Genetic cause confirmed for Connemara hoof syndrome

Distal hoof wall lesions, characteristic of HWWS.
Distal hoof wall lesions, characteristic of HWWS.Affected hoof cross section. Affected hoof cross section.

American researchers are significantly closer to identifying the genes responsible for Hoof Wall Separation Syndrome (HWSS) in Connemara ponies.

Recent findings are an important step toward ultimately developing a genetic test capable of identifying carriers, and potentially eliminating the debilitating condition from the breed in the long term.

Researchers at the University of California, Davis, performed a genome-wide association study to find out more about the condition, in which the dorsal hoof wall splits from underlying structures.

Their research proves there is a genetic basis to the condition – a view that has been questioned until now by some in the Connemara Pony breeding world.

Dr Carrie Finno, Carly Stevens, and Dr Danika Bannasch found a strong association between the disease status and polymorphisms – a genetic variant that appears in at least 1 per cent of a population – in two-megabase regions of the genome.

Sequencing of candidate genes within this region is under way to find those responsible for the syndrome.

The syndrome is an inherited condition  and typified by the dorsal hoof wall splitting away from underlying structures.

The defect develops in foals between one and six months of age. It results in afflicted ponies having to support weight on the sole of the hoof instead of the dorsal hoof wall.

Affected animals can suffer severe pain despite careful management; their quality of life can diminish and euthanasia may be necessary. Even if the condition is initially controllable, ponies may still develop laminitis over time.

The syndrome is particularly troubling for the Connemara community because the parents of affected ponies are themselves completely unaffected.

Although affected individuals do share common bloodlines, it is problematic to predict whether a particular breeding will produce a foal with unhealthy hooves.

Investigation into the underlying genetic cause of syndrome has the potential to allow breeders to make more informed decisions, and could also provide insight into the nature of the disease.

Seven month Connemara filly with HWSS showing typical lesions. This pony is walking on the sole ,not the wall, as is normal.
Seven month Connemara filly with HWSS showing typical lesions. This pony is walking on the sole, not the wall, as is normal.

Because sequencing of candidate genes within the identified region is under way, the researchers are still collecting DNA samples from both affected and unaffected Connemara ponies.

The Connemara Pony Research Group, formed to initiate research into the hoof problem and educate owners and breeders about the condition, welcomed developments.

In a blog post this week, the group said it had been aware for some time of the progress being made in the genetic research, undertaken at the specific request of the group.

“The very existence of HWSS and that it is of genetic orign of it has been questioned by many people throughout the Connemara Pony breeding world,” it said.

“Those who have not seen or personally experienced the condition are reluctant to accept that such a serious problem could be lurking within their beloved breed,” the group said.

“Understandably, the recurring call has been the need for scientific proof that this is not simply an environmental or management issue, but a genetic one.”

Now there was proof it was of genetic origin, it said.

“While this may only be the first official statement regarding research into HWSS, we believe that now is the time for an open acknowledgement that there is a cause for concern, but that there is a way out of the situation.”

he peeling walls near the bearing border.  Note the abnormal periople and the lack of shine on the hoof wall indicating that hoof wall is permeable.
The peeling walls near the bearing border. Note the abnormal periople (area below the coronet) and the lack of shine on the hoof wall indicating that the hoof wall is permeable.

The research group, with members from Germany, Denmark, Sweden, Austria, Canada, the United States, Australia and New Zealand,  said it hoped that the syndrome would make it on to the agendas of upcoming meetings in Ireland of the International Committee of Connemara Pony Societies (ICCPS) and its technical group.

“With so many delegates from breed societies around the world present for these, the research group group sincerely hopes that the UC Davis release on HWSS is enough scientific proof to acknowledge the condition, and start a constructive discussion on the issue.”

The group said UC Davis needed the help of Connemara pony owners and breeders worldwide.

“They need more samples from ponies, and the sooner they get them, the sooner we will have a test for the condition. The ICCPS are in a unique position to facilitate this,” it said, adding that an education campaign to the daughter breed societies would rapidly add to the universty’s collection of samples.

“Additionally, breed societies could vastly aid breeders and owners by co-ordinating the collection and shipment of blood samples to UC Davis. What a huge pro-active and positive step for the breed that would be.”

The group said the Connemara pony community needed to start talking now about how to deal with the problem in the expectation a test for carriers would become commercially available.

“There will be a time lag between the research phase being completed and when testing becomes commercial available, but that should not stop us from calmly, and rationally, discussing how this issue will be managed to the betterment of the breed.

“We are not the only breed society to have faced such issues – both the Fell Pony breeders and the New Forest Pony Breeders have faced similar problems over the genetic issues in their respective breeds. And, like the conditions in the above breeds, we do not believe that HWSS will be limited to just the Connemara Pony – this research will likely benefit horses worldwide,” it said.

“We know the test is coming. Let us, as a cohesive community, be positive and ready. Let the Connemara pony community worldwide be ready and waiting to embrace the new technology as soon as it becomes available.”

The calloused sole typical of HWSS and the result of walking on the sole. If the callousing is left alone the pony will stay paddock sound but will usually not be capable of any work.
The calloused sole typical of HWSS and the result of walking on the sole. If the callousing is left alone the pony will stay paddock sound but will usually not be capable of any work.

Over the past 15 years there has been increasing awareness worldwide of the syndrome in a subset of Connemara ponies.

Robert Eustace, of the Laminitis Trust in Britain, originally described the condition as “coconut-matting hooves”, as the borders of the hoof wall appear rough and frayed.

Hoof samples of affected Connemara ponies who had been referred to the trust for treatment were analysed at the University of Edinburgh, where they found a malfunction of lipid metabolism in the extracellular matrix of the hoof wall between the tubular structures of the hoof wall.

In simple terms, there seems to be a lack of “waterproof glue” holding the hoof wall tubules together.

If the condition is caused by a simple recessive gene – the current hypothesis – this means it can occur only when two carrier ponies are bred to each other.

A genetic test for the genes responsible for the syndrome would, over time, allow the level of the syndrome within the population to be reduced or eliminated without compromising genetic diversity – already a concern with an already small gene pool.

New Zealand has had only one pony identified with this condition. She was imported in utero.

Funding for the research has come from the Morris Animal Foundation, UC Davis Center for Equine Health, and Merial.

Those interested in submitting a sample should contact Carly Stevens ( or Kartika Jayashankar ( for more information.

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