Thoroughbred chestnut foal with white markings

International collaboration identifies the first case of Fragile Foal Syndrome (FFS) in the Thoroughbred

An international collaboration between the UC Davis Veterinary Genetics Laboratory (VGL) and the Royal Veterinary College (RVC) characterized the first case of Fragile Foal Syndrome (FFS) in the Thoroughbred. Previously, this condition was called Warmblood Fragile Foal Syndrome (WFFS) as it was initially discovered in Warmblood horses and, until now, the syndrome had only been diagnosed in horses from those breeds. 

The disorder is characterized as a connective tissue defect caused by a change in DNA in the procollagen-lysine, 2-oxoglutarate 5-dioxygenase1 (PLOD1) gene. The condition is inherited as an autosomal recessive trait, which means a foal will only be affected for the disorder if it has two copies of the mutation. Affected foals are typically born with an open abdomen and extensive skin lesions due to abnormally thin and fragile skin, leading them to be stillborn or euthanized shortly after birth. Subsequent work by our team and others has identified the PLOD1 variant causing this disease in many warmblood breeds, as well as, in Thoroughbreds, Knabstruppers, Haflingers and American Sport Pony. However, prior to our current study no affected horses had been reported outside of the warmblood breeds.

“Pregnancy loss, stillbirth and neonatal death remain an important source of reproductive losses for Thoroughbred horse breeders worldwide,” RVC researcher and co-author of the publication, Dr. De Mestre, explains. “Over the last 5 years, the Equine Pregnancy Laboratory at the RVC has worked alongside Rossdales Laboratories, Newmarket, developing and characterizing a biobank of clinical cases of abortion and stillbirth. We were delighted to be able to team with Dr. Bellone and vet student Alexandra Grillos enabling them to demonstrate the relevance of Fragile Foal Syndrome in the Thoroughbred. Whilst clearly a distressing condition for affected foals and their owners, the good news is that this lethal syndrome can be avoided with testing and careful mating selection.”

The study, led by our very own VGL director, Dr. Rebecca Bellone, was just published in the Equine Veterinary Journal and is the first to fully characterize a clinical case of Fragile Foal Syndrome in a Thoroughbred. Clinical confirmation of this case prompted the need for a test name revision in light of recent findings.

The VGL has updated the name of this test from WFFS to FFS to best reflect these findings and the appropriate use of this test.

This report highlights the importance of genetic testing for FFS in all horse breeds that carry the mutation to inform breeding decisions and avoid producing affected foals.

This study was performed as part of the UC Davis School of Veterinary Medicine’s Students Training in Advanced Research (STAR) Program and Global Programs International Travel Fund. To hear more about that experience, please see the “Meet the Scientist” side bar with fourth year UC Davis Veterinary Student Alexandra Grillos.

To read the recent publication go to

For more information on the condition and the test offered by the VGL, please refer to


Grillos, A., Roach, J., de Mestre, A., Foote, A., Kinglsey, N., Mienaltowski, M. and Bellone, R. (2022), First reported case of fragile foal syndrome type 1 in the Thoroughbred caused by PLOD1 c.2032G>A. Equine Veterinary Journal. Accepted Author Manuscript.