USING RELATIVE RISK ANALYSIS AND OPEN HEALTH REGISTRIES TO PLAN MATINGS
This article is important for breeders who are planning a breeding and would like to compute the odds of the proposed mating producing CA carriers. Until we have a genetic test for carriers of CA, relative risk analysis is our only tool for assessing the likelihood of producing dogs affected with CA. Dr. Bell can also assist you in the relative risk pedigree analysis. (Please see "Requesting a Risk Analysis".)
Important points covered:
1. With a simple autosomal recessive genetic disorder, pedigree analysis can be used to compute relative risk factors for carrier and affected status.
2. The objective risk factor for a parent of an affected dog is 100% chance of being a carrier. A full sibling of a carrier has a 50% chance of being a carrier. A non-affected offspring of a carrier with no affected littermates has a 50% chance of being a carrier. A non-affected full sibling to an affected dog has a 67% chance of being a carrier.
Notice that these percentages refer to carriers, not affected dogs. The chance of producing an affected dog would be even less.
3. It is erroneous to add risk factors for each parent of proven carriers. Only one parent of a carrier needs to pass on the defective gene and we cannot determine which parent is a carrier. Assigning carrier risk to both parents of a carrier also incorrectly places selective pressure against dogs based on their matings, and not their proven genetic background.
AUTOSOMAL RECESSIVE INHERITANCE
This article will be helpful in understanding how CA is inherited. Breeders especially are urged to study it carefully.
Autosomal = a pair of like chromosomes.
Recessive = two copies of a gene must be present before a dog is affected by the disease or trait, thus a carrier would have one copy of the gene to pass on to offspring but would not actually have the disease or trait.
Important points covered:
1. Simple autosomal recessive genetic conditions are inherited only from parents that are carriers for the recessive gene or are themselves affected by the condition.
2. Both affected and carrier parents can pass the recessive gene on to their offspring.
3. Both parents of an affected animal must be carriers of the recessive gene.
4. Breeding carriers will not always produce an affected offspring.
5. Clear bred to Clear will only produce Clear offspring
CA – DR. BELL & OESCA
These articles are helpful to get an overview of the problems we face, how we began, and how far we have come in working together.
Some of the important points:
1. CA is not a single kennel problem
2. CA is an autosomal recessive gene
3. The closest common ancestor in pedigrees is not necessarily a carrier
4. To keep our gene pool diverse, we should not discard dogs from breeding programs simply because they are related to a dog affected with CA.
CEREBELLAR ABIOTROPHY (CA) SYMPTOMS
This short article gives a description of the typical gait of a dog affected with CA.
CEREBELLAR ABIOTROPHY IN THE OLD ENGLISH SHEEPDOG: Its Cause And Diagnosis
This article is a great resource for understanding CA in our breed. A must read before you view the CA registry.
Important points covered are:
1. CA is caused by a degeneration of cells in the cerebellum of the brain.
2. CA causes uncoordinated movements of the limbs.
3. CA is not painful and need not shorten a dog's life.
4. CA can be diagnosed through clinical observation, examination of the cerebellum after death, and an MRI in advanced cases.
5. CA is caused by an autosomal recessive gene, which means both parents must carry the gene to produce an affected dog.
Cerebellar Abiotrophy (CA) Symptoms
(also known as Canine Ataxia)
By Dr. H. Steven Steinberg, DVM, DACVIM (Neurology)
In 1994 adult Old English Sheepdogs were presented to me with a progressive abnormality in their gait. It became evident that this problem was widespread and has been recognized in various parts of the country. The underlying cause of this disease, the genetic basis of this disease as well as defining an appropriate approach to decreasing the incidence of this disease, commonly called Cerebellar Degeneration or Cerebellar Abiotrophy (CA), is ongoing at a number of centers.
Other Cerebellar degeneration conditions have been described over the years including one in Gordon Setters described by me in 1981. The Cerebellum is part of the brain that is critical for controlling smooth, efficient, and effective movement.
This short description will define a few of the hallmarks of this condition but it must be noted at the outset, that a veterinarian and in many cases a veterinary neurologist is necessary to diagnose this condition. Many of the gait abnormalities seen with this condition are subtle and even in its most advanced form CA mimics other orthopedic and/or neuromuscular conditions. This condition seems less severe and progresses more slowly than most other Cerebellar Degenerations reported. Older affected dogs that get around reasonably well have been seen. Dogs do not die from this condition.
The most obvious abnormality in these dogs is a basewide hindlimb stance and gait. Typically when dogs walk they should move their paws towards midline. This is best appreciated when the dogs are walking away from the viewer. CA dogs tend to stand with their hindlimbs far apart and have mild to moderate circumduction. Circumduction is where the paw is carried outside of a line dropped vertically to the ground from the hip joint. The paw also strikes the ground far from the midline and usually outside of a line dropped vertically from the hip joint.
A second noteworthy gait abnormality is known as hypermetria. Evidence of hypermetria is seen in dogs with CA when they lift their legs much higher than is normal or appropriate. This is best appreciated in the forelegs. In many breeds with broad chests or white chests (like Boxers) this prancing gait is considered desirable by some in the show ring. It is difficult sometimes to appreciate this abnormality but when watching a severely affected dog from the side their carpus (wrist joint) may be raised as high as their shoulder joint before placing the paw on the ground.
In the later stages of the disease, the dogs are extremely wobbly (ataxic) and their gait problem becomes much easier to appreciate.
Although the condition has been presented in dogs who are young adults, there may be subtle changes in puppies.