Research supported by, and conducted at Nestle Purina Pet Care Co.
Summarized by the Kerry Blue Terrier Foundation
Canine hip dysplasia (CHD) is a highly prevalent, genetic, osteoarthritic disease that causes variable degrees of pain and disability throughout a dog's life. The disease is understood to be polygenic with a heritability (h2) of approximately 0.25. The purpose of the investigation was determine the influence of environmental factors specifically diet, on the phenotypic expression of this genetic disease.
Forty-eight 6-week old Labrador Retriever puppies from 7 litters were allotted by pairing to 2 groups of 24 dogs each.
The control-fed group was fed for 15 minutes, and each member of the other group (restricted-fed) was always offered 25% less of the same food given to the control-fed pair mate.
Radiography of the hip joints was done when the dogs were 30, 42 and 54 weeks of age, then yearly till end of life. The radiographs were evaluated for evidence of CHD and OA by an independednt, blinded radiologist (DNB) using criteria of the OFA scoring system. At 2, 3 and 8 years of age the hips were evaluated and scroed using the PennHIP method.
Discussion and Conclusion
Environmental factors can influence the expression of diseases of complex inheritance. In the investigation, food consumption and body weight were found to be potent factors. Keeping dogs lean did not change the genes of dogs predisposed to hip dysplasia. Rather, leanness was shown to delay or prevent the expression of radiographic signs of CHD and therefore confounded diagnosis using the OFA method at 2 years of age.
The principal risk factor for the development of hip OA has been shown to be joint laxity in the hip-extended projection this laxity is underestimated and often masked completely. The demonstrated deficiency of the hip-extended radiograph has been linked to a suspected high rate of false-negative diagnoses at 2 years of age.
The life-long study reported here, provides evidence that hip phenotypes were much worse at the end of life than at 2 years of age. The normal OFA designation of hips at 2 years of age was conservatively wrong 46% of the time when compared to end-of-life radiographic results. Even more telling, when 2 year OFA score was compared to histopathological findings in aged dogs, a 56% false-negative diagnostic rate was found.
This study refutes the popular view that holds that hip OA occurs either early in life, in the case of dysplastic dogs, or much later in the geriatric years in the case of "old age" (idiopathic) OA. The study's results show a linear increase in prevalence of OA over life. This study adds to the growing pool of studies implicating hip laxity as the principal risk factor, if not the cause, for the development of OA, irrespective of age.
DI was not influenced by diet and predicted at 2 years of age that all dogs were OA susceptible. Indeed all but one dog developed either radiographic or histologic OA by the end of the study; and tighter hips developed OA later in life, looser hips earlier. The researchers speculate that if dogs with DI< 0.3 were included in this study that no OA would have occurred over the life of the dog.
Clinical Effects of Dietary Restrictions:
- Control-fed dogs at 2 years of age had 6 times the prevalence of OA to restricted-fed-dogs
- This beneficial effect was observed for life. Radiographic OA prevalence of control-fed dogs at time of death was 83% versus restricted-fed prevalence of 50%.
- Restricted-fed dogs benefited by a delay in radiographic onset of OA, a reduction in radiographic severity of OA, and a delay if clinical signs from OA. Median therapy-free interval for control-fed: 10.3 years and for restricted-fed: 13.3 years.
- Recommendation: Dogs susceptible to CHD should be kept lean for life.
Clinical Significance of Hip Radiographic/Histopath Evaluations:
- The high false-negative rate of OFA type diagnoses at 2 years of age (46% end of life OFA type score) (56% histopath OA) warrants a more accurate test for selecting breeding stock.
- PennHip was found to predict OA susceptibility accurately. the DI was not influenced by environmental factors suggesting a higher estimate of heritability.
- Recommendation: Hip films are warranted well beyond 2 years of age, and in the case of breeding dogs, hips should be evaluated at regular intervals for life.