In the spring of 2020, a previously healthy Quarter Horse foal in Oregon rapidly grew weak and uncoordinated a few weeks after birth. But all tests for known neurologic diseases came back negative. Now, research conducted at the University of California, Davis, has yielded a diagnosis: a newly identified hereditary condition called Equine Juvenile Spinocerebellar Ataxia (EJSCA).

Although the UC Davis team initially focused on the Oregon foal, the scope of their inquiry quickly broadened. It ultimately included 11 other similarly affected youngsters, says Briana Brown, MS, a graduate student working with Carrie Finno, DVM, DACVIM (Large Animal), PhD. “We received phone calls from veterinarians in Texas about additional cases, and we discovered that these foals were related.”

Piecing together the evidence

Based on case histories, postmortem exams and extensive genetic testing of the affected foals, the researchers developed a profile of a devastating new disease—EJSCA. “Foals appear normal when they are born—they are bright, alert and responsive,” says Brown. “The onset of neurologic signs begins between one and five weeks after birth. Ataxia [incoordination] of the hind limbs begins and rapidly progresses until the foal can no longer stand. This typically happens in less than a week.”

Physiologically, Brown says, the deterioration of the myelin sheaths that protect axons cause the signs associated with EJSCA. Long, thin projections from the nerve cells (neurons), axons conduct electrical impulses to and from the brain and spinal cord. “The axons also begin to degrade,” she explains. “And malfunctioning current conduction is the physical reason for the clinical signs observed in EJSCA. Without functioning neurons in these tracts, the foals have a difficult time walking properly because their legs are not sending enough information to the brain about where the ground is or if they are off-balance.”

No treatment currently exists for EJSCA, and foals deteriorate until they can no longer walk or stand and are euthanized.

Tracing the genetic origins

The UC Davis team also determined that all the affected foals descended from a single stallion. “Our pedigree investigation supports the hypothesis that EJSCA is an autosomal recessive disease,” says Brown. “Autosomal means the mutation is not found on the X or Y chromosome. Recessive means that the individual must have two copies of the mutant form of the gene to show physical characteristics for the disease.”

Individuals with only one copy of the gene are known as carriers. They are not affected but carry the gene mutation. If two carriers are bred, however, their offspring has a 25-percent chance of inheriting a mutated gene copy from each parent and developing EJSCA. To maintain client confidentiality, pedigree information is not publicly released.

Brown and Finno urge breeders to take steps to avoid producing foals with EJSCA. “We would advise breeders to continue following best breeding practices. If they believe their stock may be a carrier for EJSCA, it is in their best interest to have them tested. That way, they do not breed that horse to another potential carrier. This disease is severe and devastating.”

The test for EJSCA is currently available only through the UC Davis Veterinary Genetics Laboratory.

Reference: “Clinicopathological and pedigree investigation of a novel spinocerebellar neurological disease in juvenile Quarter Horses in North America.” Journal of Veterinary Internal Medicine, April 2024