by Keith D. Stroyan
Around 1989 I found then-current vet research on a problem that was fairly wide-spread in field bred Labrador retrievers and wrote the article below that appeared in several retriever field trial and hunt test magazines. It seems that the puppy eye tests recommended by that research helped reduce the problem in the interim, but there are two developments since 2005. First, some research indicates that retinal folds in some parts of the eye are not associated with OculoSkeletal Dysplasia (OSD) - the official name for this kind of "dwarfism." I don't know if that work has been published. Second, the company Optigen has developed a DNA test for OSD. See the website:
http://www.optigen.com/opt9_rdosd.html
This should be a more accurate way to help make breeding decisions. I am delighted that modern science has helped with this test and DNA tests for EIC
http://www.vdl.umn.edu/ourservices/canineneuromuscular/taylor2008/home.html
and CNM
in addition to OSD.
For many years there has been a known genetic link between eye defects known as "Focal Retinal Dysplasia" and skeletal abnormalities known as dwarfism. The particular combination of symptoms is mainly found in field trial strains of Labrador retrievers. As recently as a special seminar at the Canine Eye Registry Foundation published in 1987, the best scientists could say was, "Unfortunately, it is very difficult to put out sound advice, in terms of breeding..." This has changed in an important way.
A study published by Carrig, Sponenberg, Schmidt and Tvedten in the November 1988 volume of the veterinary journal JAVMA has shown that "carriers" of the dwarfism gene can often be detected by expert eye exams. Your local veterinarian will not be able to pick this up, but the specialists who do CERF exams (widely known for PRA check-ups) can detect "folds" in the retina of a carrier dog's eye using special tools. The new findings show that an otherwise normal-looking dog that carries the dwarfism gene usually has retinal folds that are visible with these instruments. (The board that certifies ophthalmologists, ACVO , American College of Veterinary Ophthalmologists, with member addresses around the country can be obtained from the Canine Eye Registry Foundation, South Campus Courts, Building C, Purdue University, West Lafayette, IN 47907) If your pup has a normal skeleton and does not have these folds on clinical exam by a veterinary ophthalmologist, the chances are very good that he or she does not carry the dwarfism gene. It's not perfect, but in the carefully controlled scientific study only one dog out of twenty four that was a known carrier did not have clinically detectable folds.
The test can have both "false positives" and "false negatives." In general clinical observation, folds could be missed, especially if they are near a retinal blood vessel. Moreover, folds tend to fade in older dogs, so might be detected in a young dog, but missed in an older dog. Finally, folds can be present in a dog's eye for other reasons, so presence of folds is not conclusive proof that the dog carries this gene. Neither is freedom from clinically observable folds absolutely conclusive proof that the dog is free of the gene. Conclusive proof can be obtained by test breeding to a homozygous affected dog, but this involves keeping a blind dwarf dog to breed to and then "sacrificing" the puppies in order to dissect and microscopically examine the eyes. By contrast, the eye exam is a simple procedure that takes only minutes (but must be done by a specialist.) The odds of 24 to 1 are pretty good under those circumstances! Perhaps in time veterinary ophthalmologists will be able to come close to those odds in general clinical observation.
At present it appears that the most reliable eye exams for drd can be obtained on pups between the ages of 6 weeks and 6 months, with 14 weeks optimal. This will pick up cases of retinal folds that fade later in the dog's life. I do not have statistics on how many dogs will show folds as puppies and later appear clear, but I suspect that this is common. I know rumors that this has happened with some famous dogs and have seen slides of a pup with clear folds that faded to being nearly un-detectable a year later. Eye exams later in the dog's life are not as accurate for drd, but are important for the detection of other late-onset eye disorders such as PRA and cataracts. So check 'em young, check 'em middle and check 'em at 4 or 5. I test all my puppies when I get them at 7 weeks or before they leave my kennel. Then I'll check them again at 2 years when they are getting hip x-rays for OFA. Once they have one-year CERF numbers, I'll keep them current until I feel they are no longer breeding prospects.
The genetics of the drd (dwarf with retinal dysplasia)-gene are now known. Lab owners can understand it easily by analogy to the color gene. A black Lab can carry the yellow gene, but will still be black. Such a dog is B-y, heterozygous, on the color gene, meaning it has both genes, black and yellow (one from each parent). Black is dominant, so the dog is black, but when bred to a yellow Lab, the dog will produce about half yellow pups. The black heterozygous dog is a "carrier" of yellow. To be yellow a Lab must be homozygous, y-y, on the color gene. A pup can only be homozygous (y-y) if it inherits the trait from both parents. In the case of yellow, both parents can be black "carriers" of yellow (B-y), one could be a carrier (B-y) and the other "affected" (y-y), but a "free" parent (B-B) cannot produce "affected" offspring, even when bred to an "affected" individual (y-y), since one gene comes from each parent. If you suspect that your dog has produced a dwarf, you might want to get a specialist opinion, exams from both an ophthalmogist and an orthopedist to confirm drd. If a pup is a true dwarf, both parents are carriers. (Retinal folds in your pups indicate drd, but are not conclusive proof.)
The new findings of Carrig, et al, show that the dwarfism gene is dominant for the retinal folds that an ophthalmologist can detect. A heterozygous dog may be functionally fine and several field champions are reported to be carriers. The heterozygous gene has an "incompletely dominant effect," meaning that the extent of visual impairment varies from symptoms an owner might notice down to clinically undetectable folds. Special exams can usually detect the folds in pups. (In one case of twenty four in the study, a known carrier had clinically undetectable folds that could be detected when the dog's eye was dissected and examined under a microscope.) Top stud dogs or brood bitches that will be bred often could still be test bred for dwarfism to obtain conclusive proof. Also, a dog that has retinal folds, which the owner suspects may come from a cause other than drd, can be test bred to rule drd out as the cause.
A homozygous drd-gene dog usually has severely affected eyes with the retina even detached and floating. They may have cataracts, persistent hyloid remnants and severe enough additional defects that they are mis-diagnosed in the sense that their eye defects are not attributed to the drd gene. Homozygous dogs are all reported to have abnormal skeletons - dwarfism in the sense of front shoulders lower than rear - a sort of 'bulldog' appearance. In other words, drd is recessive for dwarfism. (One dwarfed dog, out of 59, had clinically normal eyes, but all homozygotes were dwarfed and no heterozygotes were.) The new findings also show that this gene is not sex linked. For years breeders have treated this condition as a recessive - and it is in the sense of its severe effects. The dominant expression of small retinal folds, often unnoticed by the dog's owner, is the new finding. Such carrier dogs may lead a successful life of retrieving, but should not be bred. Now we can find important clues before we breed. The test isn't perfect, but the good news is that eyes clear of retinal folds means that the puppy has good odds of being free of drd while retinal folds means we should suspect drd.
Some questions remain besides the practical matters of bringing the test into clinical practice. For example, there is a suspicion that carrier dogs tend to be abnormally large. This has not been proved and may never be, since probably many very large dogs are not carriers. Breeders report to me that when they have had litters with dwarfism and severe eye defects, the whole litter is affected and with many complications - stillbirths, highly varying size of the pups, difficult delivery... The study cited above suggests that some pups in these litters, the heterozygous, should have normal skeletons and only the mild eye defects of retinal folds. I would very much appreciate learning additional information of this kind from breeders. This information might not have been obtained under scientifically controlled conditions, but could help shed further light on an important genetic defect in our friend the Labrador retriever.