Aussie Genetics Fact Sheet
by C.A. Sharp
CERF exams are performed on all breeds of dog to screen for hereditary eye diseases in each breed. In Australian Shepherds, which should be examined as young puppies and annually thereafter, the most common defects are cataracts, Collie Eye Anomaly (CEA) and iris coloboma, with Persistent Pupilary Membrane (PPM) and distichaisis being of moderate concern. Progressive retinal atrophy, retinal dysplasia (independent of CEA) and glaucoma have been seen but are extremely rare. Merle occular dysgenesis (homozygous merle eye) also occurs, but since we know this is the result of breeding two merles together the source of the problem is obvious and easy to avoid.
Cataract--an opacity (clouding) of the lens. The most common in Aussies are posterior (back of the lens) polar (in the center). Hereditary cataracts will be bilateral (both eyes) but may not start at the same time in each eye. The disease can be blinding. Affected Aussies usually are diagnosed around two years of age, give or take, but some do not exhibit cataracts until several years later. Mode of inheritance is unknown.
Iris Coloboma--a missing section of iris. Almost all affected dogs are merle. A large coloboma will prevent the iris from dilating and contracting properly resulting in some discomfort and difficulty in bright light. The condition is congenital (present at birth). Mode of inheritance is unknown.
CEA--a complex of congenital defects including choroidal hypoplasia (a thinning of the vascular tissue within the eye), optic disc coloboma/staphloma (incomplete development of the optic nerve where it enters the eye), and retinal dysplasia or detachment (sections of retina--the vision reception tissue--that are not properly attached to the wall of the eye). Some dogs are only mildly affected, but those with large colobomas or retinal detachment will be blind. The defects observed can vary from one eye to the other but both will have something wrong. The disease is a recessive; if a dog has it both parents carried it.
PPM--a membrane that covers the pupil (hole in the middle of the iris) before birth which fails to completely go away, leaving tags of tissue. These may be iris-to-iris, iris-to-lens or iris-to-cornea (the clear front covering of the eye). Iris-to-iris rarely causes visual problems unless it is unusually large, but the other types can cause opacities of the lens or cornea that can be blinding. The condition is congenital and the mode of inheritance is unknown. Young puppies which exhibit PPM should be re-checked at a few months of age to see if the PPM has resolved (gone away). If it has not, the dog should not be bred.
Distichiasis--one or more abnormal eyelashes that grow toward the eye instead of away from it. It can cause painful abrasion to the cornea and may require surgical correction. It can occur at any age. Mode of inheritance is not known.
PRA, glaucoma and non-CEA retinal dysplasia are rare and the mode of inheritance unknown, though PRAs in other breeds are all recessive so the same probably would apply to Aussies. Dogs affected with any of these defects should not be bred.
A note of caution regarding diagnoses of PRA in Aussies--if the dog affected is an active working dog, retinal damage caused by blows to the head (kicks, etc) may appear similar to PRA. Make sure the dog is seen by an ophthalmologist familiar with traumataic retinal damage.
Young Aussies are checked is to screen for the congenital defects seen in CEA and for PPM. It is vital that all Aussie pups get an early screening because CEA can "go normal." Such dogs are not cured, they have only choroidal hypoplasia which can be hidden when the pigment develops in the back of the eye when the puppy is a few weeks old. The earlier you can get the check done, the better. Even 8 or 10 weeks may be too late. "Go normals" will have two genes for CEA and therefore pass one to each of their offspring if bred.