Canine Spinocerebellar Ataxia (SCA)
Canine spinocerebellar ataxia (SCA) is an inherited disorder occuring in Jack Russell Terrier, Smooth-haired Fox Terrier and Toy Fox Terrier. First time form of hereditary ataxia was reported in Smooth-Haired Fox Terrier in 1957, which was followed by report of a disease with similar symptoms in the Jack Russell Terrier in the 1973. Another form of ataxia is cerebellar ataxia. Cerebellar ataxia, however, is a different disorder than spinocerebellar ataxia and a Jack Russell Terrier can be affected with one of the forms of ataxia, not both. Different forms of ataxia differ from each other in ages at onset, clinical signs, and histopathologic changes.
Jack Russell Terriers, Parson Russell Terrier and Russell Terriers are recognized as separate breeds and it is believed they have common descend, originating from the stock of Parson Jack Russell, a 19th century dog breeder from Great Britain.
Characteristics and Symptoms
Age of onset of canine spinocerebellar ataxia is between 2 months to 9 months of age. At beginning, the dog’s owner may notice lack of coordination, pelvic limb swazing when walking, and difficulties in climbing up-stairs or jumping. SCA is a progressive disorder, and with its progress, ”prancing” type of gaint is noticed, especially in the pelvic limbs. Affected dogs may have difficulties while standing, and may fall frequently, with troubles in attempts of returning to standing position. Neurological examinations have revealed symmetric spinocerebellar ataxia, characterized by hypertermia and spasticity, especially in the pelvic limbs. Postural reactins appear to be delayed and hypertermic, while spinal reflexes are normal to increased. Brain stem auditory-wvoked potentials (BAEPs) identified abnormalities in some but not all SCA affected dogs. These abnormalities include lack of waves III, IV and V.
Post-mortem histopathological examination showed bilateral symmetrical myelopathy, myelin loss, swelling of axons, moderate diffuse gliosis and marked loss of myelinated nerve fibers.
All of the SCA affected Jack Russell Terriers exhibit myokymia. Myokymia is a involuntary tremor of muscles, where muscles become hyperactive and heat is created. The excessive heat causes hyperthermia and can result in a seizure. In SCA affected dogs it usually appears between 2 moths to 3 years of age. Common symptoms of myokymia are stifness of hind limbs and fore limbs, curled paws, facial rubbing and extremly high core body temperature. Myokymia is exhibitedby human patients with voltage-gated potassium channel genes.
Since SCA is an inherited disorder with a progressive nature, with no cure developed, most of SCA affected dogs are euthanized by 2 years of age due to poor quality of life.
Canine spinocerebellar ataxia (SCA) is caused by a point mutation in the KCNJ10, a potassium channel gene known to cause myokymia in the human patients. SCA is inherited as an autosomal recessive disorder. Healthy parents of an affected dog are obligate heterozygotes, and therefore carry one mutant allele. Heterozygotes have no symptoms. At conception, each cub has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.
Gilliam D, O’Brien DP, Coates JR, Johnson GS, Johnson GC, Mhlanga-Mutangadura T, Hansen L, Taylor JF, Schnabel RD. A Homozygous KCNJ10 Mutation in Jack Russell Terriers and Related Breeds with Spinocerebellar Ataxia with Myokymia, Seizures, or Both. J Vet Intern Med.
Rohdin C, Gilliam D, O’Leary CA, O’Brien DP, Coates JR, Johnson GS, Jaderlund KH. A KCNJ10 mutation previously identified in the Russell group of terriers also occurs in smooth-haired fox terriers with hereditary ataxia and in related breeds. Acta Vet Scand. 2015 May 23;57:26.