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Osteochondrodysplasia in Scottish Fold cats

R MALIKa, GS ALLANa, CR HOWLETTb, DE THOMPSONc, G JAMESd, C McWHIRTERe and K KENDALLf

Objective To better characterise the bone and joint problems which can develop in Scottish Fold cats.

Design Retrospective study of cases seen in five veterinary clinics and radiographic survey of cats in a cattery.

Results Six Scottish Fold cats (four castrated males, two spayed females) aged between 5 months and 6 years werepresented for signs of skeletal disease including lameness, reluctance to jump, a stiff stilted gait, short misshapen distallimbs, swelling of plantar tarsometatarsal regions and short thick inflexible tails. A further four cases (one male, threefemales, 15 months to 11 years) were identified by radiographic screening of a cattery. A diagnosis of osteochondrodys-plasia was based on characteristic radiological findings including irregularity in the size and shape of tarsal, carpal,metatarsal and metacarpal bones, phalanges and caudal vertebrae, narrowed joint spaces, and progressive new boneformation around joints of distal limbs with diffuse osteopenia of adjacent bone. A plantar exostosis caudal to the calca-neus was present in advanced cases. In all nine cases where pedigree information was available, affected catsallegedly originated from the mating of a Scottish Fold to a cat with normal ears. The severity and time of onset of phys-ical signs, and rate of progression and extent of radiographic abnormalities, varied from case to case. Limited histolog-ical observations suggested the underlying problem may be an osteochondrodysplasia, related to inadequate cartilagematuration. Clinical signs were ameliorated by administration of pentosan subcutaneously in two of three cats in which itwas trialled, and one of these also benefited from an oral glycosaminoglycan preparation.

Conclusions Clinical and radiological findings were ascribed to defective maturation and function of cartilage, partic-ularly in the distal limbs, ears and tail. As all Scottish Fold cats suffered from osteochondrodysplasia of some degree,the best solution would be to avoid using fold-eared cats for breeding and instead use Scottish shorthairs.Aust Vet J 1999;77:85-92Key words: Cat, Scottish Fold, osteochondrodysplasia, osteodystrophy, chondrodysplasia, cartilage, autosomal dominant, lameness.

Fold Scottish FoldPO OrallySFOCD Scottish Fold osteochondrodysplasiaSC Subcutaneously

The Scottish Fold is one of therarest purebred cats in Australia.The defining feature of the breed

is its ears, which fold forward (Figure 1).The ear fold is not present at birth butdevelops at 3 to 4 weeks of age. Thebreed was developed in Scotland bymating a queen, affected by a naturalmutation, to local farm cats and BritishShorthairs.1 Preliminary genetic analysessuggested that the defining geneticabnormality, which causes the auricularcartilage to fold, is inherited as a simpleautosomal dominant trait, and the fold-ear allele is designated Fd.2,3

Although fold-eared cats were firstbred in 1961 and registered in 1966,there was no mention of any skeletaldeformities until 1971 when progressivebony abnormalities and a cripplinglameness were recognised.1,4 Inheritancestudies were based on three differenttypes of matings: skeletally normal Foldsto skeletally normal Folds, affected Foldsto skeletally normal Folds, and affectedFolds to normal cats. Kittens with radio-logical lesions were only produced fromFold-to-Fold matings.5 It was inferredthat affected kittens are homozygous forthe Fd gene: thus normal cats are fdfd,skeletally normal Scottish Folds areFdfd, and Scottish Folds with osteodys-trophy are FdFd. It has been suggestedthat breeding of fold-eared cats shouldbe limited, and if used at all, they shouldbe mated only to normal-eared cats.5

The breed was banned in England in1974, but continued to evolve in theUSA where initial imports from the UKwere outcrossed to numerous otherbreeds.1,4,5 Fold-to-Fold matings wereavoided, so only about 50% of offspringhad folded ears. Kittens with normal

(prick, pert or upright) ears from thistype of mating are known as ‘Scottishshorthairs’ in North America andAustralia (synonym: Scottish Fold S/E[straight ear], Scottish Fold variant; JSternbeck personal communication).

A limited description of SFOCD wasgiven initially by Jackson,5 and moredetails for a single case were reportedrecently.6 The earliest and most consis-tent finding is a tail with an abnormallythick and inflexible base. The feet areshort, and the underlying skeletalchanges result in reduced ability tosupport weight, an abnormal gait andultimately lameness. In affectedhomozygous individuals, joint lesionsprogress until the cats are unable towalk. Lesions are evident radiographi-cally in kittens aged 7 weeks.5 The meta-physes of metatarsal and metacarpalbones are distorted with widenedphyses, and similar but less markedchanges are evident in phalanges. Thisresults in decreased length and abnormalshape of these bones and shortening ofdistal limbs. The caudal vertebrae arereduced in length, with widened

aDepartment of Veterinary Clinical Sciences, TheUniversity of Sydney, New South Wales 2006bSchool of Pathology, The University of NewSouth Wales, Kensington, New South Wales 2052cKu-Ring-Gai Veterinary Hospital, 290 BobbinHead Road, Turramurra North, New South Wales2074dSylvania Veterinary Hospital, 335 PrincesHighway, Sylvania Heights, New South Wales2224eChatswood Veterinary Clinic, 80 Sydney Street,Willoughby, New South Wales 2068fEast Chatswood Cat Clinic, 346 Penshurst Street,Willoughby, New South Wales 2068

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endplates. After age 6 months, grossplantar exostoses of tarsal and metatarsalbones become clinically and radiograph-ically evident. Histologically, tissuesfrom affected kittens show defectivebone formation at growth plates, withdisordered chondroblast proliferationand irregular groups of cells arrangedhaphazardly. Physes are grosslyexpanded with deficient ossification,irregular mineralisation, defectiveremodelling and supernumerary centresof ossification.

The original observations on SFOCDwere published in a journal not widelyavailable outside the UK. Subsequentaccounts paraphrase these observations,but consider the problem as a chon-drodysplasia.7 However, it was notedthat problems may occur later in life insome Folds, in which radiographic signsare initially absent,1 and that heterozy-gotes may develop skeletal disease that ismuch milder than observed in homozy-gous Folds.4

This article concerns six cats thatdeveloped signs of SFOCD. At least fiveof them were allegedly the progeny ofmating a Fold to a non-Fold. A radio-graphic survey in a cattery suggestedthat, contrary to contemporary wisdom,this was likely a widespread problem inthis breed.

Case reportsCase 1

A female Fold (Figure 1) brought infor vaccination was considered some-

what small (1.3 kg) for her age (14weeks). It was presented for evaluationof left hindlimb lameness 5 weeks later,weighing 1.8 kg. Lameness was thoughtto be worse in the mornings. The distalhindlimbs were abnormally short andmisshapen, with excessively curvedtoenails. Radiographs of the distalextremities and tail at 21 weeks showedchanges consistent with SFOCD,including marked malformation ofmetatarsal and metacarpal bones,phalanges and caudal vertebrae. Theproximal tail was thickened and inflex-ible (Figure 2). No treatment wasprescribed and the kitten was spayed.Radiographs at 6 and 11 monthsshowed progression of skeletal abnor-malities, with development of periartic-ular new bone, narrowed joint spacesand apparent loss of bone density indistal limbs. At this time the cat was saidto ‘seize up’ during play. Pentosan poly-sulphate (3 mg/kg SC every week for 4weeks) was prescribed. According to theowners, this reduced signs of dysfunction.Current therapy is a complex glyco-saminoglycan preparation (Cartiflex,Pharmedica, 2 g PO daily for 30 days,then 2 g twice weekly) that also appearsto reduce the cat’s discomfort.

Case 2A 6-month-old castrated Fold was

presented for an abnormal gait whichhad become noticeable over thepreceding 3 weeks. The cat’s movementshad become stiff and stilted, its back had

become arched and bilateral palpableswellings of the tarsi became obvious tothe owner. Radiographs demonstratedchanges consistent with SFOCD, andthe cat was euthanased at its owners’request. Necropsy was not permitted.

Case 3A 16-month-old male Fold (4.0 kg)

was presented for right forelimb lame-ness of 3 weeks duration. It had beenconsidered normal when vaccinated at14 weeks (1.8 kg), and castrated 6 weekslater. The affected paw was painful onpalpation. Radiographs under generalanaesthesia demonstrated changes in thelimbs consistent with SFOCD,including malformed metacarpal andmetatarsal bones and periarticular newbone formation. The cat was treatedwith pentosan polysulphate (3 mg/kgSC once weekly for 4 weeks), whichproduced some reduction in lameness.The cat died after a traffic accident 1year later and was submitted fornecropsy. Tissues were collected forhistopathological examination. Inspec-tion at necropsy showed foreshorteningof the distal limbs.

Case 4An 11-month-old male Fold was

presented for limping. The cat had beenconsidered normal when vaccinated at18 weeks and castrated at 8 months. Ithad been lame for several weeks, andappeared to avoid jumping from anyheight. Hard, symmetrical, non-painful

Figure 1. The cat in case 1. Note the forward-folding ears. Figure 2. The thickened, shortened, inflexible tail of the cat in case

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swellings were palpable aroundtarsometatarsal regions and similar butless prominent swellings were evidentaround carpometacarpal regions. Mildbilateral patellar luxation, worse on theleft, was present. Hindlimb radiographsdemonstrated changes consistent withSFOCD, including abnormalities inshape and size of metatarsal bones, withmilder changes in the distal forelimbs.Samples of synovial fluid from carpusand tarsus showed no abnormalities.

The cat was re-presented 10 monthslater because of worsening signs. Theowners considered the cat to be in pain,particularly on cold mornings. It couldnot jump, and movement was difficulteven after it had ‘warmed up’. Radiog-raphy demonstrated progression ofskeletal lesions, with periarticular newbone formation resulting in ankylosis ofdistal joints. The owners requestedeuthanasia of the cat. Tissues forhistopathological examination werecollected at necropsy.

Case 5A 1-year-old spayed Fold was

presented for right forelimb lameness ofa few days duration. During physicalexamination, pain was detected onpalpation of the forelimbs. Radiographsdemonstrated changes consistent withSFOCD, with shortening and outwardbowing of metacarpals two and five, butlittle periarticular new bone formation.Similar changes were noted also in thehindlimbs. The tail was unremarkableradiographically.

Case 6A 6-year-old castrated Fold was

presented for investigation of a loco-motor problem. The cat had beenrecently rehoused, and its new ownernoted the cat had a stiff, short and some-what immobile tail, and misshapen andsplayed claws. The cat’s gait wasabnormal, it limped occasionally andflinched and vocalised when picked up.Radiographs showed changes consistentwith SFOCD; there was little bonydeformity, but periarticular new bonewas prominent and had resulted inankylosis of intertarsal andtarsometatarsal joints. The cat wastreated with pentosan polysulphate (3mg/kg SC once weekly for 4 weeks)without discernible improvement. Signswere unchanged 1 year later. The cat’s

hindclaws need regular trimming as theytend to grow abnormally.

Further casesTo further assess the prevalence of

SFOCD, distal limbs and tails of‘asymptomatic’ breeding stock from acommercial cattery were screened radio-graphically. Five cats were examined,four Folds (three females of 4, 5 and 11years; one male of 15 months of age)and a Scottish Shorthair (male, 9 years).All Folds had radiographic changesindicative of SFOCD, ranging from verymild (15-month-old male) to severe (4-year-old female), but no abnormalitieswere detected in the Scottish Shorthair.The changes observed in the worseaffected 4-year-old female Fold weremore extensive and severe than those ofthe 11-year-old female. The tail wasaffected in all Folds except the 5-year-old female.

Mating dataPedigrees were obtained for nine Folds

with SFOCD: cases 1 to 5 and the fouraffected Folds in the cattery. All cats

originated from the mating of a Fold toa cat with normal ears, this being a Scottish shorthair in seven instances,and a British Shorthair in one. In sixcases the queen was a Fold, while inthree cases the tom was a Fold. Thusnone resulted from a Fold-to-Foldmating. Several of the cats were closelyrelated: cats in cases 2 and 3 and one ofthe cats from the cattery had the sameparents, while cats in cases 4 and 5 andanother cat from the cattery were theproduct of another parent set. Of theremaining three cats, two came fromentirely different lines.

Radiographic findingsRadiographs provided the basis for

confirming a diagnosis of SFOCD.Changes were similar in all cats, and thetwo cats subjected to serial radiographyshowed progressive disease. The rate ofprogression differed between cats, as didthe age of apparent onset of clinicalsigns. Arbitrarily, we divided the radio-graphic changes into two components.

Firstly, there were skeletal deformitieswhich were thought to result from

Figure 3. (A) Dorsopalmar radiograph of the distal forelimb of the cat in case 3 at 16months. The metacarpal bones and phalanges are shortened somewhat, the distal radialand ulnar growth plates are of an abnormal shape, and there is ‘mushrooming’ of thedistal radial metaphysis. There is also extensive periarticular new bone formation aroundcarpal and proximal metacarpal bones that has resulted in bony ankylosis. Intercarpaland carpometacarpal joint spaces appear almost completely obliterated. A dorsopalmarradiograph of a normal cat’s metacarpals is provided in (B) for comparison; note thedifference in the length of the metacarpal bones and phalanges.

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abnormal bone growth during develop-ment, and corresponded to the anatom-ical finding of shortened distal extremi-ties. These changes were conspicuous incases 1, 3 and 4, although similar butmore subtle changes could be discernedin all other cases. In case 3, for example,the metacarpal bones were shorter thannormal, of abnormal shape (proximaland distal ends disproportionately large)and of inconsistent length (metacarpalsthree and four disproportionatelyshorter) (Figure 3A). Similarly, thephalanges were of abnormal size andshape. Deformities of the distal appen-dicular skeleton were even more promi-nent in case 1: all metatarsal andmetacarpal bones were shortened andmalformed, some having a bent shapewith swollen proximal and distal ends,while phalanges were similarly affected(Figures 4 and 5). Radiographs of thetail were not performed routinely, butcats with a short thick tail invariably hadcaudal vertebrae that were shorter andwider than normal (Figure 6). Interest-ingly, the tail was unaffected in case 4and one of the cattery cats despite severeprogressive appendicular disease.

Secondly, there were additional bonechanges evident in sequential radio-graphs that suggested a progressiveankylosing polyarthropathy affectingdistal limb joints. Changes of this typewere present in all cases, but developedmore rapidly in cats with distal limbshortening (Figures 3 and 7) than incases with normally proportioned limbs(Figure 8). Pelvic limbs tended to beearlier and more affected. Periarticularnew bone formation, the most conspic-uous change, was typically first detectedalong proximal metatarsals, where itextended and merged with new boneformed on the distal tarsal bones(Figures 7 and 8). These changes wereprogressive, occurred circumferentiallyaround the tarsometatarsal joint, andeventually resulted in tarsometatarsaland intertarsal ankylosis (Figures 7 and8). The periarticular new bone was typi-cally smooth, although the extensivenew bone that formed plantar to thecalcaneus and eventually extended intoadjacent soft tissues as an exostoses wasmore irregular. Intertarsal andtarsometatarsal joint spaces becameirregular, indistinct and progressivelynarrowed, and the corresponding jointsin the forelimbs eventually became simi-

Figure 4. Dorsopalmar radiograph of thedistal hindlimb of the cat in case 1 at 6months. The metatarsal bones are shorterthan normal, misshapen asymmetrically,with swelling of the proximal and distalends and bending of shafts. Intertarsaland tarsometatarsal joint spaces seemnormal, and periarticular new bone forma-tion is not apparent.

Figure 5. Lateral radiograph of the distalhindlimb of the cat in case 1 at 6 months.The metatarsal bones are shorter thannormal, asymmetrically misshapen, withenlargement of proximal and distal endsand bending of shafts. Intertarsal andtarsometatarsal joint spaces are of rela-tively normal appearance, and periartic-ular new bone formation is not apparent.

Figure 6. Radiograph of the thickened shortened inflexible tail of the cat in case 1 at 11months. Several caudal vertebrae are shorter than normal, with enlarged bony vertebralendplates, reduced intervertebral spaces and new bone formation tending towards anky-losis of adjacent vertebrae.

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larly affected (Figure 3). As well, tarsaland metatarsal bones became less radio-dense, with irregular punctate areas ofosteolucency giving the tarsal bones amoth-eaten appearance (Figure 7).

The proximal limb joints, long bonesof the limbs and the vertebral column,except for the tail, appeared unaffected.

Histopathological findingsTissues from cases 3 and 4 were fixed

for 2 to 3 months in buffered formalin(pH 7.2) and decalcified using a rapiddecalcification procedure for 2 daysfollowed by a further 5 days in 5%formic acid. The tissues were thenroutinely dehydrated and embedded inparaffin. Sections cut at 4 and 7 µm werestained with haematoxylin and eosin.Similar histological changes wereobserved at the articular surfaces of thephalanges, metatarsal and metacarpalbones. Some interphalangeal jointssectioned were distorted and subluxated.Some bones in the distal limbs hadabnormal articular cartilage. The hyalinecartilage was thicker in places: herenecrotic cartilaginous foci were evidentalthough the cartilage maintained itsmicroscopic structure, but with chon-drocytes failing to take up stain. Nearbycells stained correctly and, on the edgeof this thicker articular cartilage, prolif-erating chondrocytes appeared to growfrom the margins towards the centre ofthe articular cartilage (Figure 9). Wherearticular cartilage was thicker it appearedto be inadequately resorbed, and theunderlying junction between cartilageand bone had a transitional zone witheither a discontinuous tide mark or a‘stuttered’ layered tide mark. Largeisolated islands of hyaline cartilage wereobserved in some places, extending fromthe articular cartilage into epiphysealbone (Figures 10 and 11), and in otherplaces microscopic foci of apparentlyviable cartilage were enclosed insubchondral bone. These findingssuggested that remodelling of cartilagewas slowed, perhaps due to delayed orinadequate maturation. Some bonesdisplayed subarticular osseous erosionsconsisting of areas of osteoclasis, accom-plished by mostly mononuclear osteo-clasts, marginated by subperiosteal boneformation.

Flaking and fibrillation of the articularcartilage was sometimes evident.Chronic synovitis accompanied these

changes, typically with a small numberof fibrous synovial villi and a minimalcellular response. The predominantproliferative cell in the villus was thesynovial membrane cell, although occa-sionally mononuclear cells were presentalso. The fibrocartilage at tendo-osseousjunctions appeared distorted without anordered junctional array. Mature bone innodules and spicules was evident in peri-articular dense connective tissue of liga-ments and joint capsules, formingosseous projections (enthesophytes).This bone appeared to be forming as aresult of an intermembranous process.

In case 3 the distal radial and ulnargrowth plates were present but function-ally closed. Growth plate cartilage chon-drocytes were small, eosinophilic, butsomewhat clustered, and each separatedby considerable matrix. The chondro-cytic morphological features of thephysis were more like that of theresting/proliferating chondrocytic zoneand these cells seemed not to haveentered the hypertrophic stage of matu-ration. Remodelling of cartilageappeared disturbed as a result of inade-quate chondrocytic maturation.

DiscussionThe defining phenotypic feature

which identifies a cat as a Scottish Foldsuggests a developmental cartilagedefect, the most obvious manifestationof which is forward folding of the ears.This presumably results from the carti-lage of the pinna lacking sufficientresilience to support the ear against theeffect of gravity. It is hardly surprisingthen that some cats with this geneticdefect have other abnormalities referableto defective cartilage function.

In order to fully describe the osteo-chondrodysplasia encountered in Scot-tish Folds it would be necessary toobserve and sample representative tissuesduring skeletal development, to moreclosely follow the process of endochon-dral ossification in distal extremitiesradiographically and microscopically.Though our histological observationsare limited, it seems that the cartilage inbones of distal limbs is replaced slowly,possibly reflecting inappropriate chon-drocytic maturation. The presence oflarge islands of epiphyseal cartilageattached to articular cartilage suggestsdefective endochondral osteogenesis.The junctions between cartilage and

bone in these regions have a transitionfrom chondrocytes to osteocytes and thismorphological appearance is encoun-tered in conditions such as aviandyschondroplasia8 and dyschon-droplastic lesions seen in fast-growinglarge and giant breeds of dogs.9 If similarchanges occur in physeal cartilagesduring development, there would be

Figure 7. Lateral radiograph of the distalhindlimb of the cat in case 2 at 6 months.The distal tibia and fibula, tarsal andmetatarsal bones and phalanges are notmarkedly deformed, although themetatarsal bones are shorter than normal,with metatarsal bones 2 and 5 beingsplayed outward. Smooth new boneformation is evident around the tarsusand proximal portion of the metatarsusand has resulted in bony ankylosis of thetarsometatarsal and intertarsal joints. Theintertarsal and tarsometatarsal jointspaces appear indistinct and may benarrowed irregularly, and tarsal boneshave a moth-eaten appearance, perhapscontributed to by overlying periarticularbone.

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delayed and dysplastic osteogenesis incertain bones, such as metacarpal andmetatarsal bones, which would result inmalformation. Thus the abnormalitiesin size and shape of bones in cats withSFOCD might occur because of defec-tive endochondral ossification in distalextremities.

Later in life, malformation andresulting abnormal mechanical forcespresumably take their toll, resulting insubarticular osteoclasis and periarticular

new bone formation at insertions oftendons and joint capsules, as well asdegenerative joint disease and synovitis.Deficiencies in the function of articularcartilage, which normally provides a fric-tionless intra-articular surface, maycontribute to premature and accelerateddegenerative joint disease. The severeperiarticular new bone formation atdistal extremities of affected cats corre-sponds to new bone formed in thetendons and joint capsules of distal

joints; this may reflect abnormal stressesabout affected joints and/or be a moredirect result of the abnormal fibrocarti-lage architecture observed in theseregions. Less severely affected cats havelittle developmental bone malformationand tend to present at an older age,when periarticular new bone formationand allied changes have moved towardsankylosis of distal joints.

The danger of producing catshomozygous for the Fd gene from Fold-

Figure 8. Dorsopalmar (A) and lateral (B) radiographs of the distal pelvic limb of the cat in case 6 at the age of 6 years. Smooth periarticularnew bone formation is evident around the tarsus and proximal portion of the metatarsus and has resulted in bony ankylosis of thetarsometatarsal and intertarsal joints. A large enthesophyte is present on the calcaneus, although this is hard to discern in the photo-graph. The intertarsal and tarsometatarsal joint spaces are indistinct and narrowed. Note that the metatarsal bones are nearly ‘normal’ inlength. Similar changes are evident in the lateral radiograph (C) of the distal thoracic limb.

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Clearly there is considerable variation in the severity ofchanges and rate of progression, as the cat in case 6 had rela-tively mild disease at 6 years, whereas cats in cases 2 and 4were sufficiently affected for owners to opt for euthanasia atages 6 and 21 months.

We suspect subclinical or mild involvement may be presentin nearly all mature heterozygous Folds. The natural course ofthis disease in affected individuals needs to be established.Such studies could be performed readily, because radiologicalexamination of the distal limbs should provide a sensitive andspecific screening tool for cats in which physical signs, such aslameness, a short immobile tail, and plantar tarsal exostoses,are absent. Further information concerning the changes in thebones and joints could be obtained non-invasively using serialcomputed tomography or magnetic resonance imaging ofaffected distal extremities.

It could be argued that some of the cats described here orig-inated from matings other than those recorded on their regis-tration papers, but the possibility that this was the case for allnine cats for which pedigrees were available seems remote.Analysis of microsatellites to determine paternity would theo-retically be able to confirm or refute this possibility,10,11 butwas not attempted. Alternatively, Scottish shorthairs used inthe matings may have been phenotypically ‘normal’ but geno-typically heterozygous for the Fd gene, due to incompletepenetration of the trait.12 This possibility is compatible withthe observation by breeders that some kittens with folded earsgrow to have apparently normal ears.13 Another possibility isthat inheritance of the Fd gene shows the genetic phenomenonof ‘anticipation’, whereby expression of the defect developsat a progressively earlier age and/or becomes more severein succeeding generations. Such phenotypic alterations

Figure 9. Photomicrograph of articular cartilage from a tarsalbone from the cat in case 3. There is variability in the staining ofthe chondrocytes at the centre of the articular cartilage: some(small arrows) stain conspicuously with haematoxylin, whereasothers (curved arrows) stain faintly suggesting karyolysis, whilesome chondrocytic lacunae appear empty (arrowhead). Theosteochondral junction is represented by a dense haematoxylinline (H). The subchondral bone is absent in one area, resorbedby osteoclasts (large arrows) and replaced by mononuclearmesenchymal cells. Haematoxylin and eosin x 75.

Figure 10. The section displays an irregular mass of cartilage (C)extending from the articular surface (A) of the tarsal bone intothe bone (B) and, although a rim of bone separates cartilagefrom the large central intra-osseous fatty marrow tissue (Fa), thisbone is thin in one region (arrow). Part of the joint surface islined by fibrovascular tissue (FV). Haematoxylin and eosin x 7.5.

Figure 11. A higher power photomicrograph of the section inFigure 10 displaying an irregular chondro-osseous junction. Thecartilaginous matrix (C) has different staining characteristicswith eosinophilic foci (E) having similarities to that of nearbyosseous matrix (B). An erosion site at the bone-cartilage junc-tion is filled by plump mesenchymal cells (arrow). Haematoxylinand eosin x 75.

to-Fold matings has been long understood by breeders whorecognise that severe signs develop early in cats of the FdFdgenotype. What is unclear, however, is the extent to whichsimilar but milder changes develop in heterozygous Folds.This report suggests that significant disease can develop inheterozygous Folds as young as 6 months, and similar conclu-sions were recently made in a case report from California.6

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have been described for several humandisorders inherited in an autosomaldominant fashion including myotonicdystrophy,14 Huntington’s disease,15

autosomal dominant polycystic kidneydisease,16 and spinocerebellar ataxia type1.17 In these disorders, progressiveamplification of DNA-triplet repeatswithin or adjacent to the disease geneoccur in successive generations, and thisinstability of DNA is associated withincreased disease severity.17 If heritableunstable DNA was involved in thegenetic inheritance of SFOCD, thephenomenon of anticipation wouldprovide an explanation as to whyheterozygous Folds seem more likely todevelop clinical signs now than in earlystudies.5

Although more work needs to bedone, the conclusion from this caseseries is that if breeders continue toproduce cats with cartilage insufficientlystrong to support the weight of the ear,some cats will inevitably develop jointand bone problems because of theunderlying cartilage defect. Prospectiveowners should be warned that Folds maydevelop musculoskeletal dysfunctionlater on, and perhaps counselled to keepcats exclusively indoors, where wear andtear on joints is likely to be less. Furtherstudies of the cartilage abnormality inaffected cats are warranted, as this mayprovide valuable insights into the struc-ture and function of cartilage in cats andother species. A tantalising possibility isthat the cartilage defect may only beevident or important at lower tempera-tures, which would explain the tendencyfor bone and cartilage dysfunction to bemost evident at the extremities.

The treatment of affected cats iscurrently unsatisfactory because thedisease is relentlessly progressive, andspecific therapy is unlikely to be devel-oped until the underlying defect isdefined. A recent report emphasised thevalue of pantarsal arthrodesis as a means

of alleviating tarsal pain in a severelyaffected cat,6 while our findings suggestthat treatment with pentosan andglycosaminoglycans can be helpful.Nonsteroidal anti-inflammatory agentsconsidered to be safe for cats, such ascarprofen,18,19 may also have a place. AsScottish Shorthairs have the same ‘wide,round eyes, sweet expression, soft voice,and fondness for affection’ as Folds,1 asuitable solution to SFOCD would beto abandon using fold-eared cats andinstead use Scottish Shorthairs, withoccasional outcrosses to British Short-hairs and domestic shorthaired cats tomaintain hybrid vigour. If this policywas adopted, the problem of SFOCDcould be eliminated in one generation.

AcknowledgmentsThe authors thank Dr Caroline

O’Leary for introducing us to theconcept of heritable unstable DNA andanticipation. The cooperation ofBajimbi cattery in facilitating thesestudies was greatly appreciated. RichardMalik is supported by the ValentineCharlton Bequest of the Post GraduateFoundation of Veterinary Science of TheUniversity of Sydney.

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(Accepted for publication 1 September 1998)

AddendumWe recently became aware of another catwith SFOCD, diagnosed by Dr HelenParry in Queensland. The cat was theresult of an accidental mating between aScottish Fold and a Devon Rex, and itwas presented for signs referable toplantar exostoses at 2.5 years of age.Characteristic radiographic findingswere present.