Recessive genes in popular dogs/lines can become embedded in a breed very quickly. In Collies, the recessive gene for Collie Eye Anomaly is now so established in the breed in the US that most examining opthalmologists agree the number of AFFECTED (carrying two genes!) animals is around 85%. Of the 15% that are "normal" (a dominant gene), many of these are carriers.
However, since CEA does not cause obvious vision impairment in most individuals, the collie breeders tend to believe it is o.k. to produce these affected animals as long as the puppies are eye tested and the worst-case scenario (the small percent with probable vision impairment) are not sold.
CEA is inherited as a simple autosomal recessive with variable expression/phenotypes (thus "good eyechecks," "bad eyechecks" in a litter of 100% affected puppies) A number of us who have worked with this for years believe there are also modifiers inherited that affect the expression of the anomaly causing more/less expression or degree of the problem.
There is also a tendency in some lines for the puppies to appear to "go normal", usually around 8-12 weeks, but this is a false normal and genetically still 100% affected. CEA in its mildest expression shows localized lack of retinal and choroidal pigmentation in an area of the fundus near the optic disc. In "go normals," as the puppy grows, pigment invades the nonpigmented area, MASKING the presence of mild CEA. The puppy will appear at the time of examination as "normal" but when bred, will produce as affected, thus leading many breeders to feel there is no rhyme or reason to the testing and to become discouraged. Breeders trying to eliminate this "go normal" tendency within their lines have their puppies' eyes examined at 4 and 5 weeks of age to determine as accurately as possible the genetic normals. Test breeding further provides assurance as these dogs reproduce the 50% of normal offspring to affected mates.
Because most affected collies can perform all normal duties/functions and are acceptable to the buying public, there has been no real push to remove this syndrome from the breed. However, PRA which causes blindness has been under a strong push to identify and remove carrier individuals through test breeding to affected PRA mates. All offspring must be neutered as all will be carriers. There is at present a study being funded to produce a DNA test for carriers and this should produce results shortly.
Often, breeders and veterinarians mistakenly believe this is a problem only in the United States. However, many of the collies in the British Isles and elsewhere are examined well past the age to catch the "go normals" and these dogs are certified falsely as "normal" while genetically affected. My own experience has indicated you can increase this tendency accidentally within the breeding pool by selecting toward "go normal" animals believing them to be true normals. Dr. Rubin in Inherited Eye Diseases in Purebred Dogs lists the number of CEA affected collies for the US (depending on region), 50% to 90%, the latter being more usual; United Kingdom, 64%; Germany, 37%; Netherlands, 41%; and Switzerland, 31%. In breedings I have personally made with imported dogs, my own and others, all with normal certifications, I have found only one to be a true normal and he was a carrier. This was not true in the past when we could count on an imported dog to be free of the anomaly.
The above information is not meant to indicate the Collie breeders are uncaring about genetic problems; only that they consider CEA to be one of the least troubling and with the least consequences. Collie breeders are presently funding not only the search for a DNA marker for PRA, but also for DM (dermatomyositis), a muscle wasting disease that expresses itself also as a skin disorder, and a number of other genetic problems identified within the breed. However, the huge numbers of Collies affected and thus pure for a RECESSIVE disorder (CEA) is meant to show how quickly and silently some of these things can develop within a breed.
The author Allene McKewen and her husband operate Reignmaker Collies in Lakeland, Florida and raise Smooth Collies as guide dogs for the blind.
Copyright 1997. No reproduction without express permission of the author.