WFFS in Thoroughbreds Prompts Renaming to Fragile Foal Syndrome

Sun, 01/09/2022 - 09:17
Health Care
Warmblood Fragile Foal Syndrome  Photo © Laboklin

A first reported case of WFFS in a Thoroughbred foal in the U.K., has prompted researchers of the Veterinary Genetics Laboratory (VGL) at UC Davies to rename "Warmblood Fragile Foal Syndrome" (WFFS) type I as  "Fragile Foal Syndrome" (FFS).


Fragile foal syndrome (FFS) type I is an inherited connective tissue defect characterized by lax and hyperextensible joints and abnormally thin, fragile skin and mucous membranes causing extensive lesions throughout the body.  This is a lethal condition. Signs of the disease are present from birth. Newborn foals are euthanized because of poor prognosis for an untreatable condition. 

FFS is inherited as an autosomal recessive defect, which means that both males and females are equally affected and that two copies of the mutation (FFS/FFS) are needed to cause the disorder. 

Horses with N/N genotype will not have fragile foal syndrome and cannot pass this fragile foal syndrome variant to their offspring.
Horses with N/FFS genotype will not have fragile foal syndrome, but are carriers. They may transmit this fragile foal syndrome variant to 50% of their offspring. Matings between two carriers will result in a 25% chance of producing a fragile foal syndrome-affected foal.
Horses with FFS/FFS genotype will have fragile foal syndrome, a fatal genetic defect of connective tissue.


The genetic disorder became public knowledge in 2018 when a U.S. breeder raised concern, which prompted the leading dressage breeding yard Hilltop Farm test all of its stallions.

Since then and under pressure of concerned breeders, major warmblood breeding societies in Europe required stallion owners to test their stallions. 

While mare owners finally found an explanation in why some of the mare were incapable of getting in foal or frequent miscarriages, stallion owners reacted vehemently against the mandatory testing of their stallions initially. 

Renowned dressage stallions such as Don Schufro, Roemer, Apache, Total U.S. and Jovian turned out to be FFS carriers. 

Thoroughbreds with FFS

Research at the VGL had identified a low allele and carrier frequency of the FFS mutation in Thoroughbreds.  In studying over 700 horses, carrier frequency was 2.4%, meaning out of 200 horses approximately 5 would have one copy of the mutation. 

The first report of an affected Thoroughbred foal in the U.K., homozygous for the fragile foal variant (FFS/FFS), has been recently published in the Equine Veterinary Journal. This is the first documented evidence of FFS caused by the PLOD1 variant in a breed outside of warmbloods, the Thoroughbred.  The report resulted from a collaboration between the UC Davis Veterinary Genetics Laboratory and the Royal Veterinary college and was written by UC Davis veterinary student, Alexandra Grillos under mentorship of VGL director Dr. Rebecca Bellone.

The identification of an affected Thoroughbred foal provides evidence that the condition is of concern to all horse breeds that carry the mutation and supports the use of genetic testing to inform breeding decisions and avoid producing affected foals. 

FFS in Other Breeds

A recent survey of 38 breeds in the USA and Europe detected the allele in 21 breeds and provided data on allele and carrier frequency in warmbloods as well as several non-warmblood breeds.

The incidence of carriers in Warmblood horses is estimated to be around 9-11%. Additionally, since the FFS allele has also been detected in the Thoroughbred, Knabstrupper, American Sport Pony, and Haflinger, albeit at low frequencies, testing could inform mating decisions to avoid breeding carriers

Read more about the latest research here

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