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Thread: Effectiveness of breeding guidelines for reducing the prevalence of syringomyelia

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    Default Effectiveness of breeding guidelines for reducing the prevalence of syringomyelia

    This is from Clare Rusbridge's blog and has an interesting diagram which illustrates why EBVs will give even those breeders that " know what is behind their dogs" better information on which to base their breeding decisions

    http://clarerusbridge-news.blogspot.com:80/


    Effectiveness of breeding guidelines for reducing the prevalence of syringomyelia


    S. P. Knowler, A. K. McFadyen, C. Rusbridge
    Veterinary Record 2011;vetrec-2011-100062

    Summary
    Several toy breed dogs are predisposed to syringomyelia (SM), a spinal cord disorder, characterised by fluid-filled cavitation. SM is a complex trait with a moderately high heritability. Selective breeding against SM is confounded by its complex inheritance, its late onset nature and high prevalence in some breeds. This study investigated the early outcome of existing SM breeding guidelines. Six hundred and forty-three dogs, 550 Cavalier King Charles spaniels (CKCS) and 93 Griffon Bruxellois (GB), were identified as having either one (454 dogs) or both parents (189 dogs) with MRI-determined SM status. Offspring without SM were more common when the parents were both clear of SM (SM-free; CKCS 70 per cent, GB 73 per cent). Conversely, offspring with SM were more likely when both parents had SM (SM-affected; CKCS 92 per cent, GB 100 per cent). A mating of one SM-free parent with an SM-affected parent was risky for SM affectedness with 77 per cent of CKCS and 46 per cent of GB offspring being SM-affected. It is recommended that all breeding dogs from breeds susceptible to SM be MRI screened; that the SM status at five years old is established; and all results submitted to a central database that can be used by dog breeders to better enable mate selection based on estimated breeding values.
    see
    http://veterinaryrecord.bmj.com/cont...vr.100062.full
    For example

    This family tree typifies the complex relationship between SM-affected asymptomatic dogs and SM-free dogs and demonstrates how hard it is for breeders to make sensible breeding decisions even when they are knowledgeable about their dogs’ background.
    The figure illustrates a three generation CKCS family tree based on the SM grades used in the 2006 International Breeding guidelines (http://www.veterinary-neurologist.co.uk - see under SM screening) *A denotes a dog that has been screened over the age of 5 years and is clear of SM. Square = male, circle= female.


    The family group is centred on a grade A male (red) without a central canal dilatation (CCD) who was 3.8 years when he had the MRI. He has an *A grade dam and a sire of unknown status. He was mated to two unrelated grade D females. Three of the offspring from one parental cross had early onset SM (E grade) however the two offspring from the other parental cross were both A grade. It is perhaps significant that for the E grade litter both maternal grandparents were D whereas the maternal grandmother of the A grade litter was *A. Not all breeders may be afford to MRI more than a few dogs and may not be provided with the MRI status any relatives of a potential mate. The EBVs for SM can be combined with those for MVD and other genetic diseases thereby providing more accurate risk information at birth and eliminating environmental influences they provide the best way to help the cavalier breed.
    Margaret C

    Cavaliers......Faith, The Ginger Tank and Woody.
    Japanese Chins.... Dandy, Benny, Bridgette and Hana.
    Remembered with love......... Tommy Tuppence and Fonzi

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    Really overwhelming evidence that using the breeding protocol for SM and scanning dogs DOES make a huge difference inw whether offspring get SM (or earlier onset SM).

    To summarise:
    • 30% of offspring HAVE SM at time of MRI when two CLEAR (A) parents are bred (eg 70% offspring are CLEAR)
    • 77% of offspring HAVE SM when ONE parent is CLEAR and ONE has SM (eg 23% are CLEAR)
    • 92% of offspring HAVE SM when BOTH parents have SM (eg two grade D dogs) (eg 8% are clear)

    Knowing this is the case, and from such a massive breed sample of both cavaliers and Brussels griffons, is very good news for breeders as it gives the a significant tool for working to reduce incidence -- but emphasises the need for all breeders and clubs to work to create worldwide low-cost scanning programmes and perhaps focus fundraising efforts in this area.

    Also I'd think other toy breeds might want to start test-scanning programmes to tackle this horrible condition right away.

    Anyone buying a puppy obviously stands a massively higher chance of having an SM-free or late and milder onset dog if they use a scanning breeder following the protocol.

    If both parents have SM -- as is a significant likelihood if neither dog has been scanned, and no older relatives either -- a puppy buyer has a great than 9 in 10 chance of having a cavalier that will grow up to have SM.


    Karlin
    Cavaliers: Jaspar Leo Lily Tansy Libby (foster) Mindy (foster)
    In memory: Lucy
    Cavalier SM Infosite:www.smcavaliers.com

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    Another really exciting element in this research is that it strongly suggests breeders can also significantly increase the chance of clear offspring by scanning their age 5+ dogs! eg if they have grandparents that are A* (eg clear at age 5+) this can help increase A's in an AxA litter and help improve chances of As when a D dog is used (for genetic diversity/good heart genes etc).

    So there are two directions from which breeders can go at this serious breed problem.

    This really is some of the most exciting, significant and directly helpful to breeders research to be produced on SM in the past decade.
    Karlin
    Cavaliers: Jaspar Leo Lily Tansy Libby (foster) Mindy (foster)
    In memory: Lucy
    Cavalier SM Infosite:www.smcavaliers.com

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    This is the table showing the results of the different matings.



    Shown as % with or without SM



    What the grades mean.


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    Thanks for those. I understand the reason the Unknown x A matings have a better result than DxA is that most breeders using 'unknowns' within this group of scanned dogs would most likely have chosen dogs from lines where a parent had already been scanned and was clear, or a sibling was clear -- eg there would be some information on the SM risk of the choice of dog perhaps being slightly better than an unscanned line. So it would have been different than breeding dogs with no scan and no related dogs scanned -- eg a more true 'unknown'.

    Nonetheless the risk of having SM affected offspring is still about double what it would be, compared to using two scanned, A grade dogs.
    Karlin
    Cavaliers: Jaspar Leo Lily Tansy Libby (foster) Mindy (foster)
    In memory: Lucy
    Cavalier SM Infosite:www.smcavaliers.com

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    I don't think that is necessarily true. You would expect U x A matings to do better than D x A simply because some of those U dogs are actually A dogs.

    The study did mention that
    Not all unscanned dogs are equal and experienced breeders may select a dog on the basis of it having relatives that are SM-free or select a stud dog that is known to have produced a high proportion of SM-free offspring: one unscanned champion stud dog was mated with 16 bitches resulting in 11 C grades, 10 A grades and only three SM-affected offspring.
    Still
    Results from this study suggested that using an SM-affected dog (grade D) with a dog of unknown status resulted in higher numbers of SM-affected dogs in the older population (75 per cent SM-affected CKCS and 59 per cent affected GB). Using an SM-free (*A or A) CKCS with an unknown dog still resulted in 43 per cent of older offspring being SM-affected (50 per cent GB) and therefore, it is recommended that all breeding dogs from breeds susceptible to SM be MRI screened.

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    Yes that's true– But with a big “but”.

    While one perhaps wouldn't expect the percentage results for Ax Unknown necessarily to be worse than the results of using D dogs, in a random population there should also have been, along with some hidden As, a high degree of dogs that would've had failure grades for breeding that should have skewed the results for Unknown dogs to a much higher percentage of affected offspring. after all, in a natural population, all indications are that a very high percentage are affected with SM.

    The fact that the dogs used in this study group came from breeders who were already choosing to scan–not a randomly selected population–would mean there would almost certainly be far better results for Unknown dogs, because breeders in a scanning group would certainly be more likely to already understand many of the issues (and dangers) of using unscanned dogs, and therefore would be deliberately choosing dogs that had other scanned, good grade dogs in its immediate circle of relatives.

    Aas you rightly note, 43% remains a very worryingly high number of affected offspring, when scanning and following the SM protocol and breeding with greater awareness of the health status of the breeding dogs (and ideally, the breeding dogs' parents) could significantly improve results.

    I think this is such exciting research –it really holds out a positive prospect for breeders and is based on such a huge breeding sample that it gives great comfort regarding significance and accuracy and implication. It's no longer a matter of just looking at the dogs from a few scanning breeders, but analyzing the results of hundreds of dogs across a large number of breeders in two breeds.
    Karlin
    Cavaliers: Jaspar Leo Lily Tansy Libby (foster) Mindy (foster)
    In memory: Lucy
    Cavalier SM Infosite:www.smcavaliers.com

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