WFFS FAQs

Warmblood Fragile Foal Syndrome is a topic of importance among breeders. Information about this genetic defect is scattered among many sources. Below are some answers to the most frequently-asked questions. 

What is Warmblood Fragile Foal Syndrome (WFFS or FFS)?

Warmblood Fragile Foal Syndrome (WFFS) is a fatal disorder caused by a genetic defect. The defective gene is one that is critical in the production of collagen. 
Affected foals have "hyperextensible, abnormally thin, fragile skin and mucous membranes (tissue that lines cavities and covers organs) that cause extensive lesions throughout the body. ...Signs of the disease are present from birth. Newborn foals have to be euthanized." [UC Davis website. See also the list of resources for breeders for other informational sites. For details of how the mutated gene interferes with the formation of healthy tissues, see "How does this genetic defect actually work?" at the bottom of this page.]

WFFS is a recessive trait, which in this case means that horses can be:

• N/N - they have two copies of the dominant allele (version of the gene). They do not have or carry WFFS and cannot pass it on to their offspring. They are clear of the mutation.
• WFFS/N - they have one allele with the WFFS mutation and one without. They are perfectly normal, but are able to pass the mutation on to their offspring some of the time. Horses who are WFFS/N are carriers of the mutation but do not have the defect.
• WFFS/WFFS - they have the defect and probably are aborted before they are born in most cases. If they are born, they are as described above and must be euthanized. There are no adult horses with WFFS/WFFS status, because they do not survive.

Because WFFS is recessive, in order to get a foal with the defect you would have to breed a carrier to a carrier. If you breed a carrier to a horse that is clear of the gene, you will never get a foal with the defect. 

How does a horse get WFFS?

A horse gets WFFS by inheriting it. The only way to get it is if both the sire and dam of the horse are carriers of WFFS. WFFS is not caused by a "bug" and is not contagious.

What is the difference between "having" WFFS and being a carrier?

Only a foal with two copies of the defective allele (versions of the gene) "has" WFFS. A fetus who has WFFS will often be reabsorbed or aborted. If a foal is born "with" WFFS, he or she will show the symptoms and will not live. 

A carrier has only one copy of the defective allele, and one copy of the normal allele. Carriers do not "have" WFFS and are normal and healthy. There have been no formal studies of carriers and possible related health issues, but it is known that there are many international competitors who are careers. 

Is WFFS really a disease?

Technically it is, because part of the definition of "disease" is "a disorder of structure or function ... not simply a direct result of physical injury." WFFS is certainly a disorder of structure and function. But it's not a disease the way most people think of a disease, like flu or E. coli or cancer. 

The clearer terms for WFFS would be syndrome, disorder, or genetic defect.

Is WFFS limited to warmbloods?

No. WFFS should probably be known as Fragile Foal Syndrome, because it has been identified not only in all warmblood populations (including those with more closed books like Trakehner and Holsteiner), but also several other breeds including Thoroughbreds. 

Is WFFS the same as HERDA?

No. HERDA (hereditary equine regional dermal asthenia) is also an autosomal recessive genetic defect, but found in American Quarter Horses (especially cutting lines) and related breeds. Unlike WFFS, it manifests usually in the second year. As with WFFS, there is no cure.

Although they are not the same, Dr. Nena Winand, who has studied both WFFS and HERDA, recommends "testing any horses at risk for HERDA before breeding to warmbloods, and not crossing carriers of these two disorders. Avoid crossing a mare that tested positive for the WFFS1 or the HERDA mutated gene with a stallion that tested positive for the WFFS1 or the HERDA mutated gene." [from Q & A with Dr. Nena Winand on WFFS]

Will my horse get WFFS from another horse who is a carrier?

No. WFFS is not caused by viruses or bacteria and is not at all contagious. It is not a "disease" in that sense; it is a genetic defect.

Do carriers get sick?

No, carriers do not get sick. As far as we currently know, being a carrier has no effect at all on the horse, and there are many highly successful horses who are carriers. 

How do I know if my horse is a carrier?

There is a DNA test that you can have done by submitting a hair sample. Click here for a list of WFFS resources. Near the bottom of the page are links for the testing labs. Make sure to use a lab that is licensed and qualified to perform the test.

Should I test my riding horses?

Even if your horse is a carrier, it should have no effect on him or her, or the training or competition career. However, testing and reporting any horse contributes to the knowledge base on WFFS and its frequency and distribution in the population. The more complete the picture, the better the information we have to use and make breeding decisions.

How long has WFFS been around?

According to the scientist who discovered the mutation, Dr. Nena Winand, the first mutation of the gene happened about 170 years ago. 

Do we know who the first horse was who had the gene mutation?

Identifying the horse who first had the mutation would require extensive research and pedigree tracking. We may never know for sure who that "original" horse was. For practical purposes today, the knowledge is irrelevant, since the mutation now exists in all warmblood breeds and some others, including some Thoroughbred populations.

How long have we known about it?

The mutation was identified in 2011: the specific gene where it occurs. There were cases described earlier, not identified at the time as WFFS, that probably were WFFS. There has been a test for it since 2013.

There has been some awareness of WFFS in Europe since it was identified, but how much is not known. It did not achieve the status of common knowledge, or the level of open discussion we have now.

In 2018 a foal by Everdale, belonging to US breeder Mary Nuttall, was born with WFFS and had to be euthanized. The breeding had been handled by Hilltop Farm, and they and Mary Nuttall promptly announced to the world that this had happened. Hilltop Farm went further and tested all their stallions, one of which - Sternlicht - was found to be a carrier, and Hilltop removed him from breeding, at least for 2018. That precipitated the current level of awareness of WFFS.

What does WFFS mean for breeders?

WFFS is a topic of importance among breeders, because it will be breeders who will be managing WFFS by the decisions they make. Much of the current discussion of WFFS concerns these questions and decisions, and opinions vary a lot. Some breeders really believe that it's a big fuss over nothing. On the opposite extreme, some believe that every carrier should be removed from breeding. Each breeder needs to understand WFFS enough to make responsible breeding decisions.

How many horses are carriers for WFFS?

Reports vary, but among warmblood populations between 9% and 19% of horses are being reported as carriers of WFFS world-wide.

Can we stamp out WFFS?

It might be possible to stamp out WFFS, but it's unlikely. It would be necessary to remove every carrier from breeding, and that is unlikely to ever happen. The costs would be prohibitive, including the loss of valuable animals from the genetic pool of warmbloods, and the cost to individual breeders of losing breeding animals.

Why are all known carriers not immediately taken from breeding?

There are several reasons to avoid removing all carriers from breeding. 

First, it would require the removal of something like 10% of the warmblood gene pool from breeding. Genetic diversity is important, and among warmbloods the diversity is not all that extensive to begin with. Losing 10% would not be good. 

Second, genetic defects happen, and there are others in this population, and new ones could develop. If you remove all carriers of all defects from breeding, you'd be looking at losing even more than 10% of your population from breeding. 

Third, you would not only be losing 10% of your breeding animals, you'd be losing some individuals that are extremely valuable in what they contribute to the improvement of sport horses. Roemer, who stood at Iron Spring Farm and was a Preferent stallion who had a strong positive influence on the quality of dressage horses, was a carrier. 

Fourth, the removal of an animal from a breeding program means a significant loss for the individual breeder, in dollar cost, time and planning invested, and an immediate loss of productivity. Having to remove a mare out of your breeding program would be a significant loss for any breeder, and for a single mare owner it would be devastating. Having to remove more than one, or your best one, would mean an additional reduction of your business's value, with the loss of important future sales.

If we don't stamp out WFFS, how do we live with it?

WFFS can be managed so that there is no risk of producing a foal with both WFFS genes: the lethal condition. An affected foal can only be conceived when both sire and dam are carriers. If breeders have a carrier mare, they must only breed her to stallions who are clear. A carrier crossed with a clear horse has a zero percent chance of producing an affected foal.

Dr. Kareen Heineking-Schütte, a veterinarian and managing director of a breeding, sales and training facility in Germany, recommends continuing to breed this generation of mares and stallions, including carriers. Then choose non-carriers for breeding stock over the next generations. She advocates "keeping preferably non-carrier fillies" and for stallions, "chose from the best sons preferably those who are non-carriers." ["Two Cents about WFFS," eurodressage] This is a gradual reduction of carriers among the breeding population that is also supported by Dr. Nena Winand, one of the world's experts on WFFS. ("See also Should Carriers be 'culled'?" below.)

Should carriers be "culled"?

To clarify terminology, "cull" is a term that means "selectively slaughter." A more accurate term in the context of WFFS would be "remove from breeding." 

To remove all carriers from breeding is not practical or advisable (see "Why are all known carriers not immediately removed from breeding?" above). According to Dr. Nena Winand, who has studies both WFFS and HERDA, "Perhaps over time outstanding horses that are non-carriers can be sought out [for breeding] and the carrier frequency can be slowly reduced if necessary, but this type of change is best effected over many generations."

Should I stop breeding my carrier mare?

While it is safe to breed a carrier mare to a clear stallion (see below, "I have a carrier mare. Is it safe to breed her"), individual breeders may wish to remove WFFS from the future generations they produce. To do that they would remove their carrier mares from their breeding program. If they then breed their clear mares only to clear stallions, their future generations will all be clear of the defect.

How do I find out if my mare is a carrier?

Testing is available, and some warmblood registries in the US have arrangements with testing labs for members to have tests done at reduced rates. Make sure to use a licensed and qualified lab for testing. Click here to view a page of WFFS resources, with testing labs listed near the end of the page.

I have a carrier mare. Is it safe to breed her?

Yes, if you choose a stallion that is clear of the defect (N/N). "Safe" means that If you breed a carrier to a clear horse, you have zero chance of having an affected foal.

I want to breed my carrier mare but only to a clear stallion. How do I find out if a stallion is a carrier?

More and more information about stallion WFFS status will be published as more people become aware of its importance in breeding decisions - and as more mare owners ask for the information. According to Dr. Winand, one of the world experts on WFFS, "Ask the stallion owner or the studbook the status of the stallion you are considering for your breeding program. If a stallion owner is not transparent about the carrier status of a stallion that should be a red flag indicating you may want to do business elsewhere."

Is it possible to know if a stallion is a carrier if he's deceased?

Maybe. If the stallion has hair on file at a lab, that hair can be tested. If semen exists, it can be tested. If both parents are known to be clear, the stallion would be clear. If the stallion has a carrier foal out of a proven clear mare, then he is a carrier.

I have a carrier stallion. Is it safe to use him?

Yes, if you breed him to mares that are clear, your stallion will never sire an affected foal. Some owners of carrier stallions are requesting proof of clear status from mare owners before they will sign a breeding contract.

Are there legal implications to be aware of with WFFS?

Legal advice is beyond the scope of this FAQ. However, there are a couple of points that could be considered.

Hiding a stallion's carrier status. There was a lawsuit filed against the owner (and others including the repro center) of a stallion who was a carrier for HERDA, similarly a genetic defect, claiming that the owner published a clear status for his stallion knowing he was a carrier. The settlement was for $60,000, according to published reports.

European Union regulations. EU regulations prohibit breeding a carrier to a carrier, if the genetic defect would cause the affected animal to suffer.

My mare is clear (N/N) for WFFS. How can her foal be a carrier?

Her foal can be a carrier if the sire was a carrier. If you breed a clear horse to a carrier, you have a 50% chance of a clear foal, and a 50% chance of a carrier foal. To guarantee a clear foal, both parents must be clear.

If my horse is clear, does that mean the parents are clear too?

No, not necessarily. If you breed a clear horse to a carrier, you will get a clear horse 50% of the time. So a clear horse could have two clear parents, one carrier parent, or even two carrier parents.

Why are so few foals born with WFFS?

Considering the percentage of breeding horses who are carriers of WFFS, we would expect one in every 400 warmblood foals to be born with WFFS (in a population that is not managed for WFFS). The frequency has not been that high. The most likely reason is that foals with WFFS do not reach full term, but are reabsorbed or aborted. 

There is also speculation that if you try to cross a carrier mare with a carrier stallion, the mare will often not get in foal. [See article by Danish breeder Jytte Kolster on eurodressage.] 


The future of WFFS is in the hands of mare owners, because they're the ones who will be making the breeding decisions. These FAQs are to help breeders learn as much as you can so you can make responsible breeding decisions that are right for your program.