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Sitemapby Noelle La Croix, DVM, Dip. ACVO
Centuries of selective matings for various phenotypic traits has generated the diverse assortment of dog breeds seen today. However, this in-breeding has also greatly diminished the genetic diversity within each specific breed. In populations of low genetic diversity, recessive alleles are more likely to pair. Paired non-lethal recessives often generate phenotypes with decreased biological fitness. This article will explain how dog breeders and veterinary ophthalmologists are trying to decrease the expression of debilitating ocular phenotypes. Our main tool is the CERF exam. In a subsequent article I will discuss the latest genetic testing for some of these ocular disorders.
The Canine Eye Registration Foundation was formed by breeders that were concerned about heritable eye diseases. The foundation worked with veterinary ophthalmologists to devise a yearly evaluation of breeding dogs, known as a CERF exam. The phenotypic appearance of each eye is evaluated during the exam and this does not imply that an ocular disorder will not subsequently develop. Therefore, dogs with phenotypically healthy eyes are cleared for one year of breeding, but there is no genotypic clearance. Breeding dogs may show phenotypic characteristics of an ocular disorder during a future CERF exam.
Each breeding club has established standards as to acceptable (passing) degrees of ocular imperfections such as distichiasis, corneal dystrophy, persistant (iris-to-iris) pupillary membranes, or entropion. The major goal of a CERF exam is to stop breeding any dogs that display potentially blinding diseases including microphthalmia, cataracts, colobomas, progressive retinal atrophy, and retinal dysplasia. The CERF exam is not used to encourage the selective breeding of “perfect” eyes, as this would probably further diminish a breed’s gene pool.
Breeding and potentially-breeding dogs are typically subject to a yearly CERF exam as the dog ages from about 4 months to 9 years. There is no minimal age limit for a CERF exam, but precise ophthalmoscopic examination of the fundi of dogs less than 21 days of age can be difficult. The canine fundus takes on its adult form between 12 and 16 weeks of age. The first CERF examination is therefore usually performed at this age. This initial exam can identify genetic ocular disorders present from birth including: microphthalmia, iris coloboma, fundic staphyloma or coloboma, retinal detachment, persistent hyloid artery, optic nerve coloboma, optic nerve hypoplasia, or micro-papilla. Most breeders (and breeder clubs) know the inheritable ocular disorders commonly found in their specific breed. They are usually eager to make breeding decisions based upon CERF exam results. However, certain breed clubs do not value or endorse CERF exams.
A dog must have an American Kennel Club (AKC) number before a CERF exam can be performed. The CERF committee will not establish a CERF number for a dog without this. The committee also prefers that the dogs be permanently identified by tattoo or implanted microchip. Dogs without permanent identification can undergo CERF exams but will be issued a CERF number with the suffix “N” (for no permanent identification).
Prior to the CERF exam the eyes of most dogs are dilated with tropicamide. This dilation allows for complete direct (slit-lamp) and indirect ophthalmoscopy. However, dilation can mask iris hypoplasia, iris coloboma, and pupillary membranes. Breeds associated with these disorders are therefore not dilated at the beginning of their CERF exam.
After a board-certified veterinary ophthalmologist performs the CERF exam, three copies of every CERF form are generated. The first (CERF copy) is sent to the CERF regardless of the dog’s evaluation. Dogs cleared for breeding are said to have “passed” and can be assigned a CERF number. Those that express phenotypic traits associated with ocular disorders are said to have ‘failed.” The American College of Veterinary Ophthalmologists (ACVO) genetics committee evaluates these CERF results annually, and determines what diseases are prevalent in each dog breed. The second (owner’s) copy can be sent to the CERF by a breeder wishing to establish a CERF number for a passing dog. The third copy is kept by the ophthalmologist as part of the dog’s medical record.
CERF examinations are performed yearly. A passing grade is only valid for 365 days post-CERF examination. This is necessary as certain diseases only present phenotypically as a dog ages. These disorders include entropion, eyelid conformation abnormalities, persistant pupillary membranes, retinal dysplasia, and choroidal dysplasia. Progressive retinal atrophy (PRA) and cataracts are may be late-onset diseases that can be identified by CERF exams. This is why some breeding animals receive CERF examinations until 9 years of age. Recently, a specific gene has been identified and associated with PRA (Figure 1). Genetic screening for PRA, and others genes associated with ocular disorders, will be discussed in my next article.
If you have any questions about the CERF exam, or genetic testing for ocular disorders, please do not hesitate to contact a veterinary ophthalmologist. We are here to help.
Noelle La Croix, DVM, Dip. ACVO
Veterinary Medical Center of Long Island
75 Sunrise Highway
West Islip, New York 11795
(631) 587-0800; fax (631) 587-2006
Figure 1: A 5–year-old male castrated Labrador retriever presenting with chronically dilated pupils and diminished night vision. These signs were this dog’s first symptoms of progressive retinal atrophy (PRA).