What is iris coloboma?

A coloboma is any area of eye tissue that fails to develop.   A dog could have a coloboma of the iris, the optic nerve, the lens, or even an eyelid.   If your dog is diagnosed with a coloboma, it’s important to know what specific tissue was involved.  Whatever kind it is, an affected dog should not be bred.  However, some types of coloboma are more serious than others for both dog and breeder.

Iris coloboma has only a small effect on the dog’s vision though a large one can interfere with contraction in response to bright light, causing the dog to squint the affected eye.  But if the coloboma is small, it will pose little difficulty for your dog.

Is there a DNA test for iris coloboma?

No.

How can I find out if my dog has iris coloboma?

Some iris colobomas are easy to see.  However, if only a small piece is missing you may not be able to.  They can be especially difficult to see in young puppies and dogs with eyes that combine blue and pigmented areas.  Pigment variation may appear to be a coloboma to the naked eye.  If you think your dog may have an iris coloboma, get it examined by a veterinary ophthalmologist.  If you go to an eye exam clinic, make sure your dog is examined before the eyes are dilated as small colobomas can be missed in dilated eyes.

Can a dog develop iris coloboma later in life?

No.  If a dog has had clear eye exams and later is found to have iris coloboma it was missed on the earlier exams.  The American College of Veterinary Ophthalmologists recommends that all Aussies should have the irises examined prior to dilation to prevent this from happening.

What does it mean for my dog if it has iris coloboma?

In most cases an iris coloboma has little if any effect on vision.  Iris coloboma does not require medication or surgery.  If the coloboma is large enough to cause the dog to squint in bright light there may be some discomfort connected to the light sensitivity.  A dog with an iris coloboma this large may misjudge distances.  If a dog is competing in high-speed sports like agility or one that is doing stockwork, whether for trials or real farm/ranch work, a larger iris coloboma could be hazardous when the dog  is negotiating obstacles at high speed or dodging flying hooves.

How common is iris coloboma in Aussies?

About one Aussie in seven has it.

Does merle-to-merle breeding cause iris coloboma?   

No.  This misconception may have arisen because they are most frequently seen in merles and double merles often have abnormal irises along with the other eye defects typical of dogs that inherit two copies of the merle gene.  

Can iris coloboma cause glaucoma?  

No.  It is not associated with glaucoma, which is extremely rare in Aussies. 

Iris coloboma inherited?

Yes, though the specifics of how it is inherited are not yet known.  Merles are more likely to be affected but it is not caused by the merle gene.  However, merle may be a risk factor when combined with whatever other gene(s) underlie the defect.

What does iris coloboma mean to my breeding program?

Dogs with iris coloboma should not be bred.  The parents and full and half siblings of an affected dog should not be bred close on the pedigree that produced it and should be bred to mates that do not have a family history of iris coloboma.

What is iris hypoplasia?

Iris hypoplasia is thin iris tissue.  This is a developmental issue and may have some relation to iris coloboma, in which some portion of the iris fails to develop at all.  However, this is not established at this time.

How can I find out if my dog has iris hypoplasia?

Have the dog examined by a veterinary ophthalmologist.  Dogs with iris hypoplasia will pass an eye exam.  However, iris hypoplasia is not always noted by the examiner and if it is very minor it might be missed.  If a dog has had clear exams with no mention of iris hypoplasia and later is found to have it, the condition either was missed nor not recorded in the prior exams.

What does it mean for my dog if it has iris hypoplasia?

Iris hypoplasia will not affect your dog’s vision.

How common is iris hypoplasia in Aussies?

About one percent have it.

Is iris hypoplasia inherited?  

Unknown.  However, if it does prove to be a mild form of the same process that causes iris coloboma then it would be inherited.

What does iris hypoplasia mean to my breeding program?

No dog worthy of breeding should be removed from a breeding program because of iris hypoplasia.  If you want to err on the side of extra caution, don’t breed it to another dog with iris hypoplasia or to dogs with a family history of iris coloboma.