Collie Eye Anomaly

Collie Eye Anomaly (CEA)

What is CEA?

CEA is more technically known as Choroidal Hypoplasia (CH). It is a recessively inherited eye disorder that causes abnormal development of the choroid - an important layer of tissue under the retina of the eye. This disease is seen most frequently in U.S. collies, but also worldwide in Rough and Smooth Collies, Border Collies, Australian Shepherds, Lancashire Heelers, and Shetland Sheepdogs. Since the choroid layer does not develop normally from the start, the primary abnormality can be diagnosed at a very young age. Regrettably, there is no treatment or cure for CEA.

In 2012 the first confirmed occurrence of CEA was reported in a Bearded Collie in the UK.

The primary problem is choroidal hypoplasia (CH). There is under-development (hypoplasia) of the eye tissue layer called the choroid. The choroid is an important layer of tissue lying under the retina which is rich in blood vessels delivering oxygen and nutrients to the retina. The choroid appears pale and thin, almost transparent, and the blood vessels of the choroid can easily be recognized in those “thin” areas. The ophthalmologist, looking at the back of the eye (the fundus) with an ophthalmoscope, typically will see an area of choroidal thinning that appears like a “window” to the underlying vessels and sclera.

CEA can be diagnosed by examination of the eye by a veterinary ophthalmologist. However it can only reliably be seen in puppies between the ages of six to ten weeks of age. Later development of the eye tends to hide these features. Thus puppies diagnosed with CEA during early screening may appear to be clear on examination as adults. These dogs are still affected and carry the genes for the condition.

MILD disease: It is easily recognizable on careful ophthalmologic examination as early as 5 to 8 weeks of age. The lesion appears as an area lateral (temporal) to the optic disc with reduction or absence of pigment so that the underlying vessels of the choroid are seen. In mildly affected dogs, choroidal thinning is the only detectable abnormality and the dog retains normal vision throughout life. However, dogs with mild disease can produce severely affected offspring.

SEVERE disease: In severely affected dogs, approximately 25% of dogs with CEA/CH, there are related problems with the health of the eye that can result in serious vision loss in some cases.

CEA/CH is not progressive in the usual sense. The essential features, choroidal hypoplasia and coloboma, are congenital – the abnormalities develop as the eye develops. These features are also stationary once ocular development is complete around 8-12 weeks of life. Retinal detachments and/or aberrant vessel formation can be congenital or develop later, in general only in eyes with colobomas. There is no cure.

BOTH the mild and severe forms of CEA/CH disease now are proven to result from the exact same gene and mutation in ALL of the affected breeds . This disease gene is located on canine chromosome number 37. The mutation acts like a RECESSIVE mutation. CEA is inherited as an autosomal recessive with close to 100% penetrance. For a dog to have symptoms of the disease it must inherit two copies of the gene, one from each parent. Dogs that carry a single copy of the gene are carriers for CEA and will pass this gene on to approximately 50% of their offspring.

Testing For CEA

Since the genere responsible for CEA has been identified, a DNA genetic test has been developed.

Laboratories, such as Embark or PawPrints Genetics provide genetic testing forCEA in Bearded Collies. There are other labs in vaarious countries that perform this test as well.

Incidence in Bearded Collies

According to the Bearded Collie Club (UK) :
" The results [of the initial study] showed one dog out of 59 tested was affected and 5 other dogs were found to be carriers. In a completely random sample, this frequency of affected and carrier dogs would indicate an incidence of CEA/CH in the population of between 3 and 4 affected dogs per thousand (0.35%) and a carrier frequency of 11.16%. However, the actual frequency of the abnormal gene is likely to be lower than this because the sample included the affected dog, its parents and three other relatives.

"It appears from current evidence that the abnormal gene came into the breed through a single dog in the relatively early years of the breed. The dog in question and its descendants were relatively little-used in breeding and not prominent in the show scene. As a result the abnormal gene has a comparatively restricted distribution within the overall population, although a number of dogs were exported and have living descendants overseas. "

Since that initial study was completed, breeders and owners in the UK have been encouraged to test their Beardies. A recent Kennel Club Health Report on DNA testing for CEA (July 2023) indicates that 290 Beardies were tested. Two dogs were affected (0.69%) and nine other dogs were identified as carrieers (3.1%). The remaining dogs were clear. It is possible that there were Beardies who were tested and whose results were not reported to The Kennel Club.

The Bearded Collie Club of America encouraged testing for CEA by members. Results, as reported on BeaCon For Health's web site:
• Testing was conducted by PawPrints; owners requested sampling kits directly from PawPrints. The collective results were reported anonymously. In late 2019 with data available on 201 dogs, the results were:
   • Carrier – 15 (12.4%)
   •   Normal – 176 (87.6%)
   •   Affected – none

• The BCCA joined the parent club partnership program at Embark in 2019. Of 168 Beardies by late 2019 (no recent reports):
    • Carriers – 6 (3.6%)
   • Normal – 162 (96.4%)
   • Affected – none

Implications for Breeders

Breeders should pay attention to protecting the genetic diversity of the breed. It is worthwhile to recall the statistical expected results of breedings with a condition with an autosomal recessive gene:

Expected results of breeding strategies for inherited recessive diseases.

Examples based on the above chart:
• If either the sire or dam is a carrier and the other parent is clear none of the offspring will be affected but about 50% will carry the mutated gene.
• If both parents are carriers about 25% of the offspring will be affected, about 50% will be carriers and about 25% will be clear.

Dogs that do not have the abnormal gene are referred to as "normal" or "clear". Dogs inheriting only one copy of the abnormal gene are known as carriers because although they appear normal, the abnormal gene is in their cells and can be carried through to the next generation. Carriers can produce affected dogs if bred to an affected dog or to another carrier. Affected dogs have two copies of the abnormal CEA gene, one inherited from each parent, and they will always pass on the abnormal gene to their progeny.

Since the incidence of CEA in Bearded Collies is low the advice to breeders is that they should only breed CEA "Clear" beardies. If carriers are used then they must bebred to clear dogs, and the resulting offspring should ALL be DNA tested before they are sold. The new owners should be made aware of the CEA genetic testing results. Owners of puppies that are carriers should be sold on Non-breeding contracts and advised against breeding the puupy.

Breeders have the ability to eliminate Collie Eye Anomaly/Choroidal hypoplasia from Bearded Collies through careful and thoughtful breeding practices.

References:

Aronson, Linda. 2017. Collie Eye Anomaly (CEA). Lighting The Way, Official Newsletter for the Bearded Collie Foundation for Health

Gaspar, Lois. 2014. Collie Eye Anomaly (CEA). The Bearded Colleague

BeaCon For Health. Lab Health Screening

The Bearded Collie Club. 2022 (updated). Collie Eye Anomaly (CEA)

The Kennel Club. DNA test - CEA/CH (Collie eye anomaly/Choroidal hypoplasia)