Aussie Genetics Fact Sheet
by C.A. Sharp

Cataracts are the most common eye disease in Australian Shepherds. They can occur for reasons other than heredity (other diseases, injury, old age) but these other causes are not common and should not be assumed to be the reason why cataracts have occurred without clear evidence to that end.

Hereditary cataracts are bilateral, meaning they occur in both eyes, but they may not appear at the same time: If a cataract is noted on one eye, it is wise to recheck in 6 months to a year to see if one develops in the other. Cataracts progress, starting as small opacities and advancing, sometimes to the point of clouding the entire lens and reducing vision to an inability to distinguish anything but extremes of light and dark. Cataracts do not cause the dog any pain and usually progress is slow enough that the dog adjusts to its vision loss. In Aussies cataracts virtually never occur in young puppies. Affected dogs most commonly present signs between 1.5 and 3, but may first show disease as late as 7 years of age. This wide range in the age of onset, coupled with the currently unknown mode of inheritance makes the disease extremely difficult to predict or eliminate.

Cataracts which occur after the juvenile period are not, necessarily, "old age cataracts." Nuclear sclerosis, the kind of cataract that is a result of the aging process, may occur as early as 8 years in rare cases but it looks distinctly different to the ophthalmologist and so will not be confused with other types of opacities.

Cataracts are classified by where in the lens they first develop. The lens is round when viewed from the front or back, thickest in the middle and tapering to the outer edge. The front is referred to as the anterior, the back is the posterior. The inside is the nucleus; the outside, the cortex, and the outer (round) edge, the equator. The center of the lens is the polar area. So, if a dog has posterior polar cortical cataracts, they are forming on the back side in the center on the outer layer. This is where the vast majority of Aussie cataracts start.

As stated earlier, the mode of inheritance remains a mystery. It is clearly not dominant or affected animals would produce at least 50% affected offspring, and unaffected animals wouldn't produce any at all. It isn't X-linked, or only males would have it. This leaves recessive or polygenic modes of inheritance. Neither is clearly indicated at present. It is also possible that our breed has not one but two or more inherited cataracts. If this turns out to be the case, it will be necessary to find ways to distinguish between types before truly effective breeding strategies can be devised.

All breeding stock must be examined by an ACVO vet annually. Affected animals should not be bred. If they have produced offspring prior to diagnosis, owners of those offspring need to be told. Parents, siblings and offspring of affected animals should only be bred with care--never to their relatives or dogs from lines where cataract is known to be a problem. If an individual produces affected offspring more than once with mates unrelated to itself or each other, serious consideration should be given to removing it from further breeding.