Genetic Testing Options for Horses

Genetic testing is a continually evolving field in equine medicine. As such, veterinarians and researchers discussed the topic during the 2019 Annual American Association of Equine Practitioners Convention, held Dec. 7-11 in Denver.

Two leading researchers in this field—Carrie Finno, DVM, PhD, Dipl, ACVIM, associate professor and director at the University of California, Davis, School of Veterinary Medicine’s Center for Equine Health; and Molly McCue, DVM, MS, PhD, Dipl. ACVIM, professor and associate dean of research at the University of Minnesota’s College of Veterinary Medicine—moderated a conversation about a variety of available (as well as not yet available) tests. They included:

Squamous cell carcinoma (SCC) Belgians and Haflingers both have a genetic link to SCC susceptibility that’s tied to ultraviolet light exposure, said Finno.

“Breeders can screen their horses for susceptibility and turn those who are susceptible out at night or wearing sun protection,” she said.

Equine neuroaxonal dystrophy (eNAD) This neurologic disorder occurs across all breeds but is most prevalent in Quarter Horses, said Finno. Its clinical signs (e.g., incoordination) mimic those of wobbler syndrome. Foals often develop it if they haven’t received enough vitamin E through fresh pasture in their first year of life.

While scientists don’t yet have a test for eNAD, they are moving toward one, said Finno.

Immune-mediated myositis (IMM) This muscle disease typically occurs in young Quarter Horses that have been recently exposed to infection or vaccination against diseases such as strangles or influenza, said Finno. A few weeks later, their muscle atrophies dramatically, as it’s being attacked by the body’s own lymphocytes (white blood cells). Fortunately, most horses respond quickly to corticosteroids.

She said a genetic test for IMM in Quarter Horses and related breeds was published in 2018.

Warmblood fragile foal syndrome (WFFS) This recessive condition results in skin splitting and evisceration in newborn foals—if they even survive to birth. Scientists have found a high (~20%) carrier frequency in some Warmbloods, said Finno.

Rather than removing affected horses that have great traits worth perpetuating from the breeding pool, “just don’t breed it to another carrier ever,” she said. “That’s the power of information—making informed breeding decisions.”

Breeders can test their horses as well as samples from aborted fetuses for WFFS.

Type 1 polysaccharide storage myopathy (PSSM1) This form of tying-up is a single gene disease prevalent in Quarter Horses, Belgians, and Percherons. “Not all horses are going to tie up, but when we diagnose horses as heterozygous (meaning they only have one copy of the allele) for PSSM1, diet and exercise changes can prevent or reduce episodes,” said McCue.

Researchers know that PSSM1 occurs in at least 30 breeds, so if you plan to breed, it’s smart to test for it, she said.

To date, there are no genetic tests for type 2 PSSM for which scientific data has been published and reviewed. Type 2 PSSM has clinical signs similar to type 1 but thus far an unknown cause.

Equine asthma This respiratory condition is a polygenic disease, said McCue, meaning many genes impact it. “There’s no research yet, but we believe it might be heritable,” she said. “There is a group currently looking at gene expression to guide treatment.”

Other Additional diseases the moderators said researchers are exploring heritability and testing for include:

  • Equine metabolic syndrome.
  • Severe untreatable calcium deficiency in Thoroughbred foals, for which a test should be available within the next year.
  • Recurrent exertional rhabdomyolysis in Thoroughbreds.
  • Atrial fibrillation in Standardbreds.
  • Idiopathic renal hematuria in Arabians.
  • Congenital microphthalmia.
  • Recurrent laryngeal neuropathy (roaring).

“The exciting thing in this field is the cost of this technology has come down so much,” said Finno. “You’re going to be seeing a lot more tests.”

Where to Test

When choosing a laboratory to submit hair samples to for testing, Finno and McCue urged attendees to use a validated lab.

“Other labs might be faster and cheaper but not as valid,” said Finno. Further, “when you go through a university lab, a portion of those funds goes back to the research.”

McCue said you can easily call and ask a lab if it has a validated license agreement for what it’s testing.

Finno encouraged breeders to continue submitting samples from affected horses (and, even better, controls—unaffected horses—from the same environment) to labs and scientists. “It’s incredibly valuable for researchers.”