Spinocerebellar Ataxia with Myokymia & Seizures in Jack/Parson/Russell Terriers
Since the mid 1990s, we have been studying a constellation of movement disorders in Jack/Parson/Russell Terriers. The situation is complicated because there appear to be several different forms of the disease in the breeds.
Click here to read a joint statement from the Animal Health Trust and University of Missouri regarding the different cerebellar ataxias in Jack/Parson Russell Terriers.
What is spinocerebellar ataxia?
The term cerebellum in Latin means “the little brain”. The cerebellum is the part of the brain responsible for coordinating movements. Ataxia comes from a Greek term meaning “without order”. When the cerebellum cannot coordinate movement, the dog can move, but the movement is poorly coordinated. They are not weak, in fact, often the movements a dog with ataxia makes are too strong. They have a goose-stepping gait and when excited or running, their legs may appear to be going every which-way. Sometimes they have problems with their balance and will fall frequently. In order for the cerebellum to control movement, it needs to get feedback about what the muscles are doing. This feedback comes to the brain through the spinal cord. When there are changes in the spinal cord in a dog with cerebellar ataxia, the disease is often call spino-cerebellar ataxia (SCA).
What is myokymia?
Myokymia is a problem with the muscles (myo- in medical terms). Dogs with this symptom have uncontrollable twitching of the muscles. The twitching tends to run through a muscle in waves, hence the –kymia portion of the word which comes from the Greek word for waves. Attacks of myokymia can be precipitated by exercise or excitement. The severity can vary from mild muscle twitching to incapacitating attacks. When severe, the dog can become rigid and develop a high body temperature. Such severe attacks can be confused with seizures, but the dog remains aware of his surroundings during an attack of myokymia. Some dogs with spinocerebellar ataxia also have true seizures.
What are the different forms of ataxia in Jack/Parson/Russell Terriers?
There appear to be at least two (probably three) different forms of cerebellar ataxia in these lines, a neonatal cerebellar ataxia and a later onset of ataxia (spinocerebellar ataxia or SCA) with or without myokymia or seizures. In the neonatal form the dogs have ataxia from the time they begin to walk. The age of onset is similar to the disease in Coton de Tulears, but they are caused by mutations in different genes.
In spinocerebellar ataxia, the dogs develop normally for the first few months of life. Then beginning at 2-6 months of age they begin to develop ataxia. There could still be other forms of ataxia as well.
What else can look like spinocerebellar ataxia?
In addition to the hereditary conditions, other diseases can cause signs of ataxia. Any disease that affects the cerebellum will produce ataxia. Canine distemper infections will frequently affect the cerebellum as can other infections. Though strokes are uncommon in dogs, they can also affect the cerebellum. In older dogs, a brain tumor in the cerebellum would produce similar signs. If necessary, your veterinarian can refer you to a board certified neurologist who can aid in diagnosing the cause of ataxia. A directory to a neurologist near you can be found at www.acvim.org under “Search for a Specialist”.
Is this a hereditary disease?
Pedigrees analyzed thus far suggest that SCA is inherited as a recessive trait. As discussed above, there appear to be multiple forms of ataxia in the breeds which can complicate the picture. In a recessive disease, both parents of an affected pup appear normal. All animals have two copies of each gene, one that is inherited from the mother and one inherited from the father. A dog that has one normal gene and one gene that causes the disease is a carrier of the trait. They show no symptoms because the one good gene is enough for their nerves and muscles to develop normally, but they will pass that bad gene on to about half of their offspring. If a carrier dog is bred to another carrier, then some of the pups (25% on average) will get a bad gene from each parent. Without one good gene to carry the day, the nerves cannot function normally and the unlucky pup has ataxia.
Is there a DNA test?
In 2012, researchers at the University of Missouri found the mutation responsible for spinocerebellar ataxia in Jack/Parson/Russell Terriers. Researchers at the Animal Health Trust in England may have also found a mutation that could explain some of the other cases of ataxia. CLICK HERE for information about the AHT test. We are collaborating to sort out these questions. A DNA test is now available for the SCA mutation identified at Missouri and can be ordered through the Orthopedic Foundation for Animals website (www.offa.org).
Owners of dogs previously sampled for this or other research projects, or with DNA banked at Missouri for any reason can request a discounted test by using the request form (see Order DNA Tests page), or contact Liz Hansen at HansenL@missouri.edu for a print version. With the DNA test carriers of the mutant gene can still be used for breeding as long as they are bred to a dog that is clear of the mutation. That way no affected dogs will be born, but the desirable genetic diversity that these dogs provide the breed will be maintained. When selecting future breeding stock, the gene status can be considered in deciding which pups to keep but does not have to be the sole factor. As discussed above there are other both hereditary and acquired causes of cerebellar ataxia. Thus not every dog with ataxia will have the mutation.