Geneticists in the US have identified a mutation that is associated with equine cerebellar abiotrophy (CA), a neurological condition found mainly in Arabian horses.
The potential causative mutation involves two overlapping candidate genes that are undergoing further study.
The research to find the mutation was carried out by Cecilia Penedo, PhD, a geneticist at the University of California, Davis Veterinary Genetics Laboratory (VGL), and Dr Leah Brault.
While there is already a marker based screening test for the condition, which causes the death of neurons in the cerebellum of affected foals, a new diagnostic DNA test has been developed by Dr Penedo.
Research began on cerebellar abiotrophy when geneticist Ann Bowling started a project many years ago at the School of Veterinary Medicine in 1985. Several breeders have assisted in identifying Arabian equine families affected by the genetic condition, and financial support has come from the Arabian Horse Foundation and others.
Results from breeding experiments carried out at UC Davis indicate a recessive mode of inheritance for the condition. This means a horse can "carry" the disease gene but not be affected. However, breedings between two carrier horses will produce an affected foal 25% of the time.
The disease is transmitted when two carrier horses are bred. A foal has a 25% chance of receiving the genes that cause the disease, which manifests itself shortly after birth. Affected horses may have mild to severe tremors of the head, and they can easily lose their balance. Horses affected by cerebellar abiotrophy are prone to falls, making them unsafe to ride and more likely to become injured.
Breedings between normal and carrier horses will not produce a CA foal although 50% of the foals are expected to be carriers.
Foals affected with CA appear normal at birth. Around six weeks of age (although sometimes as late as four months), the disease causes the death of neurons in the cerebellum of affected foals, leading to head tremor (intention tremor) and a lack of balance equilibrium (ataxia), among other neurological deficits.
Affected horses may show exaggerated action of the forelegs, a wide-based stance, and be unable to rise from a reclining position. They tend to startle easily and often fall due to ataxia.
Signs of CA are variable, and a small number of horses classified as affected by the genetic test have been reported by owners as showing no signs. CA foals that show signs of the disease are often euthanized or restricted to life as pasture pets, as they are never co-ordinated enough to be ridden safely.
The CA mutation has been found in low frequency in other breeds, such as Danish Sport Horse, Bashkir Curly, Trakehner and Welsh Pony, that have used arabian horses as foundation stock. The CA test is recommended for horses that have arabian ancestors in their pedigree.