Part of the official UK Kennel Club testing scheme in Jack Russell Terrier and Parson Russell Terrier
Late onset ataxia, otherwise known as spinocerebellar ataxia in the Parson Russell Terrier affects young dogs, usually around the age of 6-12 months. Initially the dogs’ owners may notice their dog is not very well coordinated, with hind limb swaying when walking, and difficulty in climbing stairs and jumping. As the disease progresses, a characteristic ‘prancing’ or ‘dancing’ type of gait develops, especially affecting the hind limbs. Severely affected animals frequently fall and have difficulty returning to a standing position. In most affected dogs the disease progresses with dogs finding it increasingly difficult to walk. Most dogs are euthanised on welfare grounds although a small number of dogs survive to middle age.
The single nucleotide substitution in the gene called CAPN1 that causes late onset ataxia (LOA) in Parson Russell terriers is recessive. This means that dogs that carry two copies of the mutation (homozygotes) will almost certainly develop LOA during their lives. Dogs that carry a single copy of the mutation (also known as carriers or heterozygotes) will not develop LOA as a result of the CAPN1 mutation, but they will pass the mutation onto about half of any offspring they have. Breeding dogs that will not develop LOA should be the breeder’s priority, with a reduction in mutation frequency within the whole breed being the secondary, longer-term target.
Carriers can be bred from safely, provided they are mated to a dog that has also been tested and is clear of the CAPN1 mutation (i.e. carry no copies of the mutation). If a carrier is mated to a clear dog approximately half of the resulting puppies will also be carriers, so should be tested themselves prior to breeding. Breeding carriers to tested, clear dogs is safe, in terms of avoiding dogs affected with LOA, and will help to maintain the genetic diversity of a breed. It is therefore encouraged, particularly in the first few generations following the availability of a new genetic test, so that other desirable characteristics and traits can be preserved before the frequency of the disease mutation within the breed is gradually reduced.