Warmblood Fragile Foal Syndrome FAQs

What is WFFS?

Warmblood Fragile Foal Syndrome (WFFS) is an inherited systemic connective tissue disorder that is found in Warmblood horses. Skin lacks tensile strength (extreme skin fragility characterized by tearing, ulceration, etc. from contact with normal surroundings). Lesions can occur anywhere on the body but are most noted on pressure points and in addition to skin wounds, lesions are found on the gums and other oral cavity mucous membranes and the perineum. Limb joints are lax and hyperextensible. Fetlocks are the most dramatically affected and affected foals cannot stand normally. Other symptoms may include hydrops, subcutaneous emphysema, hematomas, or premature birth.  Affected foals must be euthanized soon after birth.

WFFS is caused by a single mutation in lysyl hydroxylase 1 (LH1) gene. Mutations in LH1 are known to cause a similar disorder in humans known as Ehlers-Danlos Syndrome (EDS).  The skin defects resemble, but are much more severe than those seen in the Hereditary Equine Regional Dermal Asthenia (HERDA), which has a later onset of symptoms.


How does it occur?

Genes come in pairs with each parent contributing one copy to their offspring. With autosomal dominant genetic diseases only one defective gene is necessary to express the genetic disease in question. Such a gene can be inherited from either of the parents or from both parents. With recessive genetic diseases two copies of an abnormal gene (two copies = a pair) must be present for the horse to show signs of the genetic defect. WFFS is an autosomal recessive trait, meaning a foal can only be affected if the foal inherits the disease from both parents. Horses with only one copy of the defective gene are considered “carriers”.

How common is WFFS?

Initial estimates were that between 6%-11% of the Warmblood population are carriers of WFFS but the test has not been widely utilized and as more people test we will better understand the true carrier rate.  From our own experiences over the last few months, we are seeing a carrier rate around 10%.

How likely is my foal to inherit WFFS?

If both the sire and dam are clear of the gene mutation (N/N), then there is no chance of an offspring inheriting the mutation.  If a carrier of the recessive allele (N/FFS) is bred to a normal horse (N/N), each foal has a 50% chance of having two copies of the normal allele (N/N) and a 50% chance of being a carrier (N/FFS).  If two carriers are bred, each foal has a 25% chance of having two copies of the normal allele (N/N), a 50% chance of being a carrier (N/FFS), and a 25% chance of being affected (FFS/FFS).

So within the warmblood population, assuming a conservative 8% carrier status, the following statistics could be assumed.  Out of 100 broodmares, approximately 8 would be carriers of the WFFS allele.  If those 100 mares were bred to a carrier stallion, approximately 50 foals would inherit carrier status (N/FFS), 48 foals would NOT inherit the recessive allele and would be clear of the gene mutation (N/N), and 2 foals would be positive for the disease (FFS/FFS).

Why should I test my breeding stock?

The focus for breeders needs to be on reducing risk and avoiding carrier-to-carrier breedings (which are the only combination that can potentially result in the fatal disease being inherited).  With the vast majority of stallions and mares being untested, a breeder that tests their own mares will know either that their mare is clear of the mutation and regardless of the stallion’s status a foal would not inherit the actual disease OR that the mare is a carrier and while she herself won’t have any health risks, it will be important to know the status of potential stallion combinations for her and choose only to breed to a stallion who is himself clear of the recessive allele.

How can I test my horse for WFFS?

Testing is easy. It’s a simple hair pull (they can also test blood or semen) just like you are used to doing for DNA Proof of Parentage.  Initially Animal Genetics was the only lab in the US offering this test. 

UC Davis has just recently released their version of the test and many of the sport horse registries have negotiated discounted rates for their members.  You may not even need to pull new hair samples if your horse’s DNA is already on file at UC Davis.  Contact your registry to discuss what forms/services they can provide.  We hope that WFFS testing will become required by the Breed Associations in the future and as such should be linked to the horse’s Parentage Verification DNA results.  Utilizing UC Davis for WFFS testing now becomes our recommended protocol going forward.  

My horses tests positive as a carrier. Can I still breed my horse?

Yes, carrier horses can still be used in our breeding programs.  While there is a 50% chance they will pass along carrier status to their offspring, as long as we avoid carrier-to-carrier breedings we can prevent any potential foal losses due to WFFS.  At Hilltop, we will be requiring any mare breeding to a carrier stallion to have been tested negative/clear of WFFS.  

My horse tests clear. Does that mean it’s parents are also clear?

Unfortunately no.  We cannot assume status backwards as even two carrier individuals can produce a clear offspring.  However, we do know that two clear parents will always mean a clear offspring.

How are the registries responding?

Many of the major warmblood registries came forward early on with statements that they would be evaluating WFFS further.  As of May 9th, both the NRPS and KWPN have announced that all approved stallions will be required to be tested and publish results for WFFS.  We feel this is a smart approach – just like we test for EVA – and we hope the other registries quickly follow this lead. 

How do I know a stallion’s status?

On the pages of the stallions Hilltop Farm represents, you will find their testing status published (along with EVA status, color testing, etc).

Do I need to be concerned about WFFS status for a performance horse?

WFFS carriers have not been systematically studied. Extrapolating from what is known from human studies, this recessive allele would not be expected to have a marked affect on the health of your horse or its performance potential. If it is a mare or stallion, it would be recommended you test prior to breeding but there is no need to test geldings.