affected horses, and parameter values of 0 were assigned to the remainder of the horses.

Pedigree information—Pedigree data were provided by an animal ownership database.j The 5,157 horses were descendants of 509 sires that had a mean ± SD of 10.13 ± 19.21 (range, 1 to 251) investigated offspring and of 4,145 dams with 1.24 ± 0.55 (range, 1 to 6) investigated offspring. The number of maternal grandsires was 729; of those, 257 appeared as both sires and maternal grandsires in the pedigrees. Maternal grandsires were represented by 7.05 ± 11.66 (range, 1 to 105) investigated offspring.

Model development—Models for genetic analysis were developed on the basis of results of simple and multivariate ANOVAs. The 2 quasi-linear RNB traits were analyzed in general linear models,j and the binary traits were analyzed in generalized linear models with binomial distribution function and probit link function.k Values of P < 0.05 were considered significant.

The following effects were tested for association with the distribution of radiographic findings: date of presumptive auction (48 auction dates with 107.4 ± 31.0 [range, 67 to 176] preselected horses/auction date), year of presumptive auction (years from 1997 to 2004; 644.6 ± 65.3 [range, 548 to 740] pre-selected horses/year), sex (141 stallions, 2,847 geldings, and 2,169 mares), age (1,090 3-year-olds, 2,685 4year-olds, 1,042 5-year-olds, and 340 horses ≥ 6 years of age), interaction between sex and age (10 levels: 49 3-year-old stallions; 92 stallions ≥ 4 years of age; 595 3-year-old, 1,534 4year-old, and 572 5-year-old geldings and 146 geldings ≥ 6 years of age; 446 3-year-old, 1,097 4-year-old, and 446 5year-old mares and 180 mares ≥ 6 years of age), and season of birth (1,807 horses born from November to March, 2,026 horses born in April, and 1,324 horses born from May to October).

The models of all radiographic appearance traits included the fixed effects that were significant in the simple and multivariate ANOVAs of RNB0–7. Accordingly, effects that were significant in the simple and multivariate ANOVAs of OFMs, OFTs, and DAT were included in the models for the respective traits, according to the following equations:

article image
where yijklmn and yimn are radiographic findings for a given horse, μ is the model constant, auctioni is the fixed effect of the ith date of presumptive auction (i = 1 to 48), sex × agejk is the fixed effect of the interaction between the jth sex (j = 1 to 3) and the kth age group (k = 1 to 4), seasonl is the fixed effect of the lth season of birth (l = 1 to 3), am is the random additive genetic effect of the mth horse (m = 1 to 23,279), and eijklmn and eimn are residual.

Genetic analysis—Genetic variables were estimated in multivariate analysis in linear animal models via restricted maximum likelihood.14,l Information on all traits (ie, RNB traits and OFMs, OFTs, and DAT) was available for all horses. For all horses, complete pedigree information for 3 ancestral generations was considered, resulting in a relationship matrix composed of 23,279 horses.

Additive genetic (σ2a, cova) and residual (σ2e, cove) variance and covariance estimates from bivariate analyses were used to calculate heritabilities (h2), additive genetic (rg) correlations, and residual (re) correlations. Mean heritabilities were calculated from the 9 bivariate heritability estimates that were obtained per trait. Heritabilities and residual correlations of the binary traits were transformed onto the underlying liability scale according to described methods.15,16

Results

Mean ± SD age was 4.14 ± 0.86 years; 73.2% of the horses were 3- to 4-year-olds. Females were significantly older (4.19 ± 0.92 years) than males (geldings, 4.10 ± 0.80 years; stallions, 4.04 ± 1.02 years). Mean ± SD height at the withers (highest point of the dorsal process of the thoracic vertebrae) was 166.71 ± 3.76 cm (range, 146 to 183 cm).

Distributions of radiographic findings were summarized (Table 1). Deviations from what was considered to be a normal radiographic appearance of the forelimb navicular bones were observed in 40.57% (2,092/5,157) of the horses, mostly in the form of few (20.48%, 1,056/5,157) or several (16.97% 875/5,157) conical CSs in the central part of the distal border. In 98.66% of horses with an abnormal RNB, abnormal findings involved the distal border of the navicular bone. Osseous fragments in the fetlock joints were diagnosed in 1,433 (27.79%) horses, FTs were diagnosed in 473 (9.17%) horses, and DAT was diagnosed in 397 (7.70%) horses.

Table 1—

Distribution of findings pertaining to the radiographic appearance of the distal sesamoid (navicular) bones in the forefeet in 5,157 Hanoverian Warmblood riding horses. Horses were assigned to one of the RNB categories according to the most distinct finding in either forelimb navicular bone.

TraitCodingRNB categoryPrevalence(No. [%])
RNB0–7*003,065(59.43)
1I1,056(20.48)
2II875(16.97)
3III65(1.26)
4IV17(0.33)
5V33(0.64)
6VI26(0.50)
7VII20(0.39)
RNB0–3*003,065(59.43)
1I1,056(20.48)
2II875(16.97)
3III-VII161(3.12)
RNB0/1a00 + II-VII4,101(79.52)
1I1,056(20.48)
RNB0/1b00 + I + III-VII4,282(83.03)
1II875(16.97)
RNB0/1c00 + I + II4,996(96.88)
1III-VII161(3.12)
RNB0/1d00 + I4,121(79.91)
1II-VII1,036(20.09)
RNB0/1e003,093(59.98)
1I-VII (distal border)2,064(40.02)

0 = No abnormal radiographic findings. I = Few (1 to 4) short and conical CSs in the central portion of the distal border. II = Several (≥5) short and conical CSs in the central portion of the distal border. III = Few (1 to 4) deformed (enlarged or elongated) CSs in the central portion of the distal border. IV = Several (≥5) deformed (enlarged or elongated) CSs. V = Various markedly deformed (enlarged, elongated, branched) CSs in the central, medial, or lateral portion of the distal border. VI = Alterations of navicular bone contour (new bone formation at proximal or distal border or at medial or lateral extremity of bone). VII = Alterations in navicular bone structure (irregular pattern of spongiosa and increased or decreased bone density).

Categories used to define quasi-linear traits.

Categories used to define binary traits.

The year of presumptive auction was significantly (P = 0.02 for OFT; P < 0.01 for all other traits) related to the distribution of all traits, as was the date of presumable auction (P < 0.001). Sex was significantly related to OFTs, DAT, and all RNB traits except for RNB0/1a. Stallions had the highest risk, and mares the lowest risk, for having higher values of RNB0–7 and RNB0–3 and for being affected with OFTs, RNB0/1b, RNB0/1d, and RNB0/1e (P < 0.001). Males were more likely to be affected with DAT (P < 0.01) and RNB0/1c (P = 0.02) than females. Age was significantly related to OFTs and all RNB traits except for RNB0/1a. The probability for having higher values for RNB0–7 and RNB0–3and for being affected with RNB0/1b, RNB0/1d, and RNB0/1e increased significantly with age. Three-year-old mares born from November to March were least likely to have high values of RNB0–7 or to be affected with RNB0/1a, RNB0/1c, and RNB0/1d, whereas 4-year-old stallions born in April were most likely to have high values of RNB0–7 and to be affected with RNB0/1b or RNB0/1d.

Variance estimates of all traits differed little among the various bivariate analyses. Ranges of estimated additive genetic and residual variances were summarized (Table 2). Comparison of heritabilities of the quasi-linear RNB traits revealed slightly lower estimates for RNB0–7 (h2 = 0.133 to 0.156; SEh2 = 0.020 to 0.023) than for RNB0–3 (h2 = 0.179 to 0.193; SEh2 = 0.019 to 0.025).

Mean heritabilities and additive genetic and residual correlations between the binary RNB traits were summarized (Table 3). Transformed heritabilities ranged from h2 = 0.09 to 0.10 for RNB0/1a, from h2 = 0.24 to 0.26 for RNB0/1b, from h2 = 0.16 to 0.19 for RNB0/1c, from h2 = 0.22 to 0.25 for RNB0/1d, and from h2 = 0.33 to 0.34 for RNB0/1e. Standard errors of heritabilities were usually larger for the binary RNB traits (SEh2, 0.023 to 0.095) than for the quasi-linear RNB traits. Additive genetic correlations among RNB0/1a, RNB0/1b, RNB0/1d, and RNB0/1e were highly positive (rg ≥ 0.89; SErg ≤ 0.14). However, the additive genetic correlation between RNB0/1c and the other binary RNB traits was rg = 0.10 to 0.40 (SErg = 0.16 to 0.24). Residual correlations between the binary RNB traits ranged from re = −0.35 to 0.84 (SEre ≤ 0.02) before transformation. Transformation resulted in estimates of residual correlations that were partially outside the parameter space.

Table 2—

Ranges of estimates of additive genetic variances (σa2) and residual variances (σe2) from bivariate analyses of traits referable to the RNB of the forefeet and prevalences of OFMs, OFTs, and DAT in the same horses as in Table 1.

Traitσa2σe2
RNB0–70.1429–0.16740.9066–0.9273
RNB0–30.1262–0.13900.5830–0.5937
RNB0/1a0.0073–0.00790.1492–0.1496
RNB0/1b0.0144–0.01600.1192–0.1206
RNB0/1c0.0008–0.00090.0286–0.0288
RNB0/1d0.0165–0.01940.1355–0.1378
RNB0/1e0.0468–0.04840.1776–0.1786
OFMs0.0157–0.01600.1830–0.1833
OFTs0.0091–0.00930.0736–0.0737
DAT0.0037–0.00380.0662–0.0664

See Table 1 for key.

Table 3—

Genetic variables (estimated SEs) for different traits pertaining to the RNB in the same horses as in Tables 1 and 2, with heritabilities (mean heritabilities from bivariate analyses and transformed estimates for binary traits RNB0/1a–0/1e) in bold on the diagonal, additive genetic correlations above the diagonal, and residual correlations (transformed estimates for binary traits (RNB0/1a–0/1e) below the diagonal.

TraitRNB0/1aRNB0/1bRNB0/1cRNB0/1dRNB0/1e
RNB0/1a0.0988 (0.0244)0.9455 (0.1345)0.0993 (0.2414)0.8905 (0.1369)0.9652 (0.0461)
RNB0/1b–0.7491 (0.0307)0.2452 (0.0352)0.2012 (0.2015)0.9780 (0.0140)1.0000 (0.0001)
RNB0/1c−0.3143 (0.0432)−0.3589 (0.0493)0.1699 (0.0909)0.4008 (0.1700)0.2572 (0.1603)
RNB0/1d−0.7591 (0.0291)1.8872 (0.0066)1.2903 (0.0396)0.2320 (0.0345)0.9773 (0.0243)
RNB0/1e1.0727 (0.0169)0.8603 (0.0233)0.5290 (0.0484)0.9146 (0.0221)0.3402 (0.0383)

See Table 1 for key.

The distributions of horses with abnormal RNBs that had OFMs, OFTs, and DAT were summarized (Table 4). The number of coaffected horses ranged from 22 (RNB0/1c and DATs) to 566 (RNB0/1e and OFMs).

Table 4—

Distribution of horses with OFMs, OFTs, and DAT and various radiographic findings referable to the RNB in the same horses as in Tables 1 through 3.

TraitRNB0/1a (n=1,056)RNB0/1b (n=875)RNB0/1c (n=161)RNB0/1d (n=1,036)RNB0/1e (n=2,064)
OFMs (1,433)26927136307566
OFTs (473)928923112200
DAT (397)77762298174

See Table 1 for key.

Transformed heritabilities were moderate for OFMs (h2 = 0.14 ± 0.03), OFTs (h2 = 0.34 ± 0.05), and DATs (h2 = 0.18 ± 0.04). Results of the correlation analyses between RNB traits and OFMs, OFTs, and DAT were summarized (Table 5). Moderately negative additive genetic correlations were estimated between OFMs and RNB0/1a; between OFTs and RNB0–7, RNB0–3, RNB0/1a, RNB0/1b, and RNB0/1e; and between DATs and RNB0/1a and RNB0/1e (rg = −0.40 to −0.11; SErg, 0.04 to 0.17). Moderately positive additive genetic correlations were estimated between OFMs and RNB0/1e and between DAT and RNB0/1c (rg = 0.19 to 0.25), but the corresponding SEs were high (SErg, 0.13 to 0.20). Residual correlations between RNB traits and OFMs, OFTs, and DAT were close to 0, ranging from re = −0.04 to 0.05 (SEre, 0.02) before transformation and from re = −0.07 to 0.09 (SEre, 0.06) after transformation.

Table 5—

Additive genetic correlations (rg [SE]) and residual correlations (transformed estimates; re [SE]) among OFMs, OFTs, and DAT, and various radiographic findings referable to the RNB in the same horses as in Tables 1 through 4.

TraitOFMsOFTsDATs
rgrergrergre
RNB0–70.0403 (0.1143)−0.0210 (0.0208)−0.1076 (0.1034)0.0871 (0.0301)0.0072 (0.1409)0.0479 (0.0262)
RNB0–30.0023 (0.1125)−0.0103 (0.0223)−0.1262 (0.0953)0.0721 (0.0301)0.0247 (0.0848)0.0564 (0.0232)
RNB0/1a−0.4041 (0.1199)−0.0146 (0.0218)−0.1574 (0.1336)0.0196 (0.0338)−0.1860 (0.1711)0.0360 (0.0334)
RNB0/1b0.0336 (0.1144)0.0538 (0.0283)0.0300 (0.1077)0.0110 (0.0379)0.1874 (0.1251)0.0029 (0.0373)
RNB0/1c0.2500 (0.1950)−0.1332 (0.0447)−0.2395 (0.0354)0.1711 (0.0432)−0.2207 (0.0464)0.1686 (0.0461)
RNB0/1d0.0771 (0.1122)0.0288 (0.0254)−0.0289 (0.1085)0.0551 (0.0370)0.1162 (0.1306)0.0434 (0.0353)
RNB0/1e−0.0873 (0.1086)−0.0094 (0.0282)−0.1313 (0.0292)0.0705 (0.0222)0.0074 (0.0343)0.0666 (0.0049)

See Table 1 for key.

Discussion

The first objective of this study was to determine whether there were genetic correlations between different types of radiographic appearances of the navicular bones in the forefeet of Hanoverian Warmblood horses. The second objective was to determine whether there were genetic correlations between RNB and other radiographic findings that are frequently observed in the limbs of young riding horses.

Prevalences of 9.1% to 20.8% for osseous fragments in the fetlock and tarsocrural joints, deforming arthropathy in the tarsal joints, and pathologic changes in the navicular bones in young Hanoverian Warmbloods have been determined.7,12,13,17 However, detailed information derived from radiographic findings in individual horses was not used in previous genetic analyses5–9,12,i of the radiographic appearance of navicular bones. The fact that there was no distinction among individual radiographic findings in genetic analyses implies genetic homogeneity, but no studies have substantiated the assumption of genetic homogeneity or disproven the assumption of genetic heterogeneity for various types of radiographic findings in navicular bones. Because analyzing a combination of genetically heterogeneous conditions as 1 trait is likely to affect estimated genetic variables and biased variance and covariance estimates will result in over- or underestimation of heritabilities and genetic correlations, detailed radiographic data were used in this study.

Navicular bones were classified on the basis of the radiographic appearance of CSs and bone structure and contour. There is general agreement among investigators2,11,18–20 that these are the criteria that should be considered in evaluation of navicular bone radiographs, and several radiographic classification schemes1,11,18,20,21,g have been developed on that basis. However, a clear distinction between radiographic findings that are indicative and those that are not indicative of podotrochleosis has not been drawn.a–g Although the risk for clinical manifestation of lameness may increase with the extent of radiographic changes, presumptively pathologic changes that are detected radiographically in the navicular bone do not necessarily correlate with clinical disease.

The shape of the proximal border of the navicular bone is postulated to be hereditary and associated with radiographic findings considered to be indicative of podotrochleosis.1 By use of different classification schemes for categorizing the RNB, significant differences among paternal half-sibling groups have been reported.1,5,22,i,m Genetic parameters for all-or-none traits resembling presence or absence of radiographic findings presumed to be associated with podotrochleosis have been estimated. Reported heritability estimates were low (h2 = 0.06)9 when expressed on the observed scale, but were moderate to high (h2 = 0.20 to 0.46) when expressed on the underlying liability scale.6–8,12 In the present study, the transformed heritability estimate was low (h2 = 0.09 to 10) for presence of few, short, conical CSs in the central portion of the distal border of the navicular bone. This radiographic finding is generally considered to represent physiologic variation,2, 3,11,19,20,h and it is unlikely that it was included in previous genetic analyses. Transformed heritability estimates for the other binary traits were moderate (RNB0/1b-e; h2 = 0.17 to 0.34) and similar to the heritability estimates for the quasi-linear traits (RNB0–7 and RNB0–3; h2 = 0.14 to 0.19).

If linear models are used for genetic analysis of binary traits, underestimation of heritabilities and residual correlations must be compensated for via transformation.15,16 The applicability of transformation factors to our data was substantiated in a simulation study.17 Linear models were also used for analysis of the quasi-linear traits, although assumption of normality was clearly violated in our data. However, previous investigators23–25 failed to detect relevant differences between results of linear and threshold-model analysis of ordered categoric data. Untransformed heritability estimates of the quasi-linear traits should therefore be as reliable as transformed heritability estimates of the binary traits.

Variation of trait definition, resembling separate versus combined genetic analysis of particular radiographic features of the navicular bones, was used to test the genetic equivalence of different types of radiographic findings. Radiographic findings in the navicular bones were fit in 1 of 7 mutually exclusive categories of radiographic appearance. Given the low prevalences of radiographic findings fitting into 5 of those 7 categories, it was not possible to derive 7 distinct binary traits or estimate genetic correlations among them. Accordingly, either the 2 large categories (ie, I and II) or combinations of different categories of RNB provided the basis for binary trait definition. Combination of different forms of radiographic findings in defining binary traits reflects the common practice of simplification in animal breeding. Inclusion of mutually exclusive and combined traits in 1 set of bivariate genetic analyses results in overlap and estimation of part-to-whole correlations.

To study the effects of simplification, we evaluated 3 individual binary traits (RNB0/1a, RNB0/1b, and RNB0/1c) and 2 combined binary traits (RNB0/1d and RNB0/1e). Additive genetic correlations were mostly close to unity. However, radiographic findings that represented more extensive navicular bone remodeling indicated that there were exceptions; those radiographic findings were genetically correlated to the other traits with a maximal r value of 0.40. Because alterations in structure or contour were rarely seen in the navicular bones of the study horses, they were not analyzed individually but together with deformed CSs. Even so, the proportion of horses with these marked radiographic findings was < 5%, and the respective values for variance and covariance had large SEs. Although correlation estimates relating to these lowprevalence findings were therefore less meaningful than estimates relating to the more prevalent findings, they may be an indication of different genetic background. Choice of trait definition for genetic evaluation for RNB must account for possible genetic inhomogeneity. If breeding measures are to aim at improvement in the RNB, the choice of trait definition cannot be made exclusively on the basis of heritability estimates (ie, from estimating which trait would be expected to respond the most quickly to selection).

Osseous fragments in the fetlock and tarsocrural joints and deforming arthropathy in the tarsal joints were included in the correlation analyses because of their quantitative importance among young riding horses.7,12,13,17 The mode of trait definition with respect to the radiographic appearance of the navicular bones had a considerable influence on the correlation estimates. The finding of few short and conical CSs in the central portion of the distal border of the navicular bone is typically interpreted as variation within physiologic range.2,3,11,19,20,h Significant negative additive genetic correlations indicated that few short and conical CSs in the central portion of the distal border of the navicular bone were less likely to occur in horses affected with OFMs, OFTs, or DAT. There were further indications for a negative additive genetic correlation between most classifications of RNB of the forelimbs and OFTs. However, SEs of additive genetic correlations in many cases were as large as or larger than the respective correlation estimates.

Detailed radiographic information from larger numbers of horses is needed to obtain more reliable estimates of correlations among different types of radiographic findings in the navicular bones and between those and other relevant radiographic findings in horses. Furthermore, data from horses with clinical navicular disease as well as extensive radiographic changes should be included in future genetic analyses. Such information would likely help provide genetic justification for distinguishing between physiologic and pathologic radiographic findings. Until then, neither merely distinguishing between presence or absence of radiographic changes nor empiric distinction between normal and abnormal radiographic appearance appears to be a suitable basis for reliable genetic analyses, and a quasi-linear description of the RNB should be used instead. Such use of all available information should increase the reliability of genetic analyses, and use of reliably estimated genetic parameters is prerequisite for optimal use of the radiographic appearance of navicular bones in future breeding strategies. Genetic gain from selection may be greater with the use of RNB0/1e (ie, distinction between existence and nonexistence of radiographic changes involving the distal border of the navicular bone). However, the importance of physiologic, clinically irrelevant variation in radiographic appearance is not yet known and should be determined in future long-term studies.

Distinction between existence and nonexistence of distinct radiographic findings in the navicular bones (trait RNB0/1d in this study) has been used to demonstrate that it will be possible to decrease the prevalences of certain radiographic findings in the Hanoverian Warmblood horse population via genetic evaluation and sire selection.12,26 Refined trait definition will increase selection efficiency and accelerate breeding progress with respect to the radiographic appearance of the forelimb navicular bones in Hanoverian Warmblood riding horses.

ABBREVIATIONS

RNB

Radiographic appearance of the distal sesamoid (navicular) bones of the forelimb

CS

Canale sesamoidale

OFM

Osseous fragment in the metacarpo- or metatarsophalangeal (fetlock) joints

OFT

Osseous fragment in the tarsocrural joints

DAT

Deforming arthropathy in the tarsal (tarsocrural, proximal or distal intertarsal, or tarsometatarsal) joints

a.

Ammann E. Strukturveränderungen im Strahlbein gesunder und lahmer Pferde. Master's thesis, Department of Veterinary Medicine, University of Bern, Bern, Switzerland, 1987.

b.

Bodenmüller J. Der Wert von Röntgenaufnahmen für die Früherkennung von Podotrochlose (Strahlbeinlahmheit) bei der Ankaufsuntersuchung von Pferden. Doctoral dissertation, Department of Agricultural Sciences, University of Zürich, Zurich, Switzerland, 1983.

c.

Branscheid WJ. Untersuchungen an der Hufrolle bei Pferden mit und ohne Hufrollen-erkrankung (Podotrochlose). Doctoral dissertation, Department, University of Hohenheim, Stuttgart, Germany 1977.

d.

Langfeldt N. Statistische Untersuchungen zum Problemkreis der Podotrochlose. Ein Vergleich allgemeiner, klinischer und röntgenologischer Parameter am Patientenmaterial der Klinik für Pferde der Tierärztlichen Hochschule Hannover der Jahre 1980 bis 1984. Master's thesis, University of Veterinary Medicine Hannover, Hannover, Germany, 1986.

e.

Leuenberger H. Radiologische Untersuchungen am Strahlbein klinisch strahlbeinlahmer Pferde und Vergleich mit gesunden Pferden. Master's thesis, Department, University of Bern, Bern, Switzerland, 1989.

f.

Röstel-Peters B. Untersuchungen zur Röntgendiagnostik der Podotrochlose. Darstellbarkeit, Aussagekraft und Schematisierung von Befunden. Department of Veterinary Medicine, Freie Universität Berlin, Berlin, Germany, 1987.

g.

Seyrek-Intas D. Die Beziehung zwischen radiologischen Einzelbefunden und dem klinischen Erscheinungsbild bei der Podotrochlose des Pferdes. Department of Veterinary Medicine, Justus-Liebig-Universität Gießen, Giessen, Germany, 1993.

h.

Brunken E. Röntgenologische Verlaufsuntersuchungen am Strahlbein des Pferdes. University of Veterinary Medicine Hannover, Hannover, Germany, 1986.

i.

Astner L. Röntgenologische und klinische Untersuchung zur Erblichkeit der Podotrochlose beim Österreichischen Warmblutpferd (Feldstudie). University of Veterinary Medicine Vienna, Vienna, Austria, 1986.

j.

GLM procedure, SAS, version 9.1.3, SAS Institute Inc, Cary, NC.

k.

GENMOD procedure, SAS, version 9.1.3, SAS Institute Inc, Cary, NC.

l.

VCE5 (Variance Component Estimation), version 5.1.2, Institute for Animal Science and Animal Behavior, Federal Agricultural Research Centre, Mariensee-Neustadt, Germany.

m.

Hornig I. Radiologische Untersuchungen am Strahlbein zweijähriger Warmblutpferde. University of Bern, Bern, Switzerland, 1993.

References

  • 1

    Dik KJ, van den Broek J. Role of navicular bone shape in the pathogenesis of navicular disease: a radiological study. Equine Vet J 1995;27: 390393.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 2

    Hertsch B-W, Zeller R. Röntgenologische Veränderungen am Strahlbein und ihre Beurteilung. Prakt Tierarzt 1976;58: 1518.

  • 3

    Ueltschi G. Zur Röntgendiagnostik des Strahlbeins. Pferdeheilkunde 2002;18: 217224.

  • 4

    Van Hoogmoed LM, Snyder JR & Thomas HL, et al. Retrospective evaluation of equine prepurchase examinations performed in 1991–2000. Equine Vet J 2003;35: 375381.

    • Search Google Scholar
    • Export Citation
  • 5

    Bos H, van der Meij GJW, Dik KJ. Heredity of navicular disease. Vet Q 1986;8: 6872.

  • 6

    Koninklijke Vereniging Warmbloed Paardenstamboek Nederland [Royal Warmblood Studbook of the Netherlands]. The frequency and heredity of navicular disease, sesamoidosis, fetlock joint arthrosis, bone spavin and osteochondrosis of the hock. A radiographic progeny study. Zeist, The Netherlands: Koninklijke Vereniging Warmbloed Paardenstamboek Nederland, 1994.

    • Search Google Scholar
    • Export Citation
  • 7

    Stock KF, Hamann H, Distl O. Variance component estimation on the frequency of pathologic changes in the navicular bones of Hanoverian Warmblood horses. J Anim Breed Genet 2004;121: 289301.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 8

    Willms F. Röhe R, Kalm E. Genetische Analyse von Merkmalskomplexen in der Reitpferdezucht unter Berücksichtigung von Gliedmaßenveränderungen. 1. Mitteilung: Züchterische Bedeutung von Gliedmaßenveränderungen. Züchtungskunde 1999;71: 330345.

    • Search Google Scholar
    • Export Citation
  • 9

    Winter D, Bruns E & Glodek P, et al. Genetische Disposition von Gliedmaßen-erkrankungen bei Reitpferden. Züchtungskunde 1996;68: 92108.

  • 10

    Oxspring GE. The radiology of navicular disease, with observations on its pathology. Vet Rec 1935;15: 14331447.

  • 11

    Gerhards H, Hertsch B-W & Jahn W, et al. for the Zweite Röntgenkommission. Leitfaden für die röntgenologische Beurteilung bei der Kaufuntersuchung des Pferdes (Röntgenleitfaden). Pferdeheilkunde 2003;19: 185198.

    • Search Google Scholar
    • Export Citation
  • 12

    Stock KF, Distl O. Prediction of breeding values for osseous fragments in fetlock and hock joints, deforming arthropathy in hock joints and pathologic changes in the navicular bones of Hanoverian Warmblood horses. Livest Prod Sci 2005;92: 7794.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13

    Stock KF, Hamann H, Distl O. Variance component estimation on the frequency of deforming arthropathies in limb joints of Hanoverian Warmblood horses. J Anim Breed Genet 2004;121: 269288.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 14

    Kovac M, Groeneveld E. Garcia-Cortez A. VCE-5 user's guide and reference manual version 5.1.2. Mariensee-Neustadt, Germany: Institute for Animal Science and Animal Behavior, Federal Agricultural Research Centre, 2003.

    • Search Google Scholar
    • Export Citation
  • 15

    Dempster ER, Lerner IM. Heritability of threshold characters. Genetics 1950;35: 212235.

  • 16

    Vinson WE, White JM, Kliewer RH. Overall classification as a selection criterion for improving categorically scored components of type in Holsteins. J Dairy Sci 1976;59: 21042114.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 17

    Stock KF, Hamann H, Distl O. Estimation of genetic parameters for the prevalence of osseous fragments in limb joints of Hanoverian Warmblood horses. J Anim Breed Genet 2005;122:271280.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 18

    Huskamp B, Becker M. Diagnose und Prognose der röntgenologischen Veränderungen an den Strahlbeinen der Vordergliedmaßen der Pferde unter besonderer Berücksichtigung der Ankaufsuntersuchung. Ein Versuch zur Schematisierung der Befunde. Prakt Tierarzt 1980;10: 858863.

    • Search Google Scholar
    • Export Citation
  • 19

    Kaser-Hotz B, Ueltschi G. Radiographic appearance of the navicular bone in sound horses. Vet Radiol Ultrasound 1992;33: 917.

  • 20

    Dik KJ, Hertsch B-W, Ueltschi G. for the Erste Röntgenkommission. Ergebnisprotokoll des 1. und 2. Treffens der Röntgenkommission am 14, April 1993 in Ubrecht, und 1, July 1993 in Zürich. Gesellschaft für Pferdemedizin 1993.

  • 21

    MacGregor CM. Radiographic assessment of navicular bones, based on changes in the distal nutrient foramina. Equine Vet J 1986;18: 203206.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 22

    Philipsson J, Brendow E & Dalin G, et al. Genetic aspects of diseases and lesions in horses, inProceedings. 6th World Cong Genet Applied Livest Prod1998;24:408415.

    • Search Google Scholar
    • Export Citation
  • 23

    Matos CAP, Thomas DL & Gianola D, et al. Genetic analysis of discrete reproductive traits in sheep using linear and nonlinear models: I. Estimation of genetic parameters. J Anim Sci 1997;75: 7687.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 24

    Varona L, Misztal I, Bertrand JK. Threshold-linear versus linear-linear analysis of birth weight and calving ease using an animal model: I. Variance component estimation. J Anim Sci 1999;77: 19942002.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 25

    Varona L, Misztal I, Bertrand JK. Threshold-linear versus linear -linear analysis of birth weight and calving ease using an animal model: II. Comparison of models. J Anim Sci 1999;77: 20032027.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 26

    Stock KF, Distl O. Expected response to selection when accounting for orthopedic health traits in a population of Warmblood riding horses. Am J Vet Res 2005;66: 13711379.

    • Crossref
    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 20 0 0
Full Text Views 148 107 7
PDF Downloads 55 41 8
Advertisement