Very briefly, CEA is a condition that affects the normal anatomy of the retina and other deeper structures of the eye, this
can be irregularity in structure or even holes/pockets. In it's mildest form it will not affect the dog, in it's severest it will
cause detached retina's/complete blindness.
CEA is caused by a recessive gene which both parents must carry to produce affected pups.
An animal will have 2 copies of every gene, one coming from the sire and one from the dam.
If an animal has two normal copies of the gene, it is classed as 'normal' and cannot ever produce affected pups.
If an animal has one normal gene and one defective gene it is classed as a 'carrier', mated to another animal with the
defective gene it could produce affected pups. If mated to a 'normal' animal it will at worst produce more carriers but may
also produce some 'normals'
(Note: A carrier DOES NOT have the disease)
If an animal had two copies of the defective gene it is classed as 'Affected' and will actually have the disease. If bred from,
this animal can only produce carrier and/or affected pups.
The table below shows the likely outcome of various matings..
The results shown in grey are what everyone wants to avoid...nobody wants to produce affected pups so these are the
matings to avoid - carrier to carrier, carrier to affected, affected to affected.
Prior to the introduction of a DNA test; eye testing was first carried out on pups at around the age of 6 weeks by one of
only 30 BVA approved veterinarians around the country. This initial test is for CEA only and this period between 5 and 8
weeks of age is the best time for diagnosis of this condition. If a puppy is affected it is an indication that both of it’s parents
are carriers of the CEA gene. While the Kennel club does not impose any breeding restrictions from affected dogs or
carriers the International Sheepdog Society (ISDS) does not allow the registration of affected pups or their progeny and
does not allow the registration of puppies by parents that have produced a CEA/PRA affected pup on more than one
occasion (they allow one due to the possibility of a mis-mating). The ISDS is striving to eradicate this disease from the
breed. Puppies that successfully pass this eye test are issued with a litter certificate.
Passing an eye examinination at 6 weeks indicates the puppy is not AFFECTED by the disease but the physical
examination cannot tell you which puppies are carriers of the gene. The recent introduction of DNA testing for the CEA
gene by American laboratory, OptiGen has revolutionised breeding of border collies to a certain extent...although it can
be quite expensive, we can now DNA test all breeding stock to see which animals are affected, which are carriers of the
gene and which are DNA 'normal'. This means there need never be any more affected puppies produced...we can ensure
that by breeding carrier/affected animals to 'normal' animals the puppies cannot ever be affected by the disease. Since
this test first became available in January 2005, ALL our dogs used for breeding are DNA tested in this way.
|Collies share Collie Eye Anomaly (CEA) with several other breeds – it’s not just a problem for collies. 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.
The symptoms and signs – the clinical phenotype – can vary greatly among affected dogs within one breed,
between parent and offspring and even within a litter. This creates a difficult situation for the breeder. Learning
about the genetic cause and the course of the disease will help you understand how to manage it better and
eventually avoid it altogether with genetic testing.
The primary problem is choroidal hypoplasia (CH). There is under-development (hypoplasia) of the eye tissue
layer called the choroid. 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.
MILD disease: Mild disease is very common in U.S. collies and is present in the other breeds named above. 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. The choroidal vessels may be reduced in number and of abnormal
shape. The underlying white sclera might also be visible. Once the retina changes to its adult color around 3
months of age, the normal pigment sometimes masks the changes in the choroid (so-called “go normal” – read
more below). 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.
(The eye anomaly “merle” can be confused with choroidal hypoplasia, primarily in dogs from merle to merle
breeding and whose coat color is whiter than their littermates. Although both conditions are inherited, can
occur in the same breed and exhibit a range of fundus anomalies, there are sufficient dissimilarities for the
ophthalmologist to make the distinction.)
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. Colobomas are seen
at and near the optic nerve head as outpouchings or “pits” in the eye tissue layers. Colobomas can lead to
secondary complications such as partial or complete retinal detachments and/or growth of new but abnormal
blood vessels with hemorrhage – bleeding inside the eye. This happens in 5-10% of dogs with CEA/CH,
generally by 2 years of age, and can affect either one or both eyes. Complications of severe disease can lead
to vision loss, although this disorder only rarely threatens total blindness.
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.
Based on research done jointly by scientists at Cornell University and at The Fred Hutchinson Cancer
Research Center, 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 named above. This disease gene is located on canine
chromosome number 37 and the disease-causing mutation has been identified. The mutation acts like a
RECESSIVE mutation. That means, both parents of an affected dog must have at least one copy of the
mutation and both parents must have passed a copy of the mutation to the offspring. The affected dog is
HOMOZYGOUS RECESSIVE – that is, both copies of the gene are mutant. ALL dogs that are homozygous
recessive affected will show at least the mild form of the disease. ALL affected dogs, regardless of the actual
severity of the lesions, are homozygous for the same mutant gene.
(A dog with one mutant copy and one normal copy of the CEA/CH gene is a carrier – is heterozygous. A dog
with two copies of the normal CEA/CH gene is homozygous normal.)
The frequency of CEA/CH disease varies among breeds and by country of origin. The U.S. registration
organization, CERF, reported the incidence in the U.S. of choroidal hypoplasia, optic disc/nerve coloboma and
retinal detachment among several affected breeds over the period of 1991 to 1999. (Comparable data from
other countries isn’t available to us yet.)
Frequencies Based on CERF Eye Exams in the U.S. from 1991 to 1999
Hypoplasia Coloboma Retinal
Collie - Rough & Smooth 66.7% 8.75% 1.88%
Border Collie 2.12% 0.57% 0.06%
Shetland Sheepdog 0.39% 0.79% 0.05%
Australian Shepherd 0.22% 0.27% 0.13%
The frequency of the CEA/CH gene mutation in U.S. Rough and Smooth Collies appears to be extremely high.
In general, the frequency of affecteds in Rough and Smooth Collies is well over 50%, and in some populations
has been observed to be as high as 85-90% of dogs examined. Of the remaining, most are carriers. The
frequency of the CEA/CH gene mutation in European Shetland Sheepdogs appears to be significantly higher
than in the U.S.
The OptiGen genetic test for CEA/CH provides a powerful management tool for the breeder. This genetic test
can distinguish all three genetic states – normal, carrier and affected. With this information, the breeder can
plan matings that avoid producing any affected dogs by always selecting one parent that is normal. The other
parent can be normal, carrier or even affected, and no affected dogs will result. (See table at the end.) This
breeding recommendation is a big step forward, especially for breeds and countries where frequency of
CEA/CH is much lower. Earlier advice cautioned against breeding affected dogs, their parents, their offspring
or their siblings (unless eye exams before 3 months of age demonstrate the sib is unaffected).
Understandably, genetic testing will be a difficult tool to use for some breeders of “standard” collies (i.e.,
Rough, Smooth, Show, Standard) where the disease is very common. In some circumstances, genetically
normal – homozygous normal – collies could be difficult to find and it may not be practical for the breeder to
plan matings that include one normal dog. And, it may not be reasonable to expect complete avoidance of
CEA/CH in one generation. All the same, genetic testing is a sure-fire tool to move toward elimination of the
disease. To start, breeding a carrier to a carrier will produce an average of 25% normals, 50% carriers and
25% affecteds. With genetic testing at each subsequent generation, and with a goal of breeding normal by
carrier or normal by affected, the frequency of disease will drop and frequency of normals will increase without
loss of other desirable traits valued in collies.
Breeders should pay attention to protecting the genetic diversity of breeds that have very high frequencies of
an inherited disease. In the case of CEA/CH, the genetic test can be viewed as an adjunct to traditional
strategies for avoiding severe cases of CEA. Over the last 30 years, many animals have been examined and
those with only mild CEA (no colobomas or detachments) have been selected for breeding. The result is the
percentage of collies affected with choroidal hypoplasia remains high, but the severe grades of the disease
(colobomas and retinal detachments) have decreased due to this conscientious breeding.
Even though the ideal recommendation is to breed genetically normals, preservation of other desirable
physical traits might override the ideal in the short-term. Reduction or even elimination of the CEA/CH mutant
gene can be viewed as a longer-term goal. You should consult with your breed club for further breeding
You might ask: if the mild form and the severe form of CEA/CH disease are caused by the exact same genetic
mutation, why do some dogs have only mild disease while others have severe disease? Is the severity due to
diet, activity, or other insults like infections or trauma? So far, there are no clues that non-genetic factors are
responsible. Instead, there are probably other independently acting “modifier” genes that influence CEA/CH
gene expression. If that is so, eventually these modifier genes will be detected, although the chore will be
difficult. Possibly, by choosing mildly affected dogs and avoiding severely affecteds in a breeding program,
breeders have concentrated positive influencing independent modifier genes in their line. The CEA/CH gene
frequency may not have changed, but the disease may be partially suppressed as long as the modifying
genes are carried along. This is a risky approach, since the identity of those influencing genes – indeed even
their number and action – is a complete unknown.
You might also ask: is it true that early choroidal hypoplasia can “go normal,” that is, reverse to a normal
appearance? There are occasional reports of puppies, found to be affected as early as 5 weeks of age, that
appear to “go normal” when re-examined some months later. The abnormal features seem to disappear or
lessen due to pigment changes and masking of the thin choroid areas. (However, if a dog had a coloboma, this
will remain – it is a permanent lesion.) The majority of dogs that “go normal” are homozygous for the CEA/CH
mutation, especially if they “go normal” slowly or incompletely. A small minority, however, are heterozygous
carriers that tend to “go normal” at a very young age. Regardless, the genetic status of such dogs was and
remains constant during their lifetime, so these dogs can pass the mutant disease gene to their offspring.
Testing will help you identify the genetic status of dogs that have an ambiguous clinical diagnosis.
|Collie Eye Anomaly / Choroidal Hypoplasia
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