Collie eye anomaly
Encyclopedia
Collie eye anomaly is a congenital, inherited
, bilateral eye
disease of dog
s involving the retina
, choroid
, and sclera
. It can be a mild disease or cause blindness. CEA is caused by a simple autosomal
recessive gene defect. There is no treatment.
and Rough Collie
s, Shetland Sheepdog
s, Australian Shepherd
s, Border Collie
s, Lancashire Heeler
s, and Nova Scotia Duck Tolling Retrievers. Frequency is high in Collies and Shetland Sheepdogs, and low in Border Collies and NSDTRs. In the United States
, incidence
in the genotype
of collies has been estimated to be as high as 95 percent, with a phenotypic
incidence of 80 to 85 percent.
to express growth hormone
affects the differentiation
of other cells of the eye. The choroid, especially lateral to the optic disc
, is hypoplastic
(underdeveloped). A coloboma
, or hole, may form in or near the optic disc due to a failed closure of embryo
nic tissue. The degree of these abnormalities varies between individual dogs, and even between the same dog's eyes. CEA is inherited as an autosomal recessive trait that has a penetrance
reaching 100 percent, and has been located to canine chromosome 37.
, or intraocular hemorrhage. It can be diagnosed by fundoscopy by the age of six or seven weeks. Severe cases may be blind.
s advocate only breeding dogs with no evidence of disease, but this would eliminate a large portion of potential breeding stock. Because of this, others recommend only breeding mildly affected dogs, but this would never completely eradicate the condition. Also, mild cases of choroidal hypoplasia may become pigmented and therefore undiagnosable by the age of three to seven months. If puppies are not checked for CEA before this happens, they may be mistaken for normal and bred as such. Checking for CEA by seven weeks of age can eliminate this possibility. Diagnosis is also difficult in dogs with coat
s of dilute color because lack of pigment in the choroid of these animals can be confused with choroidal hypoplasia. Also, because of the lack of choroidal pigment, mild choroidal hypoplasia is difficult to see, and therefore cases of CEA may be missed.
Until recently, the only way to know if a dog was a carrier
was for it to produce an affected puppy. However, a genetic test for CEA became available at the beginning of 2005, developed by the Baker Institute for Animal Health, Cornell University
, and administered through OptiGen. The test can determine whether a dog is affected, a carrier, or clear, and is therefore a useful tool in determining a particular dog's suitability for breeding.
Heredity
Heredity is the passing of traits to offspring . This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause some species to evolve...
, bilateral eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...
disease of dog
Dog
The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...
s involving the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
, choroid
Choroid
The choroid, also known as the choroidea or choroid coat, is the vascular layer of the eye, containing connective tissue, and lying between the retina and the sclera. The human choroid is thickest at the far extreme rear of the eye , while in the outlying areas it narrows to 0.1 mm...
, and sclera
Sclera
The sclera , also known as the white or white of the eye, is the opaque , fibrous, protective, outer layer of the eye containing collagen and elastic fiber. In the development of the embryo, the sclera is derived from the neural crest...
. It can be a mild disease or cause blindness. CEA is caused by a simple autosomal
Autosome
An autosome is a chromosome that is not a sex chromosome, or allosome; that is to say, there is an equal number of copies of the chromosome in males and females. For example, in humans, there are 22 pairs of autosomes. In addition to autosomes, there are sex chromosomes, to be specific: X and Y...
recessive gene defect. There is no treatment.
Affected breeds
It is known to occur in SmoothSmooth Collie
The Smooth Collie is a breed of dog developed originally for herding. It is a short-coated version of the Rough Collie of Lassie fame. Some breed organisations consider the smooth-coat and rough-coat dogs to be variations of the same breed.-History:...
and Rough Collie
Rough Collie
The Rough Collie is a long coated breed of medium to large size dog that in its original form was a type of collie used and bred for herding in Scotland. Originating in the 1800's, it is now well known through the works of author Albert Payson Terhune, and through the Lassie novel, movies, and...
s, Shetland Sheepdog
Shetland Sheepdog
The Shetland Sheepdog, often known as the Sheltie, is a breed of herding dog.They are small to medium dogs, and come in a variety of colors, such as sable/white, tri-color, and blue merle. They are vocal, excitable, energetic dogs who are always willing to please and work hard...
s, Australian Shepherd
Australian Shepherd
The Australian shepherd is a breed of herding dog that was developed on ranches in the Western United States. Despite its name, the breed, commonly known as an Aussie, did not originate in Australia They acquired their name because some of the Australian sheepdogs arrived in the United States with...
s, Border Collie
Border Collie
The Border Collie is a herding dog breed developed in the Anglo-Scottish border region for herding livestock, especially sheep. It is the most widespread of the collie breeds....
s, Lancashire Heeler
Lancashire Heeler
The Lancashire Heeler is a small breed of dog developed for use as a drover and herder of cattle. The Lancashire Heeler is listed by the Kennel Club as a vulnerable breed.-Appearance:...
s, and Nova Scotia Duck Tolling Retrievers. Frequency is high in Collies and Shetland Sheepdogs, and low in Border Collies and NSDTRs. In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, incidence
Incidence (epidemiology)
Incidence is a measure of the risk of developing some new condition within a specified period of time. Although sometimes loosely expressed simply as the number of new cases during some time period, it is better expressed as a proportion or a rate with a denominator.Incidence proportion is the...
in the genotype
Genotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
of collies has been estimated to be as high as 95 percent, with a phenotypic
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
incidence of 80 to 85 percent.
Pathogenesis
CEA is caused by improper development of the eye. Failure of the cells of the posterior portion of the optic vesiclesOptic vesicles
The eyes begin to develop as a pair of diverticula from the lateral aspects of the forebrain. These diverticula make their appearance before the closure of the anterior end of the neural tube; after the closure of the tube they are known as the optic vesicles....
to express growth hormone
Growth hormone
Growth hormone is a peptide hormone that stimulates growth, cell reproduction and regeneration in humans and other animals. Growth hormone is a 191-amino acid, single-chain polypeptide that is synthesized, stored, and secreted by the somatotroph cells within the lateral wings of the anterior...
affects the differentiation
Cellular differentiation
In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...
of other cells of the eye. The choroid, especially lateral to the optic disc
Optic disc
The optic disc or optic nerve head is the location where ganglion cell axons exit the eye to form the optic nerve. There are no light sensitive rods or cones to respond to a light stimulus at this point. This causes a break in the visual field called "the blind spot" or the "physiological blind spot"...
, is hypoplastic
Hypoplasia
Hypoplasia is underdevelopment or incomplete development of a tissue or organ. Although the term is not always used precisely, it properly refers to an inadequate or below-normal number of cells. Hypoplasia is similar to aplasia, but less severe. It is technically not the opposite of hyperplasia...
(underdeveloped). A coloboma
Coloboma
A coloboma is a hole in one of the structures of the eye, such as the iris, retina, choroid or optic disc...
, or hole, may form in or near the optic disc due to a failed closure of embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
nic tissue. The degree of these abnormalities varies between individual dogs, and even between the same dog's eyes. CEA is inherited as an autosomal recessive trait that has a penetrance
Penetrance
Penetrance in genetics is the proportion of individuals carrying a particular variant of a gene that also express an associated trait . In medical genetics, the penetrance of a disease-causing mutation is the proportion of individuals with the mutation who exhibit clinical symptoms...
reaching 100 percent, and has been located to canine chromosome 37.
Signs
The most common sign of CEA is the presence of an area of undeveloped choroid (appearing as a pale spot) lateral to the optic disc. The choroid is a collection of blood vessels supplying the retina. CEA can also cause retinal or scleral coloboma, coloboma of the optic disc, retinal detachmentRetinal detachment
Retinal detachment is a disorder of the eye in which the retina peels away from its underlying layer of support tissue. Initial detachment may be localized, but without rapid treatment the entire retina may detach, leading to vision loss and blindness. It is a medical emergency.The retina is a...
, or intraocular hemorrhage. It can be diagnosed by fundoscopy by the age of six or seven weeks. Severe cases may be blind.
Breeding and testing
Controversies exist around eliminating this disorder from breeding Collies. Some veterinarianVeterinarian
A veterinary physician, colloquially called a vet, shortened from veterinarian or veterinary surgeon , is a professional who treats disease, disorder and injury in animals....
s advocate only breeding dogs with no evidence of disease, but this would eliminate a large portion of potential breeding stock. Because of this, others recommend only breeding mildly affected dogs, but this would never completely eradicate the condition. Also, mild cases of choroidal hypoplasia may become pigmented and therefore undiagnosable by the age of three to seven months. If puppies are not checked for CEA before this happens, they may be mistaken for normal and bred as such. Checking for CEA by seven weeks of age can eliminate this possibility. Diagnosis is also difficult in dogs with coat
Coat (dog)
The coat of the domestic dog refers to the hair that covers its body. A dog's coat may be a double coat, made up of a soft undercoat and a coarser topcoat, or a single coat, which lacks an undercoat...
s of dilute color because lack of pigment in the choroid of these animals can be confused with choroidal hypoplasia. Also, because of the lack of choroidal pigment, mild choroidal hypoplasia is difficult to see, and therefore cases of CEA may be missed.
Until recently, the only way to know if a dog was a carrier
Asymptomatic carrier
An asymptomatic carrier is a person or other organism that has contracted an infectious disease, but who displays no symptoms. Although unaffected by the disease themselves, carriers can transmit it to others...
was for it to produce an affected puppy. However, a genetic test for CEA became available at the beginning of 2005, developed by the Baker Institute for Animal Health, Cornell University
Cornell University
Cornell University is an Ivy League university located in Ithaca, New York, United States. It is a private land-grant university, receiving annual funding from the State of New York for certain educational missions...
, and administered through OptiGen. The test can determine whether a dog is affected, a carrier, or clear, and is therefore a useful tool in determining a particular dog's suitability for breeding.