Genetic evaluation of epilepsy in the Welsh Springer Spaniel
The Animal Health Trust (AHT) proposes to work with the Welsh Springer Spaniel Joint Health Group to gather further information about epilepsy in the breed, with a view to producing estimated breeding values (EBVs) for the disease. Health Group representatives have, so far, collated information on about 70 Welsh Springers known to have had or be related to individuals that have had epileptic seizures. In order to establish meaningful EBVs, we would like to extend the data collection to a wider sample of the population. It is estimated that there are up to 5,000 Welsh Springer Spaniels currently living in the UK and we would like to collect information on as many of these dogs as possible. A reasonable target may be to obtain information on at least 1,000 dogs. Information on both affected and unaffected dogs is required. The AHT will design a one-page short questionnaire asking for the specific information needed. One questionnaire per individual dog is required and the information requested will include owner and vet contact details, KC name and registered number of dog (or pedigree details if the dog is not registered with the KC), whether or not the dog has been diagnosed with epilepsy, and other information about possible signs of epilepsy. The questionnaire will be distributed to breeders, who in turn will be asked to distribute it among their puppy owners. Questionnaires will be returned directly to the AHT, where the information will be collated. All submitted information on individual dogs will remain entirely confidential. The AHT will work on the development of EBVs for epilepsy in the breed and, once the initial analysis is complete, will liaise with the Health Group and Breed Clubs as to how EBVs can be made available.
What are Estimated Breeding Values (EBVs)?
EBVs are an efficient way of assessing underlying genetic risk for complex diseases, which are diseases that are caused by the effects of multiple genes and environmental factors. This is in contrast to single gene conditions where the sole cause of the disease is genetic, not environmental, and is the result of a mutation in only one gene. EBVs provide an objective numerical assessment of the genetic status of an individual dog, with environmental effects removed, which allows the relative risk of different dogs to be compared on an equal basis.
The diagram below illustrates the concept. There is variation in the genetic risk of disease among five dogs due to the particular combination of genes that each inherits (genetic risk at top of diagram). Differences in the environmental effects experienced by each animal then overlay the underlying genetic risk to give the observed (phenotypic) level of disease in each of the five dogs (observed disease severity at bottom of diagram). As a result, the rank according to observed severity of disease is not the same as the rank of genetic risk. In terms of breeding decisions it is the level of genetic risk that is the most important criteria because only genes, not environmental effects, are inherited. Therefore, although ‘Snoopy’ has the lowest severity of observed disease he has a moderate underlying genetic risk. ‘Gnasher’ would be a better breeding choice, as he would produce progeny that have a lower genetic risk of disease
The calculation of EBVs requires that pedigree information and population-wide data on disease is available. Disease data must be collected on a reasonable proportion of the population, including both affected and unaffected dogs, before EBVs become effective. In addition, individual dogs must be identified and the pedigree relationships among individuals must be known. The existing BVA/KC health screening schemes (hip and elbow dysplasia, eye disease) provide ideal starting points from which to develop EBVs. It is also possible for Breed Clubs to collate breed-wide health data and for this to be used as the basis of an EBV scheme. EBVs for syringomyelia and mitral valve disease in the Cavalier King Charles Spaniel have been developed in this way.
EBVs can be obtained for all dogs in a breed – in the first instance this will be all KC registered dogs of that breed. They can be calculated for most dogs even if they have not been health screened themselves, as long as they are related to enough dogs that have been screened. The EBV of puppies at birth will be half the EBV of their sire plus half the EBV of their dam. A puppy’s EBV may be modified in the light of its own subsequent clinical record or screening results, by information coming from other relatives and eventually by information from its progeny. As information accumulates the accuracy of the dog’s EBV will increase.
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