Health Articles

Decreasing the Incidence of Patellar Luxation

by Nancy McDonald, RN, BSN

 
Although a fatal or debilitating genetic disease occasionally occurs in the Bichon Frise, it is relatively rare compared to some other breeds. Breeders likely expect a healthy litter. When the occurrence of a non-fatal defect appears such as patellar luxation or hip dysplasia, some breeders excuse it by accepting it is just that, non-fatal. More excuses include a grade I or II patellar luxation may never cause serious problems; a displaced patella may cause pain or lead to a cruciate tear but that can be repaired with surgery; mild hip dysplasia may not cause problems until the dog is aged. A Bichon with patellar luxation or hip dysplasia is not a sound animal and no excuse justifies acceptance of an unsound Bichon. Patellar luxation is inherited in a manner similar to hip dysplasia and elbow dysplasia. There is a proven technique to decreasing the incidence of hip dysplasia and elbow dysplasia and this technique will work equally well for patellar luxation.

Patellar luxation, elbow dysplasia and hip dysplasia are believed to be polygenic disorders, needing more than one pair of genes to produce an affected individual. For polygenic disorders there is no direct genetic test for the disorder or for carriers of the mutant genes. Phenotypic tests, such as radiographs, or manual examination are needed to identify those affected with the disease; thus verifying the affected animal has the genotype to cause the disease. In polygenic disorders, it is not known if the non-affected animal carries the genotype for the disease or is clear. Even two phenotypic normal parents can produce affected offspring if both parents carry the genes that cause the disorder.

In 1966 the Orthopedic Foundations for Animals (OFA) was formed to collate and disseminate information about orthopedic and genetic diseases of animals; to advise, encourage and establish control programs to lower the incidence of orthopedic and genetic diseases; to encourage and finance research in orthopedic and genetic disease in animals; and to receive funds and make grants to carry out these objectives. In association with American Kennel Club Canine Health Foundation and Morris Animal Foundation the focus of OFA has grown to include many genetic based diseases resulting in numbers of registries providing breeders and researchers with much needed information. By using these registries along with pedigrees, the incidence of diseases with complex inheritance can decrease with selective pressure based on phenotype.

As reported in The Veterinary Journal: "How the Orthopedic foundation for Animals (OFA) is Tackling Inherited Disorders in the USA: Using Hip and Elbow Dysplasia as Examples", G. Gregory Keller, Edmund Dziuk, Jerold S. Bell, scientists have proven mathematically that decrease in disease incidence occurs when using parents with better phenotypic hip conformation to produce offspring with better hips. In the study, each parent was given a value according to the known of-age hip ratings, excellent; 1 down to severe; 7. The combined parent score (CPS) for each mating was determined by adding together the numbers corresponding to hip rating for each parent. For example, two OFA excellent parents would have a CPS rating of 2; two OFA severe parents would have a CPS of 14. Hip ratings for 490,966 progeny in the OFA registry were in the study. The percentage of dysplastic progeny increased as the parental hip scores increased.

A similar study was done on 76,599 progeny in the OFA elbow registry with known sire and dam elbow ratings. Matings with the same CPS were strongly correlated with increasing percentages of dysplastic progeny. Matings involving one normal parent had significantly lower percentages of progeny with elbow dysplasia than those between two parents with elbow dysplasia. Also, matings involving one parent with Grade 1 elbow dysplasia produced significantly more elbow dysplasia than matings of parents with normal elbows.

These studies demonstrate when selection of a breeding partner based on the hip ratings of parents and grandparents (vertical or depth-of-pedigree) is combined with the individual's own ratings, the accuracy of producing offspring with improving hip ratings can be better determined. Also, selection of a mate based on hip ratings of his/her siblings (horizontal or breadth-of-pedigree) can be combined with the individual's own ratings to determine the accuracy of response.

It is known that two normal parents can produce affected offspring when a complexly inherited disease is involved. With improving parental phenotypic ratings, research data has shown that hip and elbow conformation improve. OFA vertical pedigree as well as horizontal pedigrees can provide information to the breeder helping that breeder to make informed breeding decisions. The recommendation is to beed to normal dogs with increasing normalcy of parents, grandparents, siblings and progeny. For instance, the breeder, instead of breeding to the sire chosen, may select to breed to a quality sibling of that sire with better hip ratings.

Small population breeds have concerns about genetic diversity and the effects of artificial selection on their gene pool. The removal of a large percentage of possible breeding stock for a single genetic disorder will doom any breed to extinction. As Breeders study the breadth and depth of pedigrees, fueled by varied opinions of what constitutes the ideal dog, their selection choices whether to line breed or to outcross maintain breed diversity.

The open access to OFA health database provides the ability to view an individual dog's record on all recorded health issues including test results, age of tests and resulting certification numbers. Sire and dam information are provided as well as information on full and half siblings and offspring provided the breeder tests and registers all siblings in a litter. The use of this type of information in invaluable when making decisions based on phenotypic data such a patellar luxation.

If improvement of progeny is to be made, the breeder must know the ratings of the bitch to be bred as well as the ratings of all the parents, grandparents and siblings. In addition the breeder must research all the ratings of a potential sire and relatives, and be open to choosing another sire if needed. The point is to breed into a line with better patellar ratings and less incidence of patellar luxation than the bitch. All the offspring of the litter must be tested and registered. It takes work for the responsible breeder, and working with other like-minded breeders can be a great assistance because the responsible breeder knows only breeders can improve the health of the breed.

*This article has been checked for accuracy by Dr. Jerold Bell.

*The two referenced resources by Dr. Jerold Bell may be read here:
Genetic Diversity and OFA Vet Journal, August 2011

Keller, G.G, et al. "How the Orthopedic Foundation for Animals (OFA) is Tackling Inherited Disorders in the USA: Using Hip and Elbow Dysplasia as Examples." The Veterinary Journal, Resources; page 44 0f 64

Jerold S. Bell DVM, Clinical Associate Professor of Genetics. Tufts Cummings School of Veterinary Medicine; "Small Population Breeds and Issues of Genetic Diversity". (This article was originally published in the March 2007 AKC Perspectives Delegates Newsletter.)

AKC Canine Health Foundation 2011 National Parent Club Canine Health Conference; resources pp 42 of 61. Robinson PhD, Tara Rodden, Genetics for Dummies, 2nd edition. (Wiley Publishing, Inc., Hoboken, NJ 2010)

Orthopedic Foundation for Animals; www.offa.org/pl_overview.html