A Case Study
PRA What You Can Do (Affected Dogs Needed)
DNA Testing
Progressive Retinal Atrophy
PRA GENETICS
PRA Research in IG's - Please Help
PRA Testing
Progressive Retinal Atrophy- I Still Can't Believe
It, But It Happened To Me!
Mary Ann Smart, Viva Italian Greyhounds
PRA. A disease I had heard of, thought I understood,
and which I believed did not apply to me. My dogs' eyes had always
been normal when examined by an ophthalmologist, and all my breeding
stock had CERF (Canine Eye Registry Foundation) numbers. None of my
dogs, to the best of my knowledge, was affected by or had produced
PRA. I am well aware animals not affected with a particular problem
can be carriers and produce the problem, but this particular problem
never crossed my mind when I planned Cyd's (Ch. Viva's the Carioca's)
last litter, at age seven.
Cyd had proven to be the brood bitch breeders hope for, an easy whelper, a
good mother, and she produced a large percentage of puppies that had the qualities
I seek. I chose to breed her to her grandson, Jordan, (Ch. Viva's It Sure is
Monday). She produced five puppies from that litter, four of which finished
their championships, bringing her total champion get to the impressive number
of 20!
The male I kept out of the litter proved to be a dog that I loved for many
reasons. Clay (Ch. Viva's the Dance) achieved his championship in two specialty
weekends with 4 majors, at 8 months of age. He is not only a lovely show dog,
but has a wonderful, outgoing temperament and is a real ambassador for the
breed. He also passes my acid test, which is being a joy to live with and being
beautiful in my yard, and on my couch.
By the time this litter was three years of age, several had been examined by
an ophthalmologist and been issued CERF numbers. One of the puppies, Zorro
(Ch. Viva's Dark Secret) is owned by Sherry Phillips. Zorro had a normal eye
exam at two, and at three, Sherry took him in for his annual CERF exam. After
learning the results, Sherry called me. According to the ophthalmologist, Zorro
was suspected of having PRA. Another diagnostic test, an ERG was recommended
to confirm the diagnosis.
I could not believe it. I was in complete denial. There must be a mistake,
this would be resolved shortly! All we needed was another opinion. After a
couple of long agonizing weeks, the ERG confirmed the diagnosis of PRA.
Words cannot describe my feelings....... How could my wonderful Cyd be a carrier
of PRA? Then the realization of the implications of Cyd being a carrier swept
over me. What had I done? How would this affect Cyd's offspring, and their
offspring? I was overcome by the prospect of Sherry's beloved family pet possibly
going blind.
The night I read Sherry's e-mail with Zorro's ERG results, I could not sleep.
There was obviously nothing I could do. Cyd had been spayed years ago, and
is now 10 years old. The first order of business was to call anyone who had
littermates from this litter. Clay had sired three litters of puppies, those
breeders would need to be informed. While mentally organizing my plan of action,
I remembered that there was a carrier test in Irish Setters. Someone had developed
that test! I wanted to contact the Irish Setter Club of America, and get specific
information on the development of that test.
I got up, went to my computer, and sent Sherry an e-mail message. I asked her
if she could get on the phone, or on the Internet, and find this information.
Without intending to, we had a study group. An affected dog, his offspring,
parents, littermates, and willing breeders. Sherry wasted no time, and by the
next afternoon she forwarded to me an e-mail from Dr. Greg Acland at Cornell.
She had found the individual in charge of the PRA study in progress, and YES
he was interested in this family of Italian Greyhounds.
I forwarded this message to my friend, Dr. Teri Dickinson, who had a shared
interest and to whom I had already confided the results of Zorro's eye exam.
Teri is a licensed veterinarian, and the chair of IGCA's Health Committee,
and has the credentials to mange and coordinate such an overwhelming project
with Dr. Acland and the IGCA, which needed to be involved.
All of a sudden, this unthinkable, unrelenting situation had a ray of hope.
A carrier test, the opportunity to identify carriers of an insidious disease
that lurks in our gene pool. At this point, there is no way of identifying
dogs that harbor the gene that produces this problem. Both Zorro's parents
have normal eyes, and the clinical signs do not appear in affected animals
until two and a half years of age or older. It dawned on me that I had done
nothing wrong. I had simply bred two dogs with normal eyes, and created a nightmare.
Teri contacted Dr. Acland, and in a matter of weeks, we had in our hands the
kits to draw blood for the study. Blood samples were collected from all of
Zorro's siblings, his parents, and his four offspring. As a part of the study,
all the dogs had their eyes re-examined, and ERG's were done on Zorro's offspring.
I made an ophthalmologist's appointment for my dogs. At this point, my outlook
and goals had changed for this group of dogs. Cyd had long ago been spayed,
Jordan would be neutered, but the offspring, Clay and his sister Ivy, both
had CERF numbers from March of 1996, and had a slim chance of not being carriers.
The order of priority that day when I walked into the vet's office was first
that Clay and Ivy not be affected, and hoping against hope, that eventually
Clay could be proven not to be a carrier. Cyd, Jordan, Ivy and Jordan's sire,
Sam (Ch. Viva's Leaving Cheyenne) all checked normal. Then, as though I had
not had enough bad news, my lovely Clay was diagnosed with PRA. His prognosis,
ultimately blindness. I had already prepared myself for this information, and
now resigned myself to his fate. He will grace my back yard, and my couch,
and do his most important job, be my pet for the rest of his life. He will
never be bred again. He and his father, Jordan, will be neutered.
Being a dog breeder seems to sometimes make us pay a heavy price. Our dues
seem never to be paid. A novice once asked me when she would be through paying
her dues, and I told her, "Never."
The work goes on. More affected dogs and their relatives were needed for Dr.
Acland. Phone calls were made by Sherry, Teri and me. Other affected dogs have
been found, and by the time you read this article, these dogs will have contributed
blood samples, thanks to the breeders and owners who are willing to participate.
With our cooperation, and the effort and talent of the research team, we are
well on our way to a carrier test.
In the past, when a breeder produced a dog with a genetic problem, the only
thing to do was spay/neuter the animal, removing it from the gene pool and
creating a sense of frustration, and a "back to the drawing board" attitude.
In this case, we found somewhere to turn with the PRA. I take comfort in the
knowledge that some good for our breed may come out of all the heartache caused
by this disease.
I have had many phone calls, from people interested, sympathetic or seeking
information on their related dog, and one from a friend offering to lease me
a valued bitch to breed, from an unrelated pedigree. I hope this article will
answer any questions, dispel any rumors, and raise the awareness for those
of you with an interest in the breed. If we wish to correct a problem, we must
first admit that we have problem, as denial is not an appropriate response.
PRA did not originate with my dogs! This pedigree is full of popular top producers
in the breed, some of whom had to be carriers. It is not a matter of simply
avoiding Cyd and Jordan in a pedigree, as there are many unidentified carriers
in the breed, and at present it is not possible to identify a carrier, unless
it has produced an affected offspring.
The only thing we can effectively do is to have our dogs' eyes tested by an
ophthalmologist. If you breed to someone's stud dog, ask to see their CERF
form. It only costs $7 to send the report to CERF, and $5 to renew annually.
Don't breed a bitch that does not test normal and don't buy a puppy that does
not have normal parents. In addition, don't breed offspring or littermates
of affected dogs. Don't breed dogs less than three years of age, as the onset
of PRA in this breed seems to be between two and four years.
Breeding purebred dogs is difficult at best, and to not utilize the best testing
available at this time is a little like shooting yourself in the foot. I believe
we have some responsibility to prevent when possible. In many large breeds,
OFA numbers are required before a dog can be advertised, why can't we consider
CERF just as important in IG's?
If you have an affected dog, get involved. If you have anything related to
these two carriers, you are at risk. After having spent a lot of time and effort
researching this problem, it seems to be more widespread than we realize. Many
PRA affected dogs are being "swept under the carpet."
This has completely changed what I will do in the future with my dogs. I have
Cyd great-grandchildren, and one Cyd son I still have hopes for, if a carrier
test is developed. For the next couple of years, I will show the dogs I currently
have, enjoy all of my dogs, and treasure the time I have with them.
Sherry once said "Italian Greyhounds are not an endangered species, we
need not breed the ones with a genetic disorder." I will continue to love
and protect this breed, and breeding this particular group of dogs may never
be an option again. Perhaps Cyd's greatest contributions to her breed will
not be those 20 champions as I once thought, but instead, spurring us on to
find a carrier test for PRA.
Reprinted with permission from The Italian Greyhound
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PRA - What Can You Do? (Affected Dogs Needed)
1. If you have any PRA affected dogs, they may be of
great value to the researchers studying the disease. All that is needed
is a blood sample and pertinent pedigree information. If you have an
affected dog, please contact the IGCA Health Committee (see below)
for a confidential referral to the research program.
2. Have your dogs eyes checked yearly by an board certified ophthalmologist.
While symptoms of sight loss are not apparent until a slightly later age, an
ophthalmologist can often detect changes in they eyes of affected dogs as early
as 2-3 years. Make sure to ask for a "CERF" (Canine Eye Registry
Foundation) exam. Results from these eye exams are placed in a database which
may help us learn more about the incidence of the disease.
3. Breed only dogs whose eyes are normal, and wait until three years of age
to breed. Make sure the parents of breeding stock have normal eyes, and have
not produced any PRA affected dogs.
4. Any dog who produces a PRA affected dog is a known carrier and should be
eliminated from the breeding pool. Any dog with an affected littermate has
a 75% chance of being either affected or a carrier and should not be bred.
Any offspring from known carriers have a 50% chance of being carriers, and
should be bred very carefully if at all. Affected dogs will produce 100% carriers
even if bred to normal dogs, and needless to say, should never be bred.
All of this will change, when a carrier test becomes available, as we will
then know exactly which dogs can and cannot be safely bred. In order to expedite
the research process, PLEASE contact us if you have a PRA affected dog.
All in the information you need for submitting samples
can be found on the research contacts page. If you have additional questions, please contact:
IGCA Health Committee
4 Hillcrest Dr.
Lucas, TX 75002
dickinsont@mindspring.com
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DNA Testing
At this time, no DNA test for PRA has yet been released
for IG's. The following links are to provide the
fancy more information about the DNA testing:
Optigen
Canine
Genetic Diversity Site
Canine
Genetics Site
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Progressive Retinal Atrophy
Teri Dickinson DVM
Progressive Retinal Atrophy (PRA) is an inherited condition known
to occur in many breeds of dogs. The initial onset of disease occurs
at different ages in different breeds of dogs. Dogs with PRA gradually
lose eyesight due to degeneration of the retina. There is no treatment
for PRA.
The retina is a layer of cells in the back of the eye which absorbs
light, and sends the appropriate electrical signals to the brain. Two
types of cells are involved in light reception, rods and cones. Rods
absorb light in low light situations, and are therefore essential to
night vision. Cones allow better visual discrimination and color vision
(in species with color vision), but require greater levels of light
to function.
PRA is characterized by a loss of rod cells initially. The effect
of this is that the dog may have, or appear to have, normal vision
in daylight, but is cautious and disoriented at night. This disease
is sometime referred to as "night blindness." Eventually
the cones are also affected, and the dog gradually loses day vision
as well. Most PRA affected dogs eventually become blind entirely.
Because of the dog's ability to move around well in familiar surroundings,
vision loss is often quite advanced before the owners notice the dog
has a problem. Often the vision loss becomes obvious only when the
dog is taken to a strange environment.
The eyes have no obvious signs of abnormality, and the condition is
not painful. As the disease progresses, the owner may notice the dog's
pupils remaining dilated, as the eye attempts to take in more light.
The eye may seem to "glow" in the dark. This is due to the
retina reflecting light back out through the pupil. As it loses its
layers of rods and cones, the underlying tissue allows light to be
reflected, instead of absorbed. This is called hyperreflectivity. The
eyes may resemble those of a cat, which normally has a very reflective
retina.
Diagnosis is made by doing an ophthalmic examination of the eye. The
veterinarian looks for the increased reflectivity of the retina, and
a decrease in the numbers and size of blood vessels supplying the retina.
An electroretinogram (ERG) is a special test which measures the electrical
impulses created by the retina. An ERG can be used to diagnose PRA
before the characteristic changes are seen on ophthalmic exam. While
useful, ERG is usually performed under general anesthesia, and is only
available at large veterinary teaching hospitals.
The cause of the retinal degeneration is not yet understood. However,
the genetic nature of the condition is well described. Seven different
genetic types of PRA are known, but all eventually lead to the same
result. All but one is known to be a simple recessive mode of inheritance.
This means any affected animal has two parents which are either affected
or carriers, and that at least half of an affected dogs littermates
are probably carriers. Rod-cone dysplasia-1 (rcd-1) is a type of PRA
found in Irish Setters. Rod-cone dysplasia-2 (rcd-2) occurs in collies,
rod dysplasia (rd) and early retinal degeneration (erd) occur in Norwegian
Elkhounds. Siberian Huskies have a X-linked form of the disease, which
means the gene occurs only on the X chromosome.
The most common type of PRA is progressive rod-cone dysplasia (prcd)
which is known to affect Labrador retrievers and poodles and possibly
as many as sixty other breeds. Italian Greyhounds definitely have PRA
in their gene pool, and prcd is believed to be the form of PRA affecting
Italian Greyhounds.
While PRA can be easily diagnosed, it poses a dilemma for breeders
for two reasons. First, dogs may not develop signs until late in life,
making it difficult to avoid using affected animals as breeding stock.
No research has been done specifically on IG's, but it appears the
age of onset in IG's is 3-4 years. Second, there has not been a good
way to diagnose carriers, making it very hard to remove them from the
gene pool. Test breeding a suspected carrier to an affected animal
will confirm carrier status if affected offspring are produced, but
this is not a very practical solution.
In March of 1994, Dr. Gustavo Aguirre and his associates at the Baker
Institute for Animal Health, took a giant step forward in the eradication
of PRA. They successfully identified the gene causing rcd-1 in Irish
Setters, and developed a blood test which can identify dogs with normal
DNA, dogs which are carriers, and dogs which are genetically affected
with PRA, even though they do not yet show signs. For the first time,
a method of identifying both carriers and affected individuals is now
available. This testing is being administered through the Canine
Eye Registry Foundation (CERF) at Purdue University. Dogs with
normal DNA receive a Canine DNA Registry number, and that information
is forwarded to the AKC.
At this time, there are no blood tests for the other types of PRA.
Currently Michigan State University is looking for the gene that causes
PRA in IGs. Click here for more information.
The Baker Institute is also pursuing this line of research. The Baker
Institute is one of the few research entities doing research on animal
health problems. Many research institutions make progress in animal
diseases as they search for solutions to human problems, but the Baker
Institute is dedicated solely to animal health. If you'd like to make
a contribution to assist in their work, the mailing address is:
James
A. Baker Institute for Animal Health
College of Veterinary Medicine
Cornell University
Ithaca, NY 14853
(607) 256-5600
Little is specifically known about the age of onset and incidence
of PRA in Italian Greyhounds. More data is needed in order to form
a clear picture of this disease. You can help in this process by having
a CERF exam done annually on your dogs.
Having your dog's eyes examined by your regular veterinarian during
his annual physical exam is a useful tool in helping maintain your
dog's health. The eyes often reveal signs of underlying disease. However,
to gather data about, and diagnose, inherited eye diseases in dogs,
a yearly exam should be performed by a board certified veterinary ophthalmologist.
Ophthalmologists have specialized equipment not found in generalized
veterinary practices, and years of extensive training recognizing and
diagnosing inherited eye disease. In addition, only veterinary ophthalmologists
are permitted to enter data into the CERF database.
Veterinary ophthalmologists often give discount rates for eye exams
if they are done for CERF. The advantage of a CERF exam is that a copy
of the information about every dog's eyes is sent by the veterinarian
to CERF. There it is entered into a large database which contains information
about eye disease in dogs. As more IG's have their eyes examined, more
data will be available about the diseases present in the breed.
The CERF database is what is known as a "closed" database,
which means that only statistics are released, not information about
individual dogs. The exception to this lies in the process of registration.
At the time of the exam, a copy of the data is also given to the owner.
If the owner chooses to send in his/her copy along with the identifying
information about the dog, any dog with no major inherited eye abnormalities
will receive a registration certificate, good for one year.
The names of dogs whose eyes are registered are released by CERF,
and this information is passed on to the AKC where it appears on registration
certificates, pedigrees and may now be added to a dog show catalog.
What should you do? Have as many dogs' eyes as possible examined by
a veterinary ophthalmologist, using the CERF form. If you do not know
of an ophthalmologist near you, your veterinarian or a local veterinary
association should be able to refer you to the closest one. Read your
premium lists carefully. Eye clinics are sometimes held at all-breed
shows, and offer very reduced rates.
Having CERF exams performed will add to our knowledge of inherited eye diseases
in dogs. Have the exam repeated annually, or as often as you can. As the onset
of PRA is late, a normal exam at one year may only create a false sense of
security.
Encourage people who buy dogs from you to have their dog's eyes CERF'd
as well. Even if they are not used as breeding stock, they add to our
knowledge of the gene pool, and the incidence of inherited eye diseases.
CERF publishes an informative pamphlet, Eliminating Heritable Eye
Disease in Purebred Dogs. It is available at no charge from CERF
at:
CERF
1248 Lynn Hall
Purdue University
W. Lafayette, IN 47907-1248
(765) 494-8179
Fax (765) 494-9981
Here's what we do know about inherited eye disease in IG's. From January
1991 through December 1994, a total of 343 IG's were entered in the
CERF database. Of these, 65 (19%) had one or more eye abnormalities.
Not all of these abnormalities may have been considered major, nonetheless, 1
in 5 IG's examined did not have completely normal eyes. There
are eye problems in the breed. The more we learn about them and their
incidence, the better off we are.
Food for thought. Consider this scenario. You are interested in finding
a stud dog to breed to your precious angel. You shop around and select
two five year olds you think are compatible. You ask about eye certification.
Dog A had a normal CERF exam at a dog show at 18 months. Dog B has
a current CERF number, from an exam conducted when he was 4 1/2 years
old. Additionally, his parents have current CERF numbers at 8 and 9
years of age respectively. Two of his three littermates have current
CERF numbers as well. The third was sold as a pet and was lost to follow
up. Which dog would you be more comfortable breeding to? Which is statistically
less likely to be a PRA carrier? By the way, what do you know about
your bitch and her relatives?
Copyright The Italian Greyhound
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PRA GENETICS
Teri Dickinson DVM
In discussing the inheritance of genetic diseases we
are usually limited to theoretical discussions "if Dog A, were
bred to Dog B," etc. In the case of PRA, a few courageous individuals
have stepped forward, and now allow us to examine the inheritance of
this condition, using real dogs, and real situations. For this, we
owe these people a great debt of gratitude.
The mode of inheritance of PRA (progressive retinal atrophy) has yet to be
proven in Italian Greyhounds. However, based on the research in most other
breeds, at this time, it seems safe to assume that in IG's PRA is inherited
as a simple recessive. For those of you whose Mendelian genetics courses took
place a few years ago, let's review how a simple recessive works.
For every gene an animal possesses, it has inherited DNA from each of its two
parents. In many cases, the DNA provided by one parent is different from the
DNA provided by the other parent. It is the interaction between these two pieces
of DNA that determine the animal's phenotype (what we see) and its genotype
(the actual genes it carries). This is true in dogs, as in other species, and
we will now limit our discussion to dogs.
Most genes have multiple "alleles" or choices that can be present
at that site. Take the white markings in IG's for example. This gene has four
alleles, solid, Irish marked, piebald (pied) and white. All IG's carry a copy
of that gene that they inherited from each parent. Some have two copies of
the Irish allele, some have two copies of the pied allele, and some have one
copy of pied, and one copy of Irish. A dog that has one of each appears Irish
(his phenotype), because the Irish gene is dominant and the pied gene is recessive.
So when we see the dog, he is Irish, but actually his genotype is Irish with
a pied recessive. When we breed this dog, he may produce pieds, since he is
carrying the pied recessive. His phenotype does not reveal the true story of
his genotype, and by looking at the dog, we have no way of knowing by his appearance
that he carries the pied gene. This is a common situation with a recessive
gene.
In discussing Mendelian genetics, certain common shortcuts are used to express
a dog's genotype. Each gene is assigned a letter, and various abbreviations
for each allele are constructed. Continuing with the example of white markings
in IG's, the gene is known as the "S" or spotting gene. The dominant
gene is represented by a capital letter, "S", and the recessive allele
uses lower case, "s". In the case of S, there are four alleles, so
other letters are used to identify them, S=Solid, which is dominant. Si = Irish
(white collar and legs). Sp= piebald (pied, white background with patches of
color) and sw=white. S dominant to all alleles, Si is dominant to Sp and sw,
and Sp is dominant only to sw.
A dog's genotype is expressed as a combination of the two alleles he possesses
for each gene, one inherited from each parent. For the purposes of this discussion,
let's focus on Si and Sp which are the two alleles most commonly found in IG's
in the US. A dog with a phenotype of Irish, could actually have a genotype
of SiSi (inherited the Irish gene from both parents) or SiSp, (inherited one
Irish gene and one pied gene). Because Irish is dominant, remember that the
dog's phenotype in either case is Irish. By looking at the dog, we cannot tell
if he carries pied or not. To help us predict what markings will appear on
the puppies when two dogs are bred, we use a Mendelian square. We place the
genes available from one parent on the top and from the other parent on the
left side of the square, and then fill in the square using all possible combinations.
Each square in the grid then represents the possible genetic makeup of the
offspring.
If two Irish marked dogs, both of which carry pied recessives, are bred together,
our square looks like this:
| |
Si |
Sp |
| Si |
SiSi |
SiSp |
| Sp |
SiSp |
SpSp |
From this we can conclude that one of four puppies will have a phenotype
of SiSi, and will be Irish marked, and will not carry the pied gene.
Two of the puppies will be Irish marked, but will carry the pied gene
(a genotype of SiSp or SpSi) and one of the puppies will be pied and
have a genotype of SpSp. So, 25% will be Irish, not carrying pied,
50% will be Irish, but will carry pied, and 25% will be pied. Phenotype
though would show us 75% Irish, 25% pied.
Let's take the case of Mary Ann's two dogs, Cyd and Jordan. Cyd is Irish, but
has produced pieds in the past, so we know her genotype is SiSp. Jordan is
also Irish marked, but has also produced pieds, so we know his genotype must
be SiSp as well. When bred together, we would have expected to get the 75%
Irish, 25% pied litter described above. However, when they were bred, they
produced 5 puppies, all Irish. So, why didn't our square work?
Well actually it did work. What the square produced was a prediction of all
the possible genotypes that could be created, and a statistical guess at how
many of each would appear. Which of the two genes each parent contributes is
a matter of chance. In dealing with a statistical analysis, five is a very
small number. It's like flipping a coin. If you do it enough times, it will
land heads half the time, tails half the time. But if you do it a few times,
could you possibly see heads five times in a row? Sure. Statistical predictions
only work well if you flip the coin a few thousand times.
The same principle applies here. Had Cyd and Jordan produced a few thousand
puppies, 75% would have been Irish and 25% would have been pied. Instead we
saw that five Irish puppies were produced.
Nonetheless, while all the puppies had the same phenotype (Irish) we can predict
that some had the genotype SiSi and don't carry pied, but that some have the
genotype SiSp and do carry pied. Bred to other Irish marked dogs, these dogs
may produce pieds, depending on the genetic makeup of their mate, while the
SiSi dogs cannot provide a pied gene, and will not produce pieds, even when
bred to one.
The genetics for PRA work the same way. If we assign the PRA gene the letter "P",
the dominant allele, which is for normal eyes, can be assigned the letter P,
and lower case p can be used to represent the recessive PRA affected allele.
A dog may have normal eyes (phenotype) but have a genotype of either PP (normal)
or Pp (normal but carrying an affected recessive). A PRA affected dog has a
genotype of pp.
Let's get back to Cyd and Jordan. Both have a normal phenotype. Their eyes
are normal, they have current CERF numbers, and both are well past the age
of onset of PRA. Yet bred together, they produced, not one, but two PRA affected
dogs. How did this happen? Well let's use a square and examine the situation.
Even though their phenotype does not reveal their genotype, their breeding
history does. Since each has produced an affected dog, we now know their genotype
must be Pp.
Our square then looks like this:
We have then produced genotypes of 25% normal (PP),
50% normal but carrying PRA (Pp or pP), and 25% affected (pp). Phenotypes
though are 75% normal,
25% affected. Once again, using our small sample of five dogs, we see that
25% or one and a quarter dogs would have been expected to be affected. In
this case, it's our bad luck that two were affected. The other three have
normal eyes, and therefore a normal phenotype at this time. However, looking
at our square we know that some of these "normal" dogs actually
have a phenotype of Pp, meaning that they are PRA carriers, and can pass
the PRA affected gene to their offspring.
What happens when a carrier such as Cyd, is bred to a normal dog that is not
a carrier (genotype PP)? Then the square looks like this:
We see that phenotype on all those puppies is normal
(either PP or Pp) but that 50% have a genotype of Pp and are PRA
carriers. This explains why Cyd
was bred several times prior to being bred to Jordan and never produced a
PRA affected dog. Her other mates were probably not carriers, so 50% of the
puppies were normal, 50% were carriers, but no affected dogs were ever produced.
Not until she was bred to another carrier were affected dogs produced.
In this case, Jordan is a Cyd grandson. Therefore, it is highly likely, that
the Cyd son, Sam, has a genotype of Pp, as do 50% of Cyd's offspring. Sam bred
even to a non-carrier bitch will still produce 50% carriers, and it's probable
that Jordan has a genotype of Pp, and received his PRA affected gene from his
father, Sam. This then setup the situation where for the first time, Cyd was
bred to a carrier, and affected puppies were produced.
Line breeding dogs is done because it allows us to double up on the genetic
traits we desire, and strengthen them. Unfortunately, as in this case, it sometimes
also doubles up on undesirable recessives and brings them to light. The immediate
conclusion one might reach is that we should never line breed, but is that
a valid conclusion?
Had Cyd never been linebred, and this PRA carrier state not been detected,
would that have been a good thing? Well, it would have spared Clay and Zorro
from being affected, and certainly that would have been a good thing, but what
would have been the overall affect on the gene pool for the breed?
Many people, like me, would have unknowingly been spreading the PRA affected
gene even further, by using carriers in our breeding programs, not knowing
they were carriers. Now that we know Cyd and Jordan are carriers, everyone
who has dogs downline from them needs to strongly consider their next move.
50% of Cyd's offspring are carriers. Statistically, 25% of her grandchildren,
are carriers, and 12.5% of their children are carriers. Which are which? We
have no way of knowing which dogs are carriers, which are not, unless they
have produced a PRA affected dog. There is no test for carriers. The researchers
at Cornell and Penn are working hard on it every day, and hopefully there will
be a test very soon.
In the meantime, as the owner of a Cyd granddaughter (Bernie) and a litter
of babies, I have to accept the fact that there is a 25% chance that Bernie
is a carrier. There is a 12.5% chance my babies are carriers. Since I don't
like to breed until they are three anyway, I'm hoping there will be a carrier
test before I am ready to breed any of the babies. If not, I'll probably not
breed them. The Bern is five. I'll probably spay her in the next year or so.
If a carrier test is available before then, I may breed her one more time.
If there is no test, I won't risk breeding her again. My Cyd son, Kevin, is
already neutered, and busy holding down the couch.
If anyone has any more information about PRA affected IG's and would like to
participate in the IGCA's study with Cornell, please contact me at 4 Hillcrest
Dr. Lucas TX 75002 or e-mail dickinsont@mindspring.com.
If anyone would like to make a donation to the IGCA health committee, a special
fund has been designated for the health committee to be used in this study
and others as the board designates. Please send any contributions (made out
to IGCA Health Committee) to the above address.
Reprinted with permission from The Italian Greyhound.
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PRA Research in IG's -
Please Help
Presented by the Health Committee, Italian Greyhound
Club of America
Teri Dickinson, DVM, Chair
Progressive retinal atrophy (PRA) is an inherited disease known to
affect many breeds of dogs, including Italian Greyhounds. Affected
dogs are born with normal sight. Age at onset of symptoms varies from
breed to breed. While this disease has not yet been extensively studied
in IG's, experience seems to indicate that affected dogs begin to exhibit
signs of loss of night vision at the age of 3-4 years. A PRA affected
dog will gradually experience more and more loss of functionality of
the retina, and over time may go completely blind.
Not a pretty prospect is it? The possibility that your dog, or a dog you bred,
may be affected with this, as yet, untreatable disease. So, what can you do
to prevent PRA and further the study of the disease?
In most breeds that the method of inheritance of PRA has been determined, the
disease is inherited as a simple recessive gene. In simple terms, this means
that in order for an affected dog to be born, both the parents must be carriers
of the disease. When two carriers are mated, some of the puppies receive one
copy of the affected gene from each parent, resulting in an affected puppies.
Other puppies may receive only one copy of the affected gene, and are themselves
carriers, and some will receive the normal gene from both parents, resulting
in a dog that is not affected, nor is it a carrier.
So, preventing PRA it would seem, is a simple task. Just don't use a dog that
is a carrier in a breeding program. Simple, except for one thing. Right now,
the only way to identify a carrier is if it happens to be bred to another carrier,
and produces an affected puppy. Not a pleasant way to go about it.
There is some good news on the horizon. Researchers at the Baker Institute
at Cornell University, have successfully identified the gene that causes PRA
in one breed, Irish Setters, and have created a DNA test for carriers. Irish
Setter breeders can now through a simple blood test, learn the PRA status of
their puppies, as soon as they are old enough to draw blood.
Unfortunately, there are believed to be several different genes causing PRA
in dogs, and only the one in Irish Setters has been identified. Researchers
at both Cornell and Penn State Universities are working feverishly to identify
the locations of the PRA genes in other breeds.
Even after the location of the genes is known, there is still work to be done.
It must still be determined which genes are present in which breeds, so that
when carrier tests become available, the correct carrier test can be used in
each breed.
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PRA Testing
Teri Dickinson, DVM
Chair, Health and Welfare Committee,
IGCA
2/10/05
Drs. Acland and Aguirre are still working on a DNA test
for Italian greyhounds. In there work, they ahve determined that IGs
do not have the prcd form of PRA, so the test mentioned below is not
valid in IGs. They do believe IGs have a form of PRA that is inherited
as a simple recessive (autosomal recessive) and they are continuing
the hunt for the gene that causes this as yet unidentifed form of PRA.
4/19/99
Optigen today announced
that the linkage test for the prcd form of PRA will soon be available
in two additional breeds, English Cocker Spaniels, and Chesapeake Bay
Retrievers. At this time, the researchers at Cornell are not prepared
to release the test for use in Italian greyhounds. While this news
is disappointing to the IG fancy, it is important to keep in mind that
a test which provides misleading information would be far more damaging
than no test at all.
Drs. Acland and Aguirre at the Baker Institute were initially
quite pleased with the results of this test when applied to IG's and
up until recently did believe they were close to validating the test
and releasing it for the above two breeds as well as for IG's. However,
under close, recent scrutiny, they found enough questions still remaining
about the efficacy of the test in IG's that they felt they had no choice
but to continue to look for answers, rather than releasing the test.
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