Cerebellar ataxia in Jack/Parson Russell Terriers:
Joint Statement from the Animal Health Trust & the University of Missouri
Researchers at the University of Missouri and the Animal Health Trust have been independently studying cerebellar ataxia in the Jack/Parson Russell Terriers. When both groups announced that they had found the causative mutations they were looking for, it created confusion since the results on individual dogs were different from one test versus the other. As a result, the AHT and MU collaborated to clarify the situation. We shared DNA samples and information, and each conducted further studies on those samples. These studies have recently revealed that the two research teams have identified different mutations which cause genetically distinct, but clinically similar forms of ataxia in both the Parson Russell and Jack Russell terrier breeds.
To clarify things we would like to describe some of the differences between the two forms of ataxia and suggest a consistent terminology to differentiate the two forms. We will also mention some of the other forms of ataxia that we are studying which further complicate the picture.
Late Onset Ataxia (LOA)
The disease being studied by the AHT is referred to as Late Onset Ataxia (LOA). Affected dogs begin showing signs of cerebellar ataxia between 6 and 9 months of age. The coordination difficulties become more severe and the affected dogs are often euthanized by 2 years of age due to poor quality of life. Specialized tests of hearing (BAER) can show some changes in LOA but deafness is not reported.
Spinocerebellar Ataxia (SCA) with or without Myokymia and Seizures
The disease being studied by MU is referred to as Spincerebellar Ataxia (SCA). Affected dogs also show signs of cerebellar ataxia as early as 2-6 months of age. At post-mortem examinations, degeneration can be found in the areas of the spinal cord that carry information to the cerebellum, hence the term spino-cerebellar ataxia. The coordination difficulties also progress, but in addition other signs can develop. The majority of cases also develop myokymia, an involuntary twitching of the muscles. The myokymia also becomes progressively worse with age and can result in episodes of generalized muscle spasms and over-heating. In addition, a small percentage of dogs with SCA have true epileptic seizures, some as young as 10 weeks of age. It remains to be determined if the myokymia and seizures are a variable symptom of the SCA mutation or if other genetic factors are involved. Most dogs with SCA are also euthanized young due to poor quality of life.
Neonatal Granuloprival Ataxia (NGA)
Another form of ataxia called Neonatal Granuloprival Ataxia (NGA) is also hereditary. The mutation has not yet been identified, but research is ongoing. As the name implies, coordination difficulties in dogs with NGA are apparent from the time they begin to walk, which clearly distinguishes it from the other forms. In these dogs, an area of the cerebellum called the granular layer degenerates.
Other forms of ataxia
Both labs have found a small number of dogs reported as suffering from cerebellar ataxia which began after weaning age that do not have either of the known mutation. This may represent another form of hereditary ataxia or they may represent an acquired cause of the disease.
The Animal Health Trust and the University of Missouri have discovered different mutations which cause similar, but distinct forms of the disease in Jack/Parson Russell Terriers. We are discussing offering joint testing for both mutations. We are also working to investigate those cases of ataxia which do not have either of the known mutations.