Heritability of patellar luxation in the Chihuahua and Bichon Frise breeds of dogs and effectiveness of a Swedish screening programme.

Patellar luxation is one of the more common orthopaedic diseases of dogs and is relatively frequent in some toy breeds, including the Chihuahua and Bichon Frise. Using data provided by the Swedish Kennel Club, genetic parameters, including heritability, were estimated for patellar luxation in the Chihuahua from 1999 to 2014 and in the Bichon Frise from 1997 to 2014. The effects of the current screening programmes for patellar luxation in these breeds were evaluated. Patellar luxation was defined as a binary trait, treating dogs as affected or unaffected. The edited data included 7024 records for the Chihuahua and 1071 records for the Bichon Frise. Patellar luxation was analysed using mixed linear and threshold animal models, including fixed effects of sex, birth month, birth year, age at veterinary examination, random effects of the examining veterinary surgeon, genetic effect of the individual and residual. The prevalence of patellar luxation was 23% in the Chihuahua and 12% in the Bichon Frise. Using threshold analysis, estimated heritabilities were 0.25 for the Chihuahua and 0.21 for the Bichon Frise on the observable scale, and 0.46 for the Chihuahua on the underlying scale. It was concluded that there is genetic variation in patellar luxation and that there has been a slight genetic improvement over the study period in the Chihuahua. Further genetic progress would be facilitated by selection using estimated breeding values based on veterinary screening records.