GENETICS OF INHERITED EYE DISEASEGenetic diseases are those that are passed on from parent to offspring through genes that carry the codes for each specific trait. Many of the diseases and disorders that affect the eyes have genetic factors.
How do we identify an inherited eye disease?Although there are noteworthy exceptions, most of the ocular diseases of dogs which are presumed to be hereditary have not been adequately documented. Genetic studies require examination of large numbers of related animals in order to characterize the disorder (age of onset, characteristic appearance, rate of progression) and to define the mode of inheritance (recessive, dominant). In a clinical situation, related animals are frequently not available for examination once a disorder suspected as inherited is identified in an individual dog. Maintaining a number of dogs for controlled breeding trials through several generations is a long and costly process. Both of these obstacles are compounded by the fact that many ocular conditions do not develop until later in life. Until the genetic basis of an ocular disorder is defined in a peer-reviewed published report, we rely on what statistical information is available from registry organizations, informed opinions and consensus from ACVO diplomates. We must satisfy ourselves with terms like “presumed inherited” and “suspected to be inherited.” Several companies provide information on genetic testing and greatly assist in providing more information and data to aid in defining the canine genetics of ocular diseases.
There are eye diseases in the dog for which there is evidence of a genetic or heritable cause. The American College of Veterinary Ophthalmologists has listed ten of these diseases as automatic “fails” (this means the affected dog is ineligible to receive an eye certification) because of the significance of the condition to vision and/or the very strong evidence of heritability.
Portions of the material above have been reprinted with permission of the American College of Veterinary Ophthalmologists from the publication “Ocular Conditions Presumed to be Inherited in Purebred Dogs”, 5th Edition, 2010, produced by the Genetics Committee of the American College of Veterinary Ophthalmologists, © American College of Veterinary Ophthalmologists.
EYE EVALUATION CRITERIA: WHAT TO EXPECT DURING AN EXAMOFA Eye Certification examinations are screening exams performed by board certified veterinary ophthalmologists. The exams can take place either in the veterinary office, or at a special clinic held in conjunction with another event (such as a dog show).
Bring your dog’s information to the exam so the exam form may be completed properly. Required information includes: registration number, owner’s name and contact information, dog’s registered name, date of birth, sex, breed/variety, and if applicable, permanent identification (via microchip or tattoo).
The exam is performed 30 to 40 minutes after pupil-dilating drops are placed in the eyes. The Eye Certification exam consists of indirect ophthalmoscopy and slit lamp biomicroscopy. It is not a comprehensive ocular health examination, but rather an eye screening exam. For example, Eye Certification exams do not entail measuring tear production, staining the eyes for the presence of corneal ulcers, or measuring intraocular pressures. Gonioscopy, tonometry, Schirmer tear test, electroretinography, and ultrasonography are not routinely performed; thus, dogs with goniodysgenesis, glaucoma, keratoconjunctivitis sicca, early lens luxation/subluxation or some early cases of progressive retinal atrophy might not be detected without further testing. If a serious ocular health problem (such as glaucoma) is suspected during the Eye Certification exam, the examiner will recommend a more comprehensive ocular examination. The diagnoses obtained during an OFA Eye Certification Registry exam refer only to the observable phenotype (clinical appearance) of an animal. Thus it is possible for a clinically normal animal to be a carrier (abnormalgenotype) of genetic abnormalities.