Equine Cerebellar Abiotrophy (CA)
by Stephanie Corum
Equine cerebellar abiotrophy (CA) is a condition that you may not of heard of… yet. But chances are you will become familiar with it. This genetic condition affects Arabians almost exclusively, although it has been seen in miniature horses and a few pony breeds. The condition results in the death of neurons in the cerebellum, which causes head tremors and a lack of balance. There is no treatment or cure and affected horses are usually euthanized because of the danger they pose to themselves and to others.
CA is caused by a genetic variation of unknown origin. It is a recessive gene, which means a horse can be a carrier without developing the disease. If two carriers are bred together, there is a 25% chance that the resulting foal will develop CA. Foals born with CA appear normal at birth with the onset of symptoms usually occurring by the time the foals are 6 weeks to 4 months of age. While the symptoms may vary in severity, head tremors and ataxia are usually seen. They may also have exaggerated movement of the front legs, stand base-wide and be unable to stand. The youngsters typically startle easily and fall often. Because of the neurological symptoms, CA has been confused with EPM and Wobbler's syndrome. A positive diagnosis is possible post-mortem with a histological exam of the cerebellar tissue.
To delve into a bit more detail, the cerebellum plays an important role in sensory perception, coordination and motor control. It consists of two layers, the granular (inner) layer and molecular (outer) layer. Purkinje cells are large neurons that lie between the two layers. They carry electric signals from the outer layer to the inner layer, and then signals are transmitted throughout the body. In foals with CA, the Purkinje cells begin to degenerate soon after birth, which in turn also causes the loss of granular cells. The Purkinje cells that are left are very small, and without them, the two cerebral layers become indistinct.
Research into CA is ongoing, and late last year, UC Davis came out with a DNA test that identifies gene markers associated with CA. The DNA test uses a hair root sample and is available for $50 per animal. Dr. Cecilia Penedo is leading the research at UC Davis. So far they have analyzed data from over 250 horses related to 20 affected foals. They have found that 30% of them are carriers. To assist in the research, the Arabian Horse Foundation gave UC Davis a $5,000 grant in August 2008.
Just recently, Albidayer Stud in the UAE announced that their stallion Marajj is a carrier. He was tested because a foal of his was suspected of having CA. He may be the first, but chances are there will be more to follow. Mare owners should test their mares if they plan on breeding to a carrier stallion. While gene therapy may be available years from now, this is a preventable disease by not breeding two carrier horses.
Arabian racing's own Leslie Smith has unfortunately had a first-hand encounter with CA, and it was her suggestion to make more people aware of this devastating condition. I asked her if she would tell her story.
From Leslie - “We had a beautiful grey filly born in April 2008. When she was about two months old, the filly was out in pasture with three other mares and foals, and a mare ran her into our no-climb horse wire fence. I cringed when I saw her turn her head sideways and run full tilt into the fence. I watched her get her bearings again and she seemed to be all right other than a few scratches. I continued to watch her for a couple of days and noticed nothing unusual.
“About a month later, while watching the foals "play" I noticed she was walking at an angle, and I did a double take to make sure I really saw what I thought was going on. She definitely acted like she had a few too many margaritas and was veering to one side while trying to walk straight. I called my vet and gave him the whole story about the fence incident and the symptoms. He thought it sounded like Wobblers Syndrome. He said to keep an eye on her and told me to watch for more pronounced stumbling and that "drunk walk". He suggested putting her and her dam in a separate pen so she didn't have to struggle with the others for food or get bumped around. Within another month or so she had started to shuffle more and her head started to bob quite a bit. If she reached her head forward to take food she would shake like a person with Parkinson's disease. My vet had explained that Wobblers usually starts from an area in the spinal column that is narrower than it should be and a trauma can start the symptoms to progress sooner rather than later. There is a small chance of improvement with surgery but no full recovery. My vet said the costs are high for a chance of partial recovery, the horse would never be safe to ride and you certainly wouldn't want to breed her.
“I decided to go to the UC Davis Veterinary School website to see what I could find out. I guess I was looking for some positive alternative to putting this filly down. That is how I found out about Cerebellar Abiotrophy.
“I called my vet and he had not heard of it before. I shared with him the symptoms, their article and studies they were doing on CA. I was ready to offer my filly to UC Davis for their research but she was stumbling too much to travel the 600+ miles. Due to the fact that she had more serious head bobbing and stiff legged movement as time went on, I believe she had CA rather than Wobblers. The day we put her down was very hard but I knew it had to be done.
“Since this is a hereditary problem that is more common in Arabians, where both sire and dam have to be carriers, I felt it was important to share this information, and I contacted Stephanie.
“May your 2009 foals be healthy and correct.”
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