Eye Exams & Genetic Testing Help Breeders Reduce Blindness in Labradors
Labrador Retrievers, like all breeds of dog, are susceptible to hereditary eye diseases that potentially can cause blindness. Breeders who take advantage of genetic testing and annual eye examinations are helping to prevent eye diseases.
The most common eye disease in Labradors is a late-onset form of progressive retinal atrophy known as rod-cone degeneration (prcd-PRA). Before the discovery of the gene mutation for prcd-PRA in 2005, breeders often bred carriers and affected dogs without realizing they were passing the disease onto future generations. The development of the genetic test has helped to advance breeders' ability to test their dogs and make responsible breeding decisions.
"Now, with the genetic test so easily available, we never have to produce a dog that goes blind from PRA," says Karen Helmers, D.V.M., a Labrador breeder under the Paradocs prefix and co-owner of Greencastle Veterinary Clinic in Indiana. "We can use affected and carrier dogs in our breeding programs, as long as they are bred to clear dogs. We never have to eliminate a dog solely based on its PRA status. The goal is not to eliminate carriers but to eliminate affected dogs."
The frequency of the prcd-PRA gene mutation remains high in the breed, says Sue Pearce-Kelling, president of OptiGen, the veterinary testing laboratory that offers the genetic test. "The percent of Labradors carrying at least one copy of the prcd-PRA mutation has remained above 20 percent every year," she says. "In 2011, of the Labradors tested at OptiGen, 74 percent were normal, 23 percent were carriers, and 2 percent were affected."
Genetic testing, responsible breeding and better education would help prevent PRA, Pearce-Kelling says. "It breaks my heart when I see affected test results in a litter of Labradors. The chances are very high the pups will go blind before they are 10, and PRA is a completely preventable disease."
OptiGen also offers a mutation test for retinal dysplasia/oculoskeletal dysplasia (RD/OSD) that identifies normal, affected and carrier Labrador Retrievers. Though the condition is considered rare, with around 3 percent of Labradors identified as carriers, it is devastating to those whose dogs develop the disease. Retinal dysplasia encompasses a spectrum of severities, ranging from focal or multifocal retinal folds to geographic retinal dysplasia and retinal detachment. Retinal folds typically are considered benign and have no significance unless they are associated with oculoskeletal dysplasia.
These eye diseases, as well as others for which there are no genetic tests, prompted the Labrador Retriever Club to recommend annual eye examinations by board-certified veterinary ophthalmologists and testing for inherited eye diseases for dogs that are part of a breeding program. Puppies should have eye examinations around 6 to 8 weeks of age before they are sold to new owners to check for retinal folds and the possibility of oculoskeletal dysplasia. Eye examinations, in which results are registered through the Orthopedic Foundation for Animals Eye Certification Registry or Canine Eye Registration Foundation, are among the required tests for Labradors to receive health clearances via Canine Health Information Center certification.
"The continued development of genetic testing for eye diseases in Labradors as well as other breeds will greatly benefit breeders by helping them to produce healthy animals," says Stephen Bistner, D.V.M., DACVO, of the Animal Wellness Center in Maple Grove, Minn. "The more eye clinics we hold and the more we work together to teach breeders and owners about inherited eye diseases, the more likely we are to have a greater reduction in these diseases."
The Most Widespread Form of PRA
Progressive retinal atrophy is an inherited eye disease that refers to a large group of eye diseases that involve gradual deterioration of the retina, eventually leading to blindness. The retina, located at the back of the eye, takes the light gathered and focused by the other eye structures and converts it into electrical nerve signals to send it to the optic nerve and then the brain for interpretation.
The PRA that Labradors develop, prcd-PRA, is the most widespread form based on the number of breeds it affects. Besides Labrador Retrievers, the recessive gene mutation occurs in more than 20 breeds. Experts say signs of the disease are visible by physical eye examination when dogs are between 3 and 6 years of age. Owners may initially notice their dogs having diminished sight in the evening.
In prcd-PRA, retinal receptor cells develop and function normally in early life, but at some point in mid to later life, they degenerate and die. There are are two types of receptor cells: rods and cones. The rods, which are responsible for vision in low light levels, die first. Thus, affected dogs may have difficulty seeing in dim light. The cones, which are more important for detail, movement and color vision, follow. A dog eventually loses vision in bright light until becoming completely blind.
Physical signs of prcd-PRA are determined by ophthalmoscopy, and electrical signs are detected by electroretinography. Both appear much earlier than behavioral signs. Owners may notice that their dog's eyes glow. This occurs as the receptor cells die, leaving the retina thinner and thus allowing the shiny tapetum behind it to show through more. In addition, the pupil compensates for the lack of light reception by opening up wider. The added effect of the wider pupil and brighter tapetum makes the dog's eyes glow to the point that owners may be the first to notice something different. More often, however, it is a routine eye examination that leads to discovery of the problem.
The good news is that DNA testing allows identification of carriers and affected and normal dogs. Helmers recalls having all the dogs in her kennel tested when the test was first offered. "We tested around 20 dogs," she says. "I was pretty sure they were all OK. When the results showed two were affected, I spayed them and found good homes for them. I wanted them to be in good pet homes if and when they started to go blind."
In an effort to find unidentified gene mutations, OptiGen offers free testing for Labrador Retrievers or any breed diagnosed with PRA. The disease is considered the canine version of retinitis pigmentosa in humans, which has more than 100 genetic mutations, all resulting in identical clinical signs.
"PRA in dogs probably follows a similar pattern," explains Pearce-Kelling. "There are likely to be hundreds of mutations responsible for different forms of PRA. prcd-PRA is unusual in its widespread distribution across many breeds, indicating it is an old mutation that was probably present before modern breeds segregated.
"In a couple of breeds, most notably the English Cocker Spaniel, there is a later, slower disease progression than what we see in most other breeds, including Labradors, yet there is some variation of disease onset and progression in all breeds. We are interested in any outliers, such as prcd-affected Labradors that show clinical retinal degeneration by 3 years of age or that, conversely, show no clinical signs of retinal degeneration after around 9 years of age. These outliers will hopefully point us to modifiers that affect retinal degeneration and help us learn more about therapy for treating retinal diseases in dogs and humans."
Identifying RD/OSD Carriers
When Dianne Walsh of West Chester, Pa., and her husband fostered, and then adopted, a 10-week-old yellow Labrador Retriever puppy named "Angie," they began learning about retinal dysplasia/ oculoskeletal dysplasia. Angie was born blind due to the hereditary disease. Today, one would not suspect the 8-year-old is blind.
"When Angie was a puppy, the ophthalmologist found she had no retina in one eye and only shreds of a retina in the other eye," Walsh says.
Walsh is past president of LABMED, an organization that has helped fund and care for 1,250 rescued Labradors and Labrador mixed breeds since it was founded in 1996. The Walshes couldn't help but fall in love with the blind, bowlegged puppy.
"Angie has amazed us from the start," says Walsh. "Despite being blind, she considers herself normal. She can figure out her surroundings in a matter of minutes and is a fearless explorer. We forget she's blind because she rarely bumps into anything." A type of retinal dysplasia, termed retinal folds, occurs in a benign form in many breeds. The retinal layer may fold over and prevent normal focusing. A few small folds seldom cause a change in vision, but larger areas may cause blind spots. Folds are noticeable in puppies from 8 to 16 weeks of age. Some folds may disappear in adulthood. The mode of inheritance for most retinal folds is not well characterized.
In Labrador Retrievers and Samoyeds, retinal folds may indicate RD/OSD. Research by Greg Acland, D.V.M., DACVO, and Gus Aguirre, D.V.M., DACVO, of Cornell University and the University of Pennsylvania, respectively, led to discovery of the gene mutation for OSD in 2008. Since OptiGen began offering the genetic test, more than 1,000 Labradors have been tested for the eye disease.
OSD carriers, which inherit one copy of the mutated gene, often have significant retinal dysplasia, although silent carriers without retinal folds are possible too. Some dogs with retinal folds have the benign type and are not susceptible to OSD; however, without genetic testing for OSD, they do not pass an eye examination and are not eligible for CHIC certification. Their status changes if the owner shows test results indicating the dog is not a carrier for the OSD mutation.
"Because some carriers with retinal folds exhibit a mild or partial expression of OSD and other carriers do not have folds, the type of inheritance seen in OSD is autosomal dominant with incomplete penetrance or incomplete dominance," Pearce-Kelling explains. "If Labradors with retinal folds get a normal DNA result from the RD/OSD test, then they can qualify and pass the eye examination. Though a dog doesn't have the mutation for RD/OSD, it may have an unidentified gene causing the folds."
Practicing Careful Breeding
Experts generally advise breeders not to breed dogs that develop inherited eye conditions for which there are no genetic tests. Annual eye examinations are the best way to identify affected dogs for earlier treatment and to avoid using them in breeding programs.
Among the diseases without DNA tests is entropion, an eye disorder in which the eyelid and eyelashes roll inward in an abnormal inversion. Most cases are due to a dog's facial and eyelid conformation. Signs include excessive tearing, squinting, redness and facial rubbing. In severe cases, irritation of the cornea may produce conjunctivitis, keratitis or ulceration. Surgery, which can be expensive, corrects the disorder.
"One or both eyes can be involved," Helmers says. "It is not difficult to treat with surgery, but entropion still can be passed on. I know dogs that had entropion surgery yet ended up in a breeding program, which transmits the disease to the next generation."
Labradors also can develop cataracts. Congenital cataracts are present at birth, whereas juvenile cataracts occur in dogs under 6 and senile cataracts in dogs older than 6. Cataracts are a leading cause of blindness in dogs and humans. About 100 breeds are affected by hereditary forms, and some breeds may develop more than one form. Little is known about the genetics of cataracts, a condition in which the clear lens of the eye used for focusing develops cloudy spots that gradually inhibit light from reaching the retina. The opacity inhibits the lens from focusing light onto the retina. As the lens gets cloudier, the light reaching the retina is reduced until a dog eventually becomes blind.
Labradors are among the breeds prone to a type of juvenile cataract that forms under the lens capsule, or the sac-like covering of the lens, called posterior polar subcapsular cataracts (PPSC). These occur around 1 ½ to 3 years of age. Fortunately, the PPSC type progresses slowly and seldom interferes with vision.
Surgery to remove cataracts in dogs is almost identical to cataract surgery in humans. Advances in microsurgery make the procedure appropriate for younger, healthy dogs, resulting in more retained vision and greater success. Ophthalmologists who specialize in this surgery can take the lens out of the eye and replace it with an artificial lens, restoring vision effectively.
Concerns about these diseases, plus others, prompt breeders to vigilantly have annual eye examinations and genetic testing in their breeding dogs. This practice lies at the crux of reducing eye diseases in Labrador Retrievers.
Purina appreciates the support of the Labrador Retriever Club Inc. and particularly Fran Smith, D.V.M., Ph.D., DACT, the LRC health chair, in helping to identify topics for the Purina Pro Club Labrador Retriever Update newsletter.
ACVO & OFA Establish Eye Certification Registry
The American College of Veterinary Ophthalmologists (ACVO) and Orthopedic Foundation for Animals (OFA) recently announced the establishment of an Eye Certification Registry and Clinical Database for Ophthalmic Diagnoses.
Test results from examinations by board-certified veterinary ophthalmologists will be entered in the registry and become part of the clinical database. Dogs with normal results will receive OFA eye certification numbers valid for one year. The addition of eye examination results in the OFA database makes OFA the most complete resource of canine health screening results in the world.
OFA will share a percentage of its registration fees with ACVO Vision for Animals Foundation to support research leading to elimination of ocular diseases causing vision loss and suffering in animals. For information, visit the OFA website at:www.offa.org.