One of the multiple eye disorders that can affect your dog is an inherited condition known as iris coloboma. With this condition, and for unknown reasons, development of eye tissues is incomplete in the womb. This causes the puppy to be born with a hole, split, or cleft in certain structures within the affected eye. In this case it's the iris, which is the colored portion in the front of the eye. Dogs with this condition will have a dark hole and/or an irregularly shaped iris, but their vision is usually not impaired. However, they may squint and be uncomfortable in bright light, since the coloboma prevents the iris from contracting normally upon exposure to light.
Colobomas can appear in other structures within the eye as well as the iris, specifically the choroid, eyelid, ciliary body, lens, and retina. When the iris is affected, they appear as a black round hole which is either in or directly adjacent to the iris, causing its shape to be irregular. In other cases, it can appear as a split or cleft which runs from the pupil to the edge of the iris.
Most colobomas are inherited through recessive genes and are diagnosed shortly after birth. However, they also can be linked to injuries and trauma to the eye, or appear as a complication following eye surgery. Colobomas can appear in any breed of dog, but are most common in Basenjis, Australian Sheepdogs, and other herding dogs, such as Collies. They appear in other mammals as well, most notably in humans and in cattle. They also occasionally appear in conjunction with other eye disorders such as detached retinas, juvenile and senior cataracts, distichiasis, persistent pupillary membrane, and progressive retinal atrophy.
Usually colobomas do not affect a dog's vision. When the pupil is involved, however, it can cause a double image to focus on the back of the eye, leaving the dog with blurred and/or double vision. A definitive diagnosis will be made by your veterinarian, who will examine the eye when it's both dilated and undilated. (Smaller colobomas can be missed if the eye is only examined after the pupil is dilated.)
Dogs with colobomas should not be bred, especially herd dogs that will be spending many hours a day in direct sunlight. Organizations such as the Canine Eye Registration Foundation can provide certification for breeders to guarantee that their animals' eyes are normal. Responsible breeders also are encouraged to have their litters examined by a veterinary ophthalmologist before they are placed in new homes; this can be done as early as six weeks of age.