Ophtalmo Flashcards
Which diseases are associated with the leopard complex spotting pattern?
Leopard complex spotting patterns (LP) in horses have been linked to both CSNB and ERU in Appaloosa and related breeds.
What are the clinical consequences of being LP/LP or LP/N ?
Horses that are homozygous for LP (LP/LP) have few to no spots of pigment in the white patterned area, have CSNB, and are also at highest risk of developing ERU.
Horses that are heterozygous (LP/N) have pigmented spots in the white patterned area, are not night blind, but have an increased risk for ERU compared with solid non–leopard complex spotted horses.
What is the mode of inheritence of congenital stationary night blindness (CSNB) ?
Although the coat spotting phenotype is inherited in a dominant fashion, CSNB is recessive.
Which breeds are affected by CSNB ?
Appaloosa, American miniature horse, Knabstrupper, Noriker, Pony of the Americas, Paso Fino
How to confirm congenital stationary night blindness (CSNB) ?
Horses homozygous for the leopard complex spotting mutation are night blind. Therefore, one way to diagnose horses with CSNB is by DNA testing for the LP mutation.
Definitive clinical diagnosis is determined by a negative dark electroretinography (ERG), in which the b-wave (ON bipolar cell activation) is absent and the a-wave (photoreceptor cell activation) has an increased amplitude.
How to assess the ERU risk for an Appaloosa?
Only one commercial genetic test, the LP test, is available for the Appaloosa breed. Risk for ERU based on this genetic test can be evaluated as LP/LP > LP/N > N/N.
Horses homozygous for LP are at the highest risk of developing ERU. Horses heterozygous for the LP mutation are at a higher risk of developing ERU than those without the breed-identifying white spotting pattern.
Appaloosas have been reported to be 8 times more likely to develop ERU and significantly more likely to go blind in 1 or both eyes, than any other breed.
Appaloosa horses frequently have insidious form of ERU.
How to assess the risk of posterior ERU for a Warmblood?
Posterior ERU is most often observed in warmbloods and draft breeds.
No genetic tests are currently being offered for warmbloods.
What is the silver coat color ?
Silver coat color is associated with multiple congenital ocular anomalies (MCOA), autosomal dominant mutation with incomplete penetrance.
Black horses with the silver mutation can range in phenotype from dark body with light mane and tail (A) to a light body known as light chocolate silver (B). Also note the dappling (B) that is present in some horses with the silver mutation.
(D) Chestnut horses with the silver allele only appear chestnut because silver does not dilute red pigment. Therefore, chestnut horses with or without the silver allele coat are phenotypically indistinguishable.
Breeds affected : Rocky Mountain horse, Kentucky mountain saddle horse, Kentucky mountain pleasure horse, Icelandic horse, Shetland pony, American miniature horse, Comtois, and other breeds with silver, including American quarter horse and Morgan.
A Comtois horse breeder presents his horses for sale. What are the potential eye anomalies?
Silver coat color is associated with multiple congenital ocular anomalies (MCOA), autosomal dominant mutation with incomplete penetrance.
DNA test available : silver test
Breeds affected : Rocky Mountain horse, Kentucky mountain saddle horse, Kentucky mountain pleasure horse, Icelandic horse, Shetland pony, American miniature horse, Comtois, and other breeds with silver, including American quarter horse and Morgan.
Horses heterozygous (Z/N) for the silver mutation most often have the cyst-only phenotype, whereas homozygotes (Z/Z) have multiple ocular defects.
The cysts phenotype is characterized by temporal cysts that originate from the ciliary body or the peripheral retina.
The MCOA phenotype is considered to be more severe and is characterized by ciliary or peripheral retina cysts with additional abnormalities, including iris hypoplasia, corneal globosa, megaloblepharon, retinal dysplasia, immature nuclear cataracts, and iridocorneal hypoplasia. Horses homozygous for the silver mutation (Z/Z) have impaired vision or are blind.
What is the genetic predisposition of Haflingers ?
Which European breeds are also potentially affected?
Ocular SCC can originate at the limbus, the nictitans, or the upper and lower eyelids. Haflingers are over-represented for limbal SCC, with prevalence estimated to be 35% in this breed.
An autosomal recessive mode of inheritance best explains the genetic risk for limbal SCC in the breed. Haflingers homozygous for the T allele had 5.89 times greater risk of developing limbal SCC than horses with T/C or C/C genotypes.
DNA test available : ocular SCC
The SCC genetic risk variant was also detected in 2 other closely related breeds with reported cases of ocular SCC, the Belgian and the Percheron, suggesting that it may also be an SCC risk factor in these breeds.
What are the ocular signs of this disease ?
Junctional epidermolysis bullosa (JEB) is characterized by mechanical stress–induced lesion of the skin and mucosal membranes, in draft breeds and American Saddlebreds.
This condition is a recessive disorder and carrier-to-carrier matings should be avoided. Mutations in laminin genes
cause separation of the dermis and epidermis.
Ocular manifestations : corneal ulcers
What are the inherited ocular disorders of Friesians ?
Friesian horses have several reported inherited collagen
disorders, including megaesophagus, aortic rupture, dwarfism, and hydrocephalus, and these may be related to 2 corneal disorders in the breed.
- Corneal dystrophy : characterized by stromal loss that is bilateral and has a higher occurrence in males, making an inherited X-linked recessive hypothesis likely.
-
Districhiasis : condition in which cilia aberrantly emerge from the meibomian gland openings, and, when pointing toward the cornea, these can cause corneal irritation
or ulceration.
No DNA test available
