Hereditary late-onset cerebellar ataxia described in the Jack Russell terrier and the Parson Russell terrier. It causes progressive degeneration of the Purkinje cells of the cerebellar cortex, affecting motor coordination. Affected dogs maintain mental status and sensory function, but progressively lose the ability to move normally. It is incurable and progresses over months or years.

Described in the Jack Russell terrier and Parson Russell terrier. No reliable carrier frequencies are published; it is considered a rare disease in the population as a whole, but it may be concentrated in inbred lines.

- Progressive ataxia of the limbs, more evident in the pelvic limbs
- Hypermetria when walking
- Intention tremor
- Difficulty jumping and climbing
- Preservation of mental status
- Slower progression than in infantile ataxias

Hereditary ataxias of the Russell terrier group were studied during the 2000s and 2010s, distinguishing several molecular entities. The form linked to CAPN1 (which encodes calpain 1) was described in the Parson Russell terrier (Forman et al. 2013), while the form with myokymia and seizures was associated with KCNJ10 (Gilliam et al. 2014). The association study by Gast et al. (2016) differentiated the two in practice and enabled the development of carrier DNA tests for breeders.

- Test breeding animals before mating
- Do not mate two carriers
- A carrier may be mated to a clear animal; test the offspring intended for breeding
- Do not automatically discard carriers: use them with clear animals to preserve genetic diversity

Differential diagnosis with ataxia with myokymia and seizures (SAMS) of the Russell group, caused by KCNJ10 c.627C>G (OMIA:002089-9615; Gilliam et al. 2014; Rohdin et al. 2015), and with other acquired cerebellar ataxias. The CAPN1 form has been described as 'late onset ataxia' (LOA); OMIA places the onset usually between 2 and 9 months, with slow progression. The evidence on the causal role of CAPN1 c.344G>A is limited: Forman et al. (2013) described it as a candidate variant and Gast et al. (2016) did not validate it in their cohort. MRI may show cerebellar atrophy. Supportive management.

1. Forman OP et al. 2013. Missense mutation in CAPN1 is associated with spinocerebellar ataxia in the Parson Russell Terrier dog breed. PLOS One. PMID: 23741357
2. Gast AC et al. 2016. Genome-wide association study for hereditary ataxia in the Parson Russell Terrier and DNA-testing for ataxia-associated mutations in the Parson and Jack Russell Terrier. BMC Veterinary Research. PMID: 27724896
3. OMIA:001820-9615. Ataxia, spinocerebellar, CAPN1-related. Online Mendelian Inheritance in Animals.