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Prevalence of and risk factors for hip dysplasia and cranial cruciate ligament deficiency in dogs

Tige H. WitsbergerComparative Orthopaedic Laboratory, College of Veterinary Medicine, University of Missouri, Columbia, MO 65211.

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J. Armando VillamilComparative Orthopaedic Laboratory, College of Veterinary Medicine, University of Missouri, Columbia, MO 65211.

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Loren G. SchultzComparative Orthopaedic Laboratory, College of Veterinary Medicine, University of Missouri, Columbia, MO 65211.

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Allen W. HahnComparative Orthopaedic Laboratory, College of Veterinary Medicine, University of Missouri, Columbia, MO 65211.

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James L. CookComparative Orthopaedic Laboratory, College of Veterinary Medicine, University of Missouri, Columbia, MO 65211.

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Abstract

Objective—To evaluate prevalence of and risk factors for hip dysplasia (HD) and cranial cruciate ligament deficiency (CCLD) in dogs and determine change in prevalence over time.

Design—Cross-sectional study.

Animals—1,243,681 dogs for which information was reported to the Veterinary Medical Database between 1964 and 2003.

Procedures—Information on breed, sex, and age was collected, and prevalences and odds ratios were calculated.

Results—Castrated male dogs were significantly more likely than other dogs to have HD (odds ratio [OR], 1.21), and castrated male (OR, 1.68) and spayed female (OR, 2.35) dogs were significantly more likely to have CCLD. Dogs up to 4 years old were significantly more likely to have HD (OR for dogs 2 months to 1 year old, 1.22; OR for dogs > 1 to 4 years old, 1.48), whereas dogs > 4 years old were significantly more likely to have CCLD (OR for dogs > 4 to 7 years old, 1.82; OR for dogs > 7 years old, 1.48). In general, large- and giant-breed dogs were more likely than other dogs to have HD, CCLD, or both. Prevalences of HD and CCLD increased significantly over the 4 decades for which data were examined.

Conclusions and Clinical Relevance—Results suggested that sex, age, and breed were risk factors for HD, CCLD, or both in dogs and that prevalences of HD and CCLD have increased over time.

Abstract

Objective—To evaluate prevalence of and risk factors for hip dysplasia (HD) and cranial cruciate ligament deficiency (CCLD) in dogs and determine change in prevalence over time.

Design—Cross-sectional study.

Animals—1,243,681 dogs for which information was reported to the Veterinary Medical Database between 1964 and 2003.

Procedures—Information on breed, sex, and age was collected, and prevalences and odds ratios were calculated.

Results—Castrated male dogs were significantly more likely than other dogs to have HD (odds ratio [OR], 1.21), and castrated male (OR, 1.68) and spayed female (OR, 2.35) dogs were significantly more likely to have CCLD. Dogs up to 4 years old were significantly more likely to have HD (OR for dogs 2 months to 1 year old, 1.22; OR for dogs > 1 to 4 years old, 1.48), whereas dogs > 4 years old were significantly more likely to have CCLD (OR for dogs > 4 to 7 years old, 1.82; OR for dogs > 7 years old, 1.48). In general, large- and giant-breed dogs were more likely than other dogs to have HD, CCLD, or both. Prevalences of HD and CCLD increased significantly over the 4 decades for which data were examined.

Conclusions and Clinical Relevance—Results suggested that sex, age, and breed were risk factors for HD, CCLD, or both in dogs and that prevalences of HD and CCLD have increased over time.

Hip dysplasia and CCLD are 2 of the most common orthopedic problems in dogs,1 and treatment of these conditions can be associated with substantial financial costs for dog owners.2 Importantly, dogs affected by one of these conditions can be concurrently or subsequently affected by the other. Thus, information about factors associated with whether dogs would develop HD, CCLD, or both could help veterinarians optimize their diagnostic approach and educate their dog-owning clients.

Several previous studies have reported prevalences of these conditions and risk factors for their occurrence on the basis of data from the VMDB,1,3,4 databases maintained by the Orthopedic Foundation for Animals and PennHIP,5–8 and medical records of private veterinary practices9 and veterinary medical teaching hospitals.10–13 All of these previous studies, however, examined prevalence of and risk factors for HD or CCLD individually, and to our knowledge, there have been no studies of prevalence of or risk factors for HD and CCLD occurring together. In addition, these previous studies did not examine temporal changes in the prevalence of HD or CCLD.

Therefore, the purposes of the study reported here were to evaluate prevalences of HD and CCLD, alone and together, in dogs; to determine whether sex, age, and breed were risk factors for the occurrence of HD, CCLD, or both; and to determine changes in prevalences of HD and CCLD over time. We hypothesized that HD and CCLD were more common in large- and giant-breed dogs and that prevalences of HD and CCLD had increased significantly over time due to increased recognition of both HD and CCLD.

Materials and Methods

Data for the present study were obtained from the VMDB, which consisted of a collection of medical record information supplied by 27 veterinary medical teaching hospitals in North America. The database was searched for information on all dogs examined for any reason at participating institutions between 1964 and 2003. Individual dogs were included in the study only if information on sex, age, and breed had been included; each dog was included only once. Dogs < 2 months old were excluded from the study.

For all dogs in the study, information was obtained on whether HD, CCLD, or both had ever been diagnosed. Multiple coding options for HD and CCLD were available when submitting information to the VMDB. Thus, dogs were considered to have HD if they had a coded diagnosis of HD, hip joint osteoarthritis, hip joint arthrosis, degenerative changes of the hip joint, or hip joint pain. Dogs were considered to have CCLD if they had a coded diagnosis of cranial cruciate ligament tear, cranial cruciate ligament rupture, stifle joint laxity, or stifle joint arthritis.

For analyses involving sex, categories that were used consisted of sexually intact female, sexually intact male, spayed female, and castrated male. For analyses involving age, categories that were used consisted of 2 months to 1 year, > 1 to 4 years, > 4 to 7 years, and > 7 years. For analyses involving breed, only those breeds represented by ≥ 4,000 individuals were evaluated. For analyses involving changes in prevalence over time, data were divided into four 10-year intervals (1964 to 1973, 1974 to 1983, 1984 to 1993, and 1994 to 2003).

Prevalence was calculated by dividing the number of dogs with HD, CCLD, or both by the total number of dogs at risk. Sex, age, and breed were evaluated as possible risk factors for HD, CCLD, or both by calculating ORs and their 95% CIs. Odds ratios represented the odds of the condition in a specific group of dogs, compared with the odds of the condition in all other dogs in the population. An OR > 1 indicated that dogs in that group were more likely to develop the condition than were dogs that were not in that group; an OR < 1 indicated that dogs in that group were less likely to develop the condition than were dogs that were not in that group. Likelihood ratio χ2 tests were used to compare prevalences of HD, CCLD, and both among groups. Standard softwarea was used for all analyses. Values of P < 0.05 were considered significant.

Results

Information was obtained on 1,243,681 dogs examined at the participating institutions between 1964 and 2003. Hip dysplasia had been diagnosed in 43,825 (3.52%) dogs, CCLD had been diagnosed in 31,698 (2.55%), and HD and CCLD had been diagnosed, concurrently or subsequently, in 3,756 (0.30%).

Sex—Sex was identified as a risk factor for HD and CCLD. Sexually intact female and spayed female dogs were significantly less likely than other dogs to be identified as having HD (Table 1). In contrast, castrated male dogs were significantly more likely than other dogs to be identified as having HD. The odds of HD in sexually intact male dogs was not significantly different from the odds of HD in other dogs. Spayed female and castrated male dogs were significantly more likely than other dogs to be identified as having CCLD, whereas sexually intact female and sexually intact male dogs were significantly less likely than other dogs to be identified as having CCLD.

Table 1—

Prevalences of HD and CCLD in 1,243,681 dogs, grouped on the basis of sex and neuter status, that had been examined at veterinary medical teaching hospitals in North America between 1964 and 2003.

Dogs with HDDogs with CCLD
GroupTotal No. of dogsNo. (%)OR (95% CI)No. (%)OR (95% CI)
Sexually intact female307,95710,324 (3.35)0.93 (0.91-0.96)4,614 (1.55)0.51 (0.49-0.53)
Spayed female319,72511,010 (3.44)0.97 (0.95-0.99)14,004 (4.54)2.35 (2.30-2.40)
Sexually intact male458,52515,984 (3.49)0.98 (0.96-1.00)6,948 (1.57)0.47 (0.46-0.49)
Castrated male157,4746,506 (4.13)1.21 (1.18-1.24)6,132 (4.06)1.68 (1.63-1.73)

The OR represent the odds of HD or CCLD in that group, compared with the odds in all other dogs not in that group. Odds ratios for which the 95% CI does not contain 1 are significantly (P < 0.05) different from 1.

Age—Age was identified as a risk factor for HD, CCLD, and both HD and CCLD. Dogs between 2 months and 1 year old and dogs > 1 to 4 years old were significantly more likely than other dogs to be identified as having HD (Table 2), whereas dogs > 4 to 7 years old and dogs > 7 years old were significantly less likely than other dogs to be identified as having HD. In contrast, dogs > 4 to 7 years old and dogs > 7 years old were more likely than other dogs to be identified as having CCLD, and dogs 2 months to 1 year old and dogs > 1 to 4 years old were less likely than other dogs to be identified as having CCLD. Finally, dogs > 1 to 4 years old, dogs > 4 to 7 years old, and dogs > 7 years old were more likely than other dogs to be identified as having both HD and CCLD, whereas dogs 2 months to 1 year old were less likely than other dogs to be identified as having both HD and CCLD.

Table 2—

Prevalences of HD, CCLD, and both HD and CCLD, identified concurrently or subsequently, in 1,243,681 dogs, grouped on the basis of age, that had been examined at veterinary medical teaching hospitals in North America between 1964 and 2003.

Dogs with HDDogs with CCLD  
AgeTotal No. of dogsNo. (%)OR (95% CI)No. (%)OR (95% CI)No. (%)OR (95% CI)
2 months to 1 year262,01010,699 (4.08)1.22 (1.19-1.25)1,094 (0.42)0.13 (0.12-0.14)139 (0.05)0.14 (0.12-0.17)
> 1 to 4 years357,84116,232 (4.54)1.48 (1.45-1.51)8,529 (2.38)0.91 (0.89-0.93)1,166 (0.33)1.11 (1.04-1.19)
> 4 to 7 years248,2176,951 (2.80)0.75 (0.73-0.77)9,783 (3.94)1.82 (1.78-1.87)1,096 (0.44)1.66 (1.54-1.78)
> 7 years375,6139,942 (2.65)0.67 (0.65-0.68)12,292 (3.27)1.48 (1.45-1.51)1,355 (0.36)1.31 (1.22-1.40)

Breed—Breed was identified as a risk factor for HD, CCLD, and both HD and CCLD. Breeds with the highest odds of being identified as having HD included the Newfoundland, Saint Bernard, Old English Sheepdog, Rottweiler, and German Shepherd Dog (Table 3). Breeds identified as having a significantly lower odds of having HD, compared with other dogs, included the Miniature Schnauzer, Chihuahua, Maltese, Toy Poodle, Miniature Dachshund, and Dachshund.

Table 3—

Prevalence of HD in dogs, grouped on the basis of breed, that had been examined at veterinary medical teaching hospitals in North America between 1964 and 2003

BreedNo. of dogsNo. (%) with HDOR (95% CI)Pvalue
Newfoundland5,005   
 Saint Bernard   
 Old English Sheepdog   
 Rottweiler   
 German Shepherd Dog   
 Samoyed   
 Golden Retriever   
 Alaskan Malamute   
 Labrador Retriever   
 Chow Chow   
 Airedale Terrier   
 English Setter   
 Bulldog   
 English Springer Spaniel   
 Border Collie   
 Brittany Spaniel   
 Irish Setter   
 Great Dane   
 Weimaraner   
 Australian Shepherd   
 German Shorthaired Pointer   
 Standard Poodle   
 English Pointer   
 Afghan Hound   
 Boxer   
 Siberian Husky   
 American Staffordshire Terrier   
 Shetland Sheepdog   
 Shar-Pei   
 Basset Hound   
 Dutch Pug   
 Dalmatian   
 Collie   
 Doberman Pinscher   
 Lhasa Apso   
 Pekinese   
 American Cocker Spaniel   
 Shih Tzu   
 West Highland White Terrier   
 Beagle   
 Pomeranian   
 Fox Terrier   
 Miniature Poodle   
 Greyhound   
 Boston Terrier   
 Yorkshire Terrier   
 Toy Poodle   
 Maltese   
 Chihuahua   
 Miniature Schnauzer   
 Miniature Dachshund   
 Dachshund   
 Scottish Terrier   

Breeds with the highest odds of being identified as having CCLD included the Newfoundland, Rottweiler, Labrador Retriever, Bulldog, Boxer, Chow Chow, American Staffordshire Terrier, and Saint Bernard (Table 4). Breeds identified as having a significantly lower odds of having CCLD, compared with other dogs, included the Miniature Dachshund, Dachshund, Greyhound, Shih Tzu, Miniature Schnauzer, and Pekingese.

Table 4—

Prevalence of CCLD in dogs, grouped on the basis of breed, that had been examined at veterinary medical teaching hospitals in North America between 1964 and 2003

BreedNo. of dogs.No. (%) with CCLDOR (95% CI)Pvalue 
Newfoundland4,551405 (8.90)3.77 (3.40-4.18)< 0.001 
Rottweiler18,8861,566 (8.29)3.58 (3.40-3.78)< 0.001 
Labrador Retriever72,3644,192 (5.79)2.56 (2.47-2.64)< 0.001 
Bulldog9,983532 (5.33)2.17 (1.99-2.37)< 0.001 
Boxer15,962836 (5.24)2.14 (2.00-2.30)< 0.001 
Chow Chow7,903340 (4.30)1.73 (1.55-1.93)< 0.001 
American Staffordshire Terrier5,684230 (4.05)1.62 (1.42-1.82)< 0.001 
Saint Bernard10,148362 (3.57)1.42 (1.28-1.58)< 0.001 
Alaskan Malamute5,047164 (3.25)1.29 (1.10-1.50)0.018 
Airedale Terrier5,029162 (3.22)1.27 (1.09-1.49)0.027 
American Cocker Spaniel44,7331,057 (2.36)0.92 (0.87-0.98)0.092 
German Shorthaired Pointer8,545259 (3.03)1.20 (1.06-1.35)0.045 
Miniature Poodle49,2061,454 (2.95)1.17 (1.11-1.24)< 0.001 
Golden Retriever44,1851,239 (2.80)1.11 (1.05-1.17)0.007 
Great Dane14,315396 (2.77)1.09 (0.98-1.20)0.430 
Dalmatian9,338254 (2.72)1.07 (0.94-1.21)0.774 
West Highland White Terrier6,525178 (2.73)1.07 (0.92-1.25)0.838 
Samoyed7,399186 (2.51)0.99 (0.85-1.14)0.998 
Toy Poodle17,808441 (2.48)0.97 (0.88-1.07)0.944 
Beagle18,842454 (2.41)0.94 (0.86-1.04)0.684 
English Springer Spaniel13,260319 (2.41)0.94 (0.84-1.05)0.776 
Yorkshire Terrier13,912316 (2.27)0.89 (0.79-0.99)0.226 
German Shepherd Dog72,6131,630 (2.24)0.87 (0.83-0.92)< 0.001 
Australian Shepherd8,874190 (2.14)0.84 (0.72-0.97)0.113 
Siberian Husky11,272239 (2.12)0.83 (0.73-0.94)0.038 
English Pointer7,426155 (2.09)0.81 (0.69-0.95)0.094 
Chihuahua12,818261 (2.04)0.79 (0.70-0.90)0.003 
Shar-Pei5,028102 (2.03)0.79 (0.65-0.96)0.139 
Border Collie5,109102 (2.00)0.78 (0.64-0.95)0.098 
Brittany Spaniel9,318180 (1.93)0.75 (0.65-0.87)0.002 
Standard Poodle12,054226 (1.87)0.73 (0.64-0.83)< 0.001 
Doberman Pinscher34,330634 (1.85)0.71 (0.66-0.77)< 0.001 
Weimaraner5,654100 (1.77)0.69 (0.56-0.84)0.003 
Lhasa Apso10,112177 (1.75)0.68 (0.59-0.79)< 0.001 
English Setter7,518114 (1.52)0.59 (0.49-0.71)< 0.001 
Maltese5,34077 (1.44)0.56 (0.45-0.70)< 0.001 
Pomeranian9,133131 (1.43)0.55 (0.47-0.66)< 0.001 
Shetland Sheepdog16,732224 (1.34)0.52 (0.45-0.59)< 0.001 
Scottish Terrier5,98579 (1.32)0.51 (0.41-0.64)< 0.001 
Boston Terrier10,063131 (1.30)0.50 (0.42-0.60)< 0.001 
Fox Terrier5,82073 (1.25)0.48 (0.38-0.61)< 0.001 
Basset Hound10,392115 (1.11)0.43 (0.35-0.51)< 0.001 
Old English Sheepdog6,18560 (0.97)0.37 (0.29-0.48)< 0.001 
Afghan Hound4,35241 (0.94)0.36 (0.27-0.49)< 0.001 
Irish Setter16,745149 (0.89)0.34 (0.29-0.40)< 0.001 
Collie21,219162 (0.76)0.29 (0.25-.034)< 0.001 
Dutch Pug6,02245 (0.75)0.29 (0.21-0.38)< 0.001 
Pekinese10,47974 (0.71)0.27 (0.21-0.34)< 0.001 
Miniature Schnauzer20,004126 (0.63)0.24 (0.20-0.29)< 0.001 
Shih Tzu11,26964 (0.57)0.22 (0.17-0.28)< 0.001 
Greyhound5,44530 (0.55)0.21 (0.15-0.30)< 0.001 
Dachshund32,32967 (0.21)0.08 (0.06-0.10)< 0.001 
Miniature Dachshund7,54916 (0.21)0.08 (0.05-0.13)< 0.001 

Breeds with the highest odds of being identified as having both HD and CCLD included the Newfoundland, Rottweiler, Saint Bernard, Bulldog, Labrador Retriever, and Chow Chow (Table 5). Breeds identified as having a significantly lower odds of having both HD and CCLD, compared with other dogs, included the Miniature Dachshund, Miniature Schnauzer, Chihuahua, Fox Terrier, Afghan Hound, and Greyhound.

Table 5—

Prevalence of HD and CCLD, identified concurrently or subsequently, in dogs, grouped on the basis of breed, that had been examined at veterinary medical teaching hospitals in North America between 1964 and 2003.

BreedNo. of dogsNo. (%) with HD and CCLDOR (95% CI)Pvalue 
Newfoundland4,268122 (2.86)10.01 (8.33-12.02)< 0.001 
Rottweiler17,555235 (1.34)4.71 (4.12-5.38)< 0.001 
Saint Bernard9,904118 (1.19)4.08 (3.39-4.90)< 0.001 
Bulldog9,556105 (1.10)3.74 (3.08-4.55)< 0.001 
Labrador Retriever68,738566 (0.82)3.05 (2.79-3.34)< 0.001 
Chow Chow7,61855 (0.72)2.42 (1.85-3.16)< 0.001 
German Shepherd Dog71,411428 (0.60)2.12 (1.91-2.34)< 0.001 
Boxer15,21690 (0.59)1.99 (1.61-2.45)< 0.001 
Golden Retriever43,167221 (0.51)1.74 (1.52-2.00)< 0.001 
Airedale Terrier4,89124 (0.49)1.63 (1.09-2.44)0.121 
Alaskan Malamute4,90623 (0.47)1.56 (1.03-2.35)0.208 
Samoyed7,24431 (0.43)1.42 (1.00-2.03)0.280 
American Staffordshire Terrier5,47723 (0.42)1.39 (0.92-2.10)0.468 
Old English Sheepdog6,14621 (0.34)1.13 (0.74-1.74)0.956 
Great Dane13,96344 (0.32)1.04 (0.78-1.41)0.994 
Brittany Spaniel9,16426 (0.28)0.94 (0.64-1.38)0.992 
English Springer Spaniel12,97736 (0.28)0.92 (0.66-1.27)0.967 
Basset Hound10,30326 (0.25)0.83 (0.57-1.23)0.837 
German Shorthaired Pointer8,30620 (0.24)0.80 (0.51-1.24)0.791 
Australian Shepherd8,70218 (0.21)0.68 (0.43-1.09)0.451 
Siberian Husky11,05421 (0.19)0.63 (0.41-0.96)0.199 
Irish Setter16,62327 (0.16)0.53 (0.37-0.78)0.012 
Border Collie5,0158 (0.16)0.53 (0.26-1.05)0.335 
West Highland White Terrier6,35710 (0.16)0.52 (0.28-0.97)0.217 
Doberman Pinscher33,74953 (0.16)0.51 (0.39-0.67)< 0.001 
English Pointer7,28110 (0.14)0.45 (0.24-0.84)0.086 
Dalmatian9,09612 (0.13)0.43 (0.25-0.77)0.032 
Shetland Sheepdog16,52921 (0.13)0.42 (0.27-0.64)0.001 
Shar-Pei4,9326 (0.12)0.40 (0.18-0.89)0.148 
English Setter7,4139 (0.12)0.40 (0.21-0.77)0.044 
Dutch Pug5,9847 (0.12)0.39 (0.18-0.81)0.077 
Lhasa Apso9,94611 (0.11)0.36 (0.20-0.66)0.007 
American Cocker Spaniel43,71943 (0.10)0.32 (0.23-0.43)< 0.001 
Beagle18,40618 (0.10)0.32 (0.20-0.51)< 0.001 
Weimaraner5,5595 (0.09)0.30 (0.12-0.71)0.040 
Standard Poodle11,83810 (0.08)0.28 (0.15-0.52)0.000 
Pomeranian9,0097 (0.08)0.26 (0.12-0.54)0.002 
Maltese5,2674 (0.08)0.25 (0.09-0.67)0.030 
Collie21,07316 (0.08)0.25 (0.15-0.40)< 0.001 
Yorkshire Terrier13,60610 (0.07)0.24 (0.13-0.45)< 0.001 
Shih Tzu11,2127 (0.06)0.20 (0.10-0.43)< 0.001 
Boston Terrier9,9386 (0.06)0.20 (0.09-0.44)0.000 
Pekinese10,4105 (0.05)0.16 (0.07-0.38)< 0.001 
Miniature Poodle47,77220 (0.04)0.13 (0.09-0.21)< 0.001 
Scottish Terrier5,9082 (0.03)0.11 (0.03-0.45)0.003 
Toy Poodle17,3725 (0.03)0.09 (0.04-0.23)< 0.001 
Miniature Dachshund7,5352 (0.03)0.09 (0.02-0.35)0.000 
Afghan Hound4,3121 (0.02)0.08 (0.01-0.54)0.011 
Miniature Schnauzer19,8824 (0.02)0.07 (0.02-0.17)< 0.001 
Chihuahua12,5592 (0.02)0.05 (0.01-0.21)< 0.001 
Dachshund32,2653 (0.01)0.03 (0.01-0.09)< 0.001 
Fox Terrier5,7470 (0)N/AN/A 
Greyhound5,4150 (0)N/AN/A 

Change in prevalence over time—For each of the 4 time periods examined, prevalence of HD during that period was significantly different from prevalence during the other periods (Table 6). Prevalence of HD was lower during the period from 1974 to 1983 than the prevalence during the period from 1964 to 1973 but increased thereafter. Similarly, for each of the 4 time periods examined, prevalence of CCLD during that period was significantly different from prevalence during the other periods. Prevalence of CCLD increased steadily during the 4 periods examined.

Table 6—

Prevalences of HD and CCLD as a function of time in dogs that had been examined at veterinary medical teaching hospitals in North America between 1964 and 2003

Time periodTotal No. of dogsNo. (%) of dogs with HDNo. (%) of dogs with CCLD  
1964-1973136,8386,363 (4.65)2,470 (1.81)  
1974-1983324,43410,471 (3.23)6,685 (2.06)  
1984-1993407,94415,057 (3.69)11,311 (2.77)  
1994-2003230,41511,923 (5.17)11,216 (4.87)  

Discussion

Results of the present study suggested that sex, age, and breed were risk factors for HD, CCLD, or both in dogs. Castrated male dogs were significantly more likely than other dogs to have HD, and castrated male and spayed female dogs were significantly more likely to have CCLD. Dogs up to 4 years old were significantly more likely to have HD, whereas dogs > 4 years old were significantly more likely to have CCLD, and dogs > 1 year old were more likely to have both HD and CCLD. In general, large- and giant-breed dogs were more likely than other dogs to have HD, CCLD, or both. Finally, prevalences of HD and CCLD had increased significantly over the 4 decades for which data were examined.

Data for the present study were obtained by searching the records of the VMDB, which provided information on dogs examined for any reason at any of the 27 participating veterinary medical teaching hospitals in North America, including dogs examined because of a specific disease or condition and overtly healthy dogs undergoing routine examination. Although the population of dogs included in the VMDB did not necessarily represent the general population of dogs in North America, data in the VMDB were obtained from a large number of institutions covering a wide range of hospital, patient, and client demographics. In addition, the data entry protocol for the VMDB allowed for entry of multiple diagnostic codes for any individual dog, so that dogs examined for a nonorthopedic problem in which HD or CCLD was identified serendipitously would still be recorded in the database as having this condition. As a result, we believe our findings have broad applicability to the population of dogs in North America.

Previous studies1,3–13 examining prevalences of and risk factors for HD and CCLD in dogs have generally used information from more restricted databases. Use of a source such as the Orthopedic Foundation for Animals database may be useful in determining the prevalence of HD in that diagnoses were made by board-certified radiologists,7 but the data may not be representative of the general population of dogs because of selection bias. To minimize the effects of selection bias, we elected to use information from the VMDB in the belief that this information would be more representative of the general population of dogs. In addition, use of the VMDB allowed us to examine changes in prevalence over time.

The overall prevalence of HD in the present study (3.52%) was substantially lower than prevalences reported in previous studies.5–8,10 For example, a previous study5 involving data from a single veterinary medical teaching hospital found that the prevalence of HD in Rottweilers was 41% to 53% and the prevalence in Golden Retrievers was 69% to 73%, depending on the definition of HD. In the present study, prevalences of HD in these breeds were 11.8% and 9.3%, respectively. The cause of this large disparity in prevalence of HD between the present and previous studies is unknown. It is possible that a diagnosis of HD was not entered into the VMDB for some dogs that did in fact have the condition. This may have occurred if clinical signs of HD were not evident at the time the dog was examined at the veterinary medical teaching hospital or if diagnostic testing for HD was not performed because other conditions were accorded a higher priority. Importantly, however, the coding protocol for the VMDB allowed for diagnoses other than those associated with the primary problem to be recorded. Nevertheless, it seems likely that not all dogs with HD were identified as having this condition in the VMDB. Thus, breed prevalences reported in the present study may be falsely low. Even if this were the case, ORs calculated in the present study should not have been greatly affected by this reporting bias, as all of the groups should have been affected to the same extent.

In the present study, prevalences of HD and CCLD increased significantly over the 40-year study period, although prevalence of HD did decrease during the period from 1974 to 1983. Previous studies14,15 have reported decreases in HD prevalence over time, but they did not examine the prevalence of HD over as long a time period as used in the present study. Although the causes of this increase in prevalence are not known, we do not believe that the increase in prevalence of HD or the increase in prevalence of CCLD over time was due to a true increase in the underlying frequencies of these conditions. Rather, we suspect that the increases in prevalence were due to increased recognition of these conditions by veterinarians, making it more likely that dogs with these conditions would be identified. In particular, given the reported success of selective breeding programs specifically directed at decreasing the prevalence of HD in dogs,14,15 we do not believe that the frequency of HD truly increased over the period of our study. In the case of CCLD, we cannot think of any reasons for why the frequency of this condition would have increased in the general population of dogs during our study period and believe that increased recognition of CCLD as an important orthopedic disease was the cause of the increase in prevalence that we found. Although we did not examine obesity, it has been associated with HD and CCLD. Thus, we cannot rule out the possibility that an increase in obese dogs over time has actually led to the increase in the frequency of CCLD and HD diagnoses that we saw.

We were not able to determine in the present study why castrated male and spayed female dogs were more likely than other dogs to have CCLD; however, previous studies have found similar increases in the odds13 and prevalence3 of CCLD in neutered dogs. This may, in part, be associated with the greater likelihood of obesity in neutered dogs, in that numerous studies16–18 have shown that obesity is significantly more common in neutered dogs, and a relationship between obesity and CCLD has been reported.3,13 Unfortunately, information on body weight or body condition score could not be reliably obtained from the VMDB, so we could not assess whether either of these were risk factors for CCLD. On the other hand, it is possible that owners of neutered dogs were more likely than owners of sexually intact dogs to have their dogs evaluated for orthopedic disease and treated for CCLD, which would have falsely decreased the apparent prevalence of CCLD in sexually intact dogs and altered our calculated ORs. Also, it is possible that a relatively larger proportion of dogs representing breeds with a high prevalence of CCLD, such as Rottweilers and Labrador Retrievers, were neutered, compared with dogs representing breeds with a low prevalence of CCLD, which also would have altered our ORs. However, when we examined sex differences within specific breeds, such as Rottweilers and Labrador Retrievers, we found the same significant predisposition for CCLD among neutered dogs.

In the present study, dogs ≤ 4 years old were more likely to have HD than were older dogs and dogs > 4 years old were less likely to have HD than were younger dogs. Because HD is a developmental disease, dogs that have not developed clinical signs of HD by 4 years of age are less likely to be identified as having HD after this time. Conversely, dogs > 4 years old were more likely than younger dogs in the present study to have CCLD, whereas dogs ≤ 4 years old were less likely to have CCLD than older dogs. The higher prevalence of CCLD in older dogs was in agreement with results of a previous study.3 In contrast, another study13 found that large-breed dogs were predisposed to develop CCLD at a young age, and similar findings have been reported elsewhere.9,12 Unfortunately, our data were not subdivided by breed and age, so we were unable to determine whether large-breed dogs were more likely to develop CCLD at a young age.

We also found in the present study that dogs > 1 year old were more likely than younger dogs to have both HD and CCLD. Further research is needed to delineate the reasons for this. However, we suggest that a possible explanation for this is that when older dogs are evaluated because of hind limb lameness, veterinarians tend to test for both HD and CCLD.

Several limitations of using data from the VMDB for this type of study must be acknowledged. Most importantly, diagnoses entered into the database cannot be retrospectively confirmed or denied; therefore, all results rely on correct reporting of data. In addition, only information from dogs examined at one of the participating veterinary medical teaching hospitals is included in the VMDB. Therefore, information for dogs whose owners were not referred to a veterinary medical teaching hospital and for dogs whose owners did not pursue diagnostic testing at a veterinary medical teaching hospital is not included. Thus, although the large number of cases included in the present study resulted in more precise estimates of prevalence and ORs, it also allowed for magnification of any inherent biases in data from the VMDB in relation to the general population of dogs. For example, it is possible that fewer small-breed dogs with CCLD were examined at veterinary medical teaching hospitals because these dogs were treated by their referring veterinarians, thereby falsely decreasing the prevalence of CCLD in small-breed dogs. Although the VMDB cannot account for this possibility, it does allow for coding of multiple diagnoses for each patient. Therefore, a small-breed dog that was examined for a reason unrelated to orthopedic disease and that was being treated or had been treated by the referring veterinarian for HD or CCLD could still be recorded as having this condition.

ABBREVIATIONS

CCLD

Cranial cruciate ligament deficiency

CI

Confidence interval

HD

Hip dysplasia

OR

Odds ratio

VMDB

Veterinary Medical Database

a.

Excel 2003, Microsoft Corp, Redmond, Wash.

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Contributor Notes

Dr. Witsberger's present address is the Veterinary Medical Teaching Hospital, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX 77843.

Address correspondence to Dr. Cook.