Health screening for inherited disease in the dog.

Increasingly, breeders are being encouraged to screen all potential breeding stock for inherited diseases that affect their breed before the dogs are used in their mating programmes. Consequently, the number of such screening programmes is increasing. The vast majority of them are based on a clinical examination by an experienced vet to check for clinical signs of the disease in an individual dog; in other words to check whether a dog is affected by a breed-specific inherited condition. This can, of course, give the breeder invaluable information that can be used when deciding whether or not to breed from a dog.

However, the majority of canine inherited disease results from what are called recessive mutations. This means that a dog needs to inherit two copies of the mutant gene, one from its dam and the other from its sire, to be clinically affected. There will be clinically normal dogs that carry one copy of a particular mutant gene and a normal second copy. Carriers are clinically normal, but they will pass their mutant gene onto approximately 50% of their offspring if bred from.

This complication needs to be considered by breeders who use clinical screening programmes on their breeding stock. Breeders cannot assume that if both parents are clinically clear, then all of the puppies will be clinically clear; both parents could be carrying recessive mutations that could ‘double up’ in the same puppy to cause it to be clinically affected. If a disease is the result of a single gene mutation, as many of the inherited diseases in the dog are, then if you mate two clinically normal carriers together, each puppy will have a 1 in 4 chance of being affected. The trouble is, clinical screening will not identify your carriers. Even in the more complex diseases that involve mutations in more than one different gene, the mutations are usually recessive, and so clinically clear dogs can still be carrying single mutations in one or more of the genes involved, and they will pass these on to some of their offspring. So, to emphasise again, breeders cannot assume that if both parents are clinically clear, then all of the puppies will be clinically clear.

Fortunately, we are now developing a new generation of DNA-based tests for some of the inherited diseases in the dog. These are far more definitive and give the breeder more precise information. A DNA test for an inherited disease will tell you whether your dog is normal, a carrier or affected; DNA tests can discriminate between carriers and affecteds. At the moment, the DNA tests that are being developed relate to single gene disorders, but we will soon be faced with DNA tests for the multiple genes involved in complex, polygenic diseases.

So, to summarise, clinical screening programmes remain an extremely valuable tool for the dog breeder, particularly since we are a long way away from having DNA tests for all of the known canine inherited conditions, and breeders should be strongly encouraged to use them where available on their breeding stock. However, as mentioned above, breeders should not assume that if both parents are clear in such clinical screening, then all of their puppies will be clinically clear.