PRA Is One of Several Inherited Eye Diseases
Despite living in Norway, an ocean and a continent away from the U.S., Line Leret manages the website for the Health & Genetics Committee of the Papillon Club of America. A recent posting - and one that Leret takes to heart - is a request for blood samples from Papillons diagnosed with progressive retinal atrophy (PRA), an inherited eye disorder that can lead to blindness.
The blood samples, along with those from healthy, unaffected Papillons, will be used by researchers at Michigan State University Veterinary Medical Center to study the genetics of PRA in Papillons. Their goal is to discover the causative mutations, although it is possible that more than one form of PRA occurs in Papillons, which would make the research challenging.
PRA is the disease that got Leret involved with the Health & Genetics Committee as a volunteer. It started in 1993 when Leret, of Jessheim Norway, noticed that her 3-year-old Papillon, "Speedy," had begun proceeding slowly and cautiously on stairs, especially on gray days with poor lighting. The behavior represented classic signs of PRA. Leret had heard about the eye disease but couldn't find any Papillon owners whose dogs had PRA.
Though PRA is not painful and the outward appearance of the eye is often normal, the eye disease has a gradual progression, which is why early signs are often overlooked. The retina, located at the back of the eye, takes the light gathered and focused by other eye structures and converts it into electrical nerve signals to send to the optic nerve and then the brain for interpretation. Though there are many different types of PRA caused by different mutations, all have similar signs.
Affected dogs first experience night blindness due to degeneration of the rod cells inside the eye that help them see in dim light. Dogs especially may have trouble going from a lit to a dark environment. They may stumble on stairs, especially in unfamiliar surroundings. This is followed by degeneration of the cone cells and the inability to see bright light.
A veterinary ophthalmologist can screen for PRA before signs become obvious. An electroretinagram (ERG) is used to measure the electrical response to light by the rod and cone cells in the back of the eye. In later stages, a veterinary ophthalmologist can look at the back of the eye through an ophthalmoscope to diagnose PRA.
Speedy's PRA progressed slowly, which gave Leret time to help the toy dog adjust. She gave him directional cues to help navigate and was careful not to move furniture or leave items in his path. "Speedy almost never bumped into anything," Leret says. "If he was unsure of what was in front of him, he just stopped and waited for me to tell him where to go."
When Speedy died at age 11, he was virtually blind, although he could still make out objects if the light was bright enough. As she mourned the loss of her beloved dog, Leret became dedicated to wanting to learn more about PRA. As she got involved with the Papillon Club of America's Health & Genetics Committee, she found a purpose in helping to advance the genetic research via managing the website.
A Large Group of Genetic Conditions
Progressive retinal atrophy is a large group of genetically distinct eye conditions affecting many breeds of dog that involve gradual deterioration of the retina and eventually lead to blindness. In humans, more than 200 mutations are believed to cause retinal disease, but in dogs far fewer mutations have been discovered.
Simon Petersen-Jones, D.V.M., Ph.D., the Myers-Dunlap Endowed Chair in Canine Health and professor of comparative ophthalmology at Michigan State University, is the lead investigator of the genetics study of PRA in Papillons and Italian Greyhounds. "PRA in Italian Greyhounds appears to be recessively inherited, but it is not that simple in Papillons," Petersen-Jones says. "One form causes an early-onset vision loss, and a second form appears to cause a slower loss of vision."
The research involves a candidate gene approach looking at genes known to cause PRA in other breeds to identify possible genetic markers and a genomewide association study comparing the genes of affected dogs with healthy ones.
Collecting blood samples to study DNA is imperative. "We need more samples from dogs diagnosed by PRA and from older, normal dogs that have recently had an eye examination," says Petersen-Jones.
DNA tests that detect the progressive rod-cone degeneration (prcd) form of PRA already are available for Silky Terriers and Yorkshire Terriers. Several toy breeds are susceptible to this form. The DNA tests are offered through OptiGen, a genetic testing laboratory in Ithaca, N.Y.
Sue Pearce-Kelling, president of OptiGen, says it is likely there is more than one type of PRA in Yorkies. "We are working on more DNA tests and welcome samples from any purebred dog diagnosed by a veterinary ophthalmologist as having PRA," she says.
Other Inherited Eye Diseases
PRA is not the only hereditary eye disease affecting toy breeds (See Eye Disorders in Toy Breeds on page 2.) The American College of Veterinary Ophthalmologists (ACVO) publishes Ocular Disorders Proven or Suspected to be Hereditary in Dogs, listing several inherited eye diseases.
Parent clubs also track eye diseases and may require a CERF (Canine Eye Registration Foundation) examination for a dog to receive CHIC (Canine Health Information Center) certification. CERF testing must be performed by a board-certified veterinary ophthalmologist, and results should be registered with CERF.
Though there still is much to learn about canine eye diseases, breeders share a goal of wanting to produce healthy dogs that retain their vision throughout their lives. Careful screening and selection of breeding partners help to reduce the incidence of inherited eye diseases. The potential of producing healthy, seeing dogs is the reward.