Hoof Wall Separation, (HWSD)

[connemaranz]

HWSD is a genetic hoof condition found in Connemara ponies. It is a homozygous recessive condition; this means the mutated gene has to be inherited from both parents for the disease to be present. There is now a genetic test to determine whether ponies are ‘carriers’ of the mutated gene. By avoiding the mating of two carriers, foals will not be born with the condition.

For information on HWSD http://connemara-pony.blogspot.com

On how to test your ponies http://connemara-pony.blogspot.com/p...d-testing.html

For a list of test results http://connemara-pony.blogspot.com/p...ed-ponies.html

If you have a Connemara pony (or a partbred Connemara) which shows with hoof wall issues that do not respond to dietary supplements or hoof treatments for White Line Disease/fungal conditions, then HWSD may just be the problem. By testing such ponies you will get a definitve answer.

It would be a really positive step if breeders were to have their breeding stock tested before the start of the up-coming breeding season.

This condition has been around for many years, but unacknowledged by people ‘in authority’ who wanted to keep it ‘secret squirrel’, as they felt that it would reflect badly upon the breed. All breeds have genetic issues of one kind or another. At least there is now the test for HWSD the results of which can stop the production of affected ponies while permitting the continued use of carrier ponies for breeding.
It is very important that carrier ponies are given the opportunity to breed on thereby preventing the further loss of genetic diversity. It is because of this loss of genetic diversity that the problem has arisen in the first place.

[pcwertb]

It is my understanding that partbreds can’t have actual HWSD, as the condition is homozygous recessive and this specific condition is limited to the connemara breed. A partbred could be a carrier, but not display the traits of the disease. Same with ponies who are carriers, they show as N/HWSD. A pony without the gene (free from disease) will show as N/N and an affected pony is HWSD/HWSD. I have tested my one purebred pony and she is N/N. I have one left to test, who is currently open, UC Davis does the testing.

[DawnJL]

Partbreds with Connemara ancestry on BOTH sire and dam side can be affected with HWSD if they inherited the recessive allele from both parents.

[pcwertb]

Correct, but not sure how many second generation partbreds are out there. Most commonly, one sees an F1 cross of TB/Connemara or WB/Connemara. Such a cross can only carry one recessive gene.

More information can be found on the facebook page called HWSS (Hoof Wall Separation Syndrome), even though it’s now been renamed HWSD (disease) but breeders are posting their results publicly on this page. Breeders from the US, UK, Europe…

Responsible breeders are getting the word out.

[DawnJL]

Just trying to clarify the information. We have the same scenario with part-bred Arabs and the deleterious recessive disorders. Most of those are also F1 crosses. The problem arises when folks who don’t understand the inheritance pattern, assume that only purebreds are ever affected. While the statistical chance may be small, if there are part-bred Connemaras out there in sport pony registries or otherwise with purebred Connemaras on both sire and dam side, it’s certainly possible.

The most important thing now is to test breeding stock and breed forward responsibly. The prospect of using the test to avoid ever producing another pony affected by this devastating condition is just awesome.

[MuskokaLakesConnemaras]

Just wanted to once again say a HUGE thank you to those folks that made the research and subsequent test available. To be able to 100% avoid this makes breeding that much less risky.
So thank you VERY much.

[stoicfish]

It is very important that carrier ponies are given the opportunity to breed on thereby preventing the further loss of genetic diversity. It is because of this loss of genetic diversity that the problem has arisen in the first place.

I think posting this for informational purposes is great.

However, the above statement is problematic. Or at least not complete.
Even if people test and then breed so they do not create a homozygous foal, it will eventually greatly reduce genetic diversity in the future. There will be lots of pairings that will not be possible. And if well used stallions carry it, it will just increase the percentage of the population that are carriers.
I would say if your going to be responsible about eliminating the disease, that you test, breed but in one or two generations, cull out the carriers from breeding. Give people like 10 years of mandatory testing, and a deadline and the disease will be eliminated.
It will healthier, genetically for the population if there are no breeding restrictions in the long run.

• saw my typo above. Will leave it but meant isolate instead of cull.

[connemaranz]

To eliminate any genetic disease from a population will require 7-10 Generations, not years. For anyone who is sufficiently interested in breeding strategies then getting and reading a copy of ‘Managing Breeds for a Secure Future’ by Sponenberg and Bixby is recommended reading.
One also has to be aware that what the situation is, in one country or geographical area with regards to HWSD incidence, does not translate in the same manner to other jurisdictions. This is expecially so with regards to loss of genetic diversity and also the involvement of part breds in the susceptibility to HWSD matrix. Cross breeding Connemara ponies for performance purposes is extensive in France, the UK and Ireland. In fact the part-bred industry in these countries is actually numerically greater than the pure bred industry. There has already been VGL tested HWSD/HWSD part breds confirmed in UK and Ireland.

[stoicfish]

Short of buying the book, would you please give me a synopsis of why it would take 7-10 generations?

I think this is a great topic. And definitely this is not the only congenital disease in horse breeds.

[DawnJL]

Population genetics is the field of study that looks at the frequency of particular genes in breeding populations and at the overall genetic diversity of those populations. In order to maximize genetic diversity, breeding strategies need to consider the distribution of all the genetic variations that exist in the population. Culling all carriers of a single allele from a population within a generation or less, doesn’t just affect the frequency of that particular allele by taking it to zero; it also results in shifting the distribution of many other alleles to make them either more or less commonly distributed than before. That kind of abrupt shift can result in losing (or at least making very rare) some alleles while making other more common. In order to strike a balance between diminishing the undesired allele while preserving desired genetic diversity; the selection against that single gene needs to be done over multiple generations.

In terms of population genetics, there isn’t much difference between ceasing use of all carriers immediately and doing so over 10 years. Here’s an example of what happened when Basenji breeders banded together to make what they thought was the most responsible choice regarding a recessive allele for which a test had been developed: “Bad Genes”

For those who want a more in depth look here’s a scientific article on the topic: Changes in disease gene frequency over time with differential genotypic fitness and various control strategies

[JB]

Dawn, does it change things when the population is very large, vs relatively small I’m thinking of HYPP and the QH and Paint. QH is a very, very numerous breed, and I would think that eliminating all HYPP in a single generation might not really have a big effect, but maybe I’m wrong, so I’d love your thought on that

[DawnJL]

The population size, overall genetic diversity of that population, and the starting frequency all have an influence. HYPP is also different because heterozygotes are negatively impacted by the presence of the allele not just homozygotes.

[connemaranz]

Short answer is that if all carriers are removed from a breeding population such as the Connemara pony which on population statistic perspective a rare breed then this brings the breed closer to extinction by loss of breeding animals and genetic variability.
“Carrier stallions can (and should) be mated to non-carrier mares to produce replacements, with the eventual goal of generating sons that are not carriers so that the bloodlines are not lost but the offending single gene is. This can take several generations to accomplish, but breeders should all be encouraged to move toward that goal while preserving the genetic integirty of the breed by not discarding carriers too hastily”. Sponenberg and Bixby 2007.
The 7-10 generation estimation comes from the geneticists at Davis based on the prevalence stats coming through from the ponies that have been tested to date from around the world.

[stoicfish]

In terms of population genetics, there isn’t much difference between ceasing use of all carriers immediately and doing so over 10 years.

I said mandatory testing in the next ten years, then set a deadline. I also mispoke when I said cull. I agree that you cannot just cull all the affected horses in one or 2 generations.
But I am not sure why you couldn’t succeed (depending on the initial %) within 3 or 4 generations.
I get that you want to preserve the rest of the genetics (assuming the horse in question has genetics that is desirable and wouldn’t be culled anyway).
But 10 gens is at minimum of 40 years, what incentive is that to select or have a goal of n/n horses for any one breeder?

So according to the data provided it is around 15-20% of horses that are carriers.

from 1999 with the breeding stallion number standing at 1,043 and the breeding mare numbers at 11,621.
: http://www.cpbs.ie/history/

That is 2300 animals that need to make genetic contributions. If you take an average of 2 or 3 foals per mare and cut that in half for male/female. Then cut that in half again as that would be the number of foals that should be carriers, about 1700. But also 1700 mares that carry those original mares genetics that are not affected. This is without using a carrier stallion.
A carrier stallion can profoundly affect the breed, especially with frozen and a small population.
Look at the Holsteiners and try and find one without Corde in the last 5 gens. If he was a carrier, you wouldn’t have many horses left that were not at least hetro for the issue.

So how does the issue get resolved without setting a deadline that would affect breeders in their lifetime?

[connemaranz]

Because the different populations of Connemara ponies in widely scattered parts of the globe are by definition discreet populations. There is absolutely no comparison with the annual production of Ireland (approx 3500 ponies/year) and the maybe 20 (if you are lucky) in the likes of New Zealand and Finland. However what does have to be remembered is that with these 3500 ponies/year the majority already have over respresented sire lines in their back breeding and thus constitute a genetic bottleneck in their own right.
What might be considered a viable option in one population would cause desparate loss of genetic diversity in another. There is absolutely no comparison with the extensive QH production (and HYPP and HERDA situation) in the US and a small population such as the Connemara pony worldwide. Dawn J-L is spot on with her understanding and analysis of the situation. The principles she talks about have been proven - no supposition - but proven, especially where populations have been brought back from near extinction by conservation breeding programmes. That is a fundamental difference between ‘breeders’ of a specific type of domesticated animal and conservation scientitsts; these do indeed work on 40 year timelines.
It is a great shame that the majority of ‘breeders’ do not look forward at least 5 generations when they make their breeding choices. Most only look at the foal due next year - not where that foal will fit into the scheme of things as a grandparent or great grandparent. If they did, and had done so in the past, then the situation we now find the breed in, with regards to HWSD (and also tracheal collapse) would probably never have arisen in the first place.

[stoicfish]

It is a great shame that the majority of ‘breeders’ do not look forward at least 5 generations when they make their breeding choices. Most only look at the foal due next year - not where that foal will fit into the scheme of things as a grandparent or great grandparent. If they did, and had done so in the past then the situation we now find the breed in, with regards to HWSD (and also tracheal collapse) would probably never have arisen in the first place

I very much agree with this.
I understand the science but wonder about the logistics. The science is based on modeling, but modeling with a selective process based on the science and working towards an end.
Individuals, like you stated will not make selections based on that process but their own ends. If there are no limitations in breeding, what makes the 40yr mark any different than the population genetics that are present now?
One popular carrier stallion with frozen could inoculate the populations in the different areas and greatly increase the overall percentages.

[DawnJL]

Except that carrier status does not have a neutral or positive value. Fewer breeders will choose to use a carrier for breeding than a comparable clear individual. So there is generally some negative selective pressure associated with carrier status. In animal breeds where we can analyze how testing for deleterious recessive alleles has impacted breeding choices, we see that breeders who use carriers are more likely to retain clear offspring for breeding stock and sell carrier offspring for performance (pets in the case of dogs). Only carrier individuals that are exceptional will have an opportunity to reproduce at close to the rate of a comparable clear individual; and their clear offspring will generally still have higher value as breeding stock than carrier offspring (particularly in the case of stallions)

[Janet]

Testing is already required in the US for Stallions.

According to the latest issue of “American Connemara”- “All stallions must be tested and their reports on file by Feb 1, 2015”.

There is a list of about 64 ponies that have been tested, including some well known stallions.

On the list there is
1 “affected”
11 “N/HWSD”
the rest are “N/N”

I have a halfbred mare (Conn xTB) by a known carrier. I intend to have her tested, but have not got around to it.

This stallion was a very successful Eventer, so has lots of good genes to add to the breed, as long as it can be done without propogating the HWSD.

[stoicfish]

[QUOTE=Dawn J-L;7996689]
Except that carrier status does not have a neutral or positive value. Fewer breeders will choose to use a carrier for breeding than a comparable clear individual. So there is generally some negative selective pressure associated with carrier status. In animal breeds where we can analyze how testing for deleterious recessive alleles has impacted breeding choices, we see that breeders who use carriers are more likely to retain clear offspring for breeding stock and sell carrier offspring for performance (pets in the case of dogs). Only carrier individuals that are exceptional will have an opportunity to reproduce at close to the rate of a comparable clear individual; and their clear offspring will generally still have higher value as breeding stock than carrier offspring (particularly in the case of stallions)[/QUOTE]

I hope your right.

[columbus]

In QH the issue of HERDA is closer to that of HWSD as the carrier state is not affected. I QH there is resistance to eliminating HERDA as they believe that genetics associated with performance success may be closely linked with the HERDA gene. It could very well be the case in Connemaras as well. The gene presented in a desirable successful bloodline that people had a demand for because of the success of the ponies. In QHs is is in the very successful Poco Bueno bloodline which is directly linked to success in working cattle and high performance lines. There is no denying that the gene became more common because of the success of the horses who possess the defective gene. Are the genes of success closely tied to the genes of the defect? There are certainly lines of Connemaras who have an extraordinary talent for jumping. The carrier state is managable. Can the success continue when the gene is eliminated. PatO