title: Seasonal variation in the hip score of dogs as assessed by the New Zealand Veterinary Association hip dysplasia scheme.
abstract: OBJECTIVE
  To determine whether there is a seasonal variation in the phenotypic hip score of dogs born in New Zealand as assessed by the New Zealand Veterinary Association (NZVA) canine hip dysplasia (CHD) scheme.
  
  
  METHODS
  Data from dogs born in New Zealand between 1988 and 2009 that have been scored for CHD were retrospectively evaluated for the effect of month of birth on radiographic phenotype. Data included both the total score and the subtotal score, comprising Norberg's angle, the subluxation score and changes to the cranial acetabular edge, for each dog. Datasets were created for all breeds combined and for the four most populous breeds using the scheme (German Shepherd dog, Labrador Retriever, Golden Retriever and Rottweiler) and stratified according to month of birth and season. Due to the skewed nature of the data, a Kruskal-Wallis Rank Sum test was used to test for statistical significance. Additionally, χ² analysis was performed using the median of each dataset (proportion above/below the median). The null hypothesis was that there would be no effect of month of birth, and hence seasonality, on hip phenotype for dogs born and scored in New Zealand by the NZVA.
  
  
  RESULTS
  For all breeds combined, month of birth had an effect on total and subtotal NZVA CHD scores (p<0.001) with a lower total hip score in the autumn months of March and April than other months. When individual large breed data were analysed, there was an effect of month of birth on total and subtotal scores for the Labrador Retriever and the Rottweiler (p ≤ 0.05), but not the German Shepherd dog or Golden Retriever breeds.
  
  
  CONCLUSIONS
  Being born in the autumn was associated with a protective effect on hip phenotype in some breeds. These results suggest that weather and/or another seasonal factor may have a significant environmental effect on the phenotype of the coxofemoral joint.
  
  
  CONCLUSIONS
  The protective effect of being born in autumn suggests that a decreased level of exercise during subsequent development over winter may positively impact on final coxofemoral joint conformation. Whilst statistically significant, the magnitude of the sparing effect is not likely to be clinically relevant. However, this study, in concert with other studies, may suggest that the effects of exercise can be manipulated to improve hip phenotype.
authors.affiliation: Massey University Veterinary Teaching Hospital, Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Private Bag 11222, Palmerston North 4442, New Zealand. a.j.worth@massey.ac.nz
authors.forename: A J
authors.initials: AJ
authors.lastname: Worth
authors.forename: J P
authors.initials: JP
authors.lastname: Bridges
authors.forename: N J
authors.initials: NJ
authors.lastname: Cave
authors.forename: G
authors.initials: G
authors.lastname: Jones
mesh_terms: D017677:Age Distribution; D000818:Animals; D004285:Dogs; D006619:Hip Dysplasia, Canine / Q000175:diagnosis* / Q000453:epidemiology / Q000473:pathology; D006621:Hip Joint / Q000254:growth & development; D009520:New Zealand / Q000453:epidemiology; D012189:Retrospective Studies; D012307:Risk Factors; D012621:Seasons; D012958:Societies, Scientific; D014730:Veterinary Medicine / Q000706:statistics & numerical data