Heritability of Patellar Luxation in the Chihuahua and Bichon Frise Breeds of Dogs and Effectiveness of a Swedish Screening Programme

Patellar luxation is when the patella of a canine dislocates from its normal position, which is normally in the trochlear groove of the femur, during everyday movement. Small toy breeds tend to have a higher risk of developing this issue when compared to medium and large sized dogs.


The Swedish Kennel Club implemented a study a central screening programme to examine the stifle joints of both dog breeds older than 12 months of age. Age at examination was grouped in periods of 6 months and dogs examined at 72 months or older were all grouped together. The screening was an examination from a veterinarian that rated the stifle joints of each dog from 0-3. 0 being completely unaffected to 3 being permanently dislocated. Chihuahuas and Bichon Frises are now required to be screened in order to mate and produce offspring. This was implemented in 2005.


The final data they used had records for 7024 for Chihuahuas and 1071 Bichon Frise all from from 1997-2014. In order to get a statistical view of the data, they defined patellar luxation as a binary trait; either they were affected or unaffected. They used multiple types of statistical analyses to examine the possibility of a genetic trend.


Results from this initial central screening programmes showed that 23% of Chihuahuas had some degree of patellar luxation And 12% of the Bichon Frises had it. Females of both breed were more likely to have patellar luxation. In Chihuahuas it was twice as likely and the Bichon Frise females were 4 times as likely to have patellar luxation. The Chihuahua was at higher risk of having patellar luxation when compared to the Bichon Frise. When the condition was present, the patellar luxation also tended to be more severe in Chihuahuas. The number of Chihuahuas with patellar luxation in the study tended increased with the increase of age at examination as well.


When both parents had a scored 0, their offspring had a significantly lower rate of patellar luxation. 82.5% of offspring were deemed unaffected, compared to 70% or 59%, where one or both parents are affected by patellar luxation, respectively. The use of threshold analysis found the variance components of heritabilities for both breeds and estimated .25 for Chihuahuas and .21 for Bichon Frises on the observable scale. On the underlying scale it was .46 for Chihuahuas. There was no significant phenotypic trends in either breed, but there was a small genetic trend for the Chihuahua for the overall period studied.


To conclude this study demonstrates that this condition is commonly present in both Chihuahuas and Bichon Frises. This also showed that Chihuahuas and Bichon Frises have heritabilities for patellar luxation in Sweden. These estimated heritability are low to moderate, which indicates the difficulty of gaining substantial genetic progress when based on only phenotypic selection. This study also exhibits that the current screening programme in Sweden has resulted in a small reduction in the occurrence of patellar luxation in Chihuahuas and Bichon Frises offspring. This screening also is effective as a start for genetic selection to improve both breeds at population level.



Summary by: Katherine Handley




K. Nilsson, S. Zanders, S. Malm, “Heritability of patellar luxation in the Chihuahua and Bichon Frise breeds of dogs and effectiveness of a Swedish screening programme”, The Veterinary Journal, Volume 234, 2018, Pages 136-141



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