INHERITED EPILEPSY AND WORKING AUSSIES:
MYTHS, FACTS, AND QUESTIONS
An interview with C.A. Sharp by Kay Spencer
C.A.Sharp is the president of the Australian Shepherd Genetics Institute, an organization ‘dedicated to the increase and diffusion of knowledge of genetics in the Australian Shepherd, and the inherited diseases from which it sometimes suffers.’ She is a science writer and an internationally recognized lay expert on canine genetics and hereditary diseases.
Q: Why should breeders concern themselves with inherited epilepsy if they stay away from conformation lines? Aren’t working lines pretty free of epilepsy?
C.A.: Unfortunately for stockdog breeders, it is not only in the conformation lines or those dogs with a conformation dog back in their pedigree. On the positive side, it is not as frequent in any of the working lines as it is among the conformation dogs. However, one single popular sire who carries epilepsy genes could change that.
Q: What about the rumor that epilepsy came into working lines with those illegal Border Collie crosses. How true is that?
C.A.: If epilepsy had only been reported in those lines, this might be the case. However it has been found in lines that are not at all suspect for possible Border Collie crosses.
Remember that the very early dogs behind the conformation lines can also be found behind some of the working lines. A good visual reference for this is the web chart from All About Aussies . That excellent resource shows the interrelations among the various lines in early breed history. Genes those early dogs had could be found in both working and show lines.
Another point to ponder regarding the Border Collie origin theory is that our breed is very young. First registration occurred in 1957. Before that they were all ranch dogs and most people breeding them were more interested in having dogs that could do the work than the niceties of pure lines. It is extremely likely that part of our breed’s heritage includes dogs from almost every breed and type of stockdog extant in the American West in the first half of the 20th century and probably a few other types of dog as well.
Look at the historical pictures of very early dogs in the first ASCA Yearbook. Head and body type, coat and color were all over the map. Even without the recent alleged Border Collie crosses, there is probably some Border Collie (among other things) behind most of our dogs. They may or may not have been a source of epilepsy genes.
Q: What about trauma? Can’t seizures in working Aussies mostly be attributed to head trauma from being kicked, etc.? Can seizures be started by a kick but still be attributable to inherited epilepsy?
C.A.: Head trauma may cause a seizure immediately after a blow or, due to brain swelling, within a couple days. However, unless there was permanent brain damage it will not cause ongoing seizures for the rest of the dog’s life. Head injuries that severe are rare and are apt to cause more problems that just seizures.
A dog’s skull is much better protected from injury than yours or mine. Dogs are descended from wolves, which make their living catching big critters (sometimes very big critters) with hooves. Kicks are a common hazard. If wolves’ skulls were readily subject to permanent brain damage gained in the course of hunting, they would likely have gone extinct long ago and we wouldn’t have dogs today.
The strong muscles that close the jaw attach along the ridges of bone along the top and back of the skull, wrap around the cheekbone (providing leverage for stronger grip) and attach to the jaw. These are sizable muscles in a dog with a relatively normal (wolf-like) skull shape, as is the case with Aussies. The brain case is wrapped in these muscles, cushioning the brain from blows, whether by a moose soon to be wolf food or a steer bound for your own dinner table.
A research study on mice found that head injuries could trigger the onset of epilepsy in genetically susceptible individuals. It is possible this might happen in dogs and other species, as well.
Q: I thought that IE has been proven to be a recessive trait. So, how does that work? What is a recessive exactly?
C.A.: First of all, it has not yet been proven to be recessive. One research group thought it might be based on analysis of the genealogies for two extended families, however they have yet to find anything in the DNA which proves it.
Epilepsy may be recessive. For our dogs’ sake I hope it is. A single gene will be easier for the researchers to find. However, it may also be multi-gene, or polygenic, as hip dysplasia is.
There is also a possibility that since our breed is of fairly recent origin and has a broad founder base, we may have more than one genetically distinct form of epilepsy. This appears to be the case for cataracts, another common inherited problem in the breed. A mutation has been found that causes 90% of Aussie cataracts. The remaining 10% are likely due to other, as yet unidentified, genes.
If we are lucky and Aussie epilepsy is recessive, it would pass in the same manner as red color. Dogs that were healthy could produce it if they were both carriers. If only one parent was a carrier, all the puppies would be normal though some of them would also be carriers because they would get the gene from their carrier parent. If a parent was affected, which can easily happen with epilepsy since it may not arise until after the dog has been bred, all the puppies would inherit a copy of the gene. Whether they would be affected or not would depend on the status of the other parent.
Q: Can dogs can “carry” and pass on the genetics for epilepsy for generations without ever having produced any epilepsy? How does that work?
C.A.: If the mode of inheritance is recessive or polygenic, yes. If frequency of the gene is low in a population, most matings will not produce the disease because only a few dogs carry it. Low frequency appears to be the case at the moment in working lines.
If epilepsy is polygenic it can also skip multiple generations because you have to put together the proper combination of genes to get the trait. This has been called the BINGO effect. If your dog has BIN, it will be healthy and it won’t produce epilepsy unless you breed to a bitch that has the GO. ( editor’s note: read C.A.’s article on polygenetic inheritance)
Q: Explain how linebreeding to a carrier of epilepsy will eventually manifest epilepsy in the progeny of that line.
C.A.: People linebreed because they want to increase the probability that they can get two copies of genes for the good traits a dog has. By having the dog, and perhaps some of his close relatives, one or more times on both sides of a pedigree a breeder increases the likelihood that this will happen. Unfortunately the flip side of that is that you also increase the probability that you will get two copies of genes you don’t want and every dog, even the very best, has some of those.
There is no such thing as a dog that is genetically clear for all the traits, including diseases, that you do not want. The tighter your pedigrees, the more likely you are to find out what those unwanted things are.
Q: How would breeding strategies change if there was a DNA test which identified carriers?
C.A.: We can use the DNA tests that are already out there as examples. At present there are tests for Collie Eye Anomaly, a somewhat common eye problem; Progressive Retinal Atrophy, a rare eye problem; and MDR1, the extremely common drug sensitivity gene connected to severe and even fatal reactions to Ivermectin, Acepromazine, Imodium, and other drugs.
These tests tell you the genetic status of your dog, whether it has two normal copies of the gene or if it has one or two copies of the mutated version. With a serious, often late-onset disease like epilepsy which has significant quality of life and economic consequences, you could identify affected dogs early enough that you could make sure they would never be bred.
If a dog were a carrier, you might use it for breeding, but only with mates that had two copies of the normal version of the gene. That way you would not produce any affected offspring and at the same time you could preserve the good qualities the dog has.
Q: Until such a test is available, what are the strategies we can use right now, to minimize our chances of producing epileptic pups?
C.A.: Because this disease is so serious, even lethal in some cases, and because both the dog and the people who live with it must deal with a potentially difficult, distressing disease that can be extremely expensive to treat, or may result in euthanasia should the owners be unable or unwilling to treat the dog, I recommend that no first-step relative of a dog with epilepsy be bred. First step relatives are parents, offspring and siblings. Other relatives might be bred so long as great care is taken to avoid mating them with dogs with a family history of epilepsy.
I use the term family history because you cannot consider only the direct ancestors that appear in a pedigree. You also need to know the status of the siblings, cousins, aunts/uncles, etc.
Q: What about genetic diversity? What is it, and why is so important?
C.A.: Genetic diversity is the amount of genetic variation within a population. The population under consideration might be a species, a breed, or some portion of a breed, like all the dogs in a particular country or the dogs from a certain line. The smaller a population is, the lower the available genetic variation tends to be because some gene variants will have been lost to that group through a process called genetic drift.
Aussies are a very populous breed with many more founders (upwards of 350) than most breeds have. Unfortunately, breed history since the time of those founders in the early to mid-20 th century has reduced diversity.
The breed’s average coefficient of inbreeding (COI) is presently running around 13.5%. This is more closely related than a half-sibling mating where there is no other relationship than the common parent.
Some of this is normal in the development of a breed. As early breeders focused on what traits were and were not appropriate for Aussies, dogs deemed inappropriate, like those with lots of white, for example, were passed by. The white marking gene version that causes extreme white, as you see in many other breeds of dog, is now at extremely low frequency. It is very unusual today to see a mostly-white Aussie that is not a double merle.
Most of the diversity reduction in Aussies can be seen in the present-day structure of the breed. The vast majority are of the conformation lines. These dogs actually represent a single mega-line, as they all trace back largely to the output of one kennel (Flintridge) in the late 1960s and early 1970s.
The balance of the breed is, for the most part, represented by the working lines. There are also performance dogs used for agility and other events, however I don’t think anyone has developed a distinct line of performance dogs as of yet.
The working lines are more varied than the show lines. Some are tightly bred while others are much less so. I have noted, both through looking at pedigrees and in talking to stockdog breeders, a tendency to keep a number of the lines largely separate, the argument being that dogs from other lines are not adequate to the specific needs of the breeder or breeders of the line under discussion.
When you take a relatively small population of dogs, as the stockdog lines are, and you further subdivide it by more-or-less exclusive bloodlines, each of those bloodlines will have only a small amount of the genetic diversity available in the breed as a whole. The exclusive lines will also have less diversity than what is available in the overall stockdog population.
The problem that lack of genetic diversity presents is that your dogs may not have the genetic tools to deal with new conditions. Those conditions may be new strains of livestock, different management practices, or a new infectious disease. The problem might also be an insufficient supply of the good versions of a gene that can cause disease or deformity.
Editor’s Note: more questions about epilepsy in Aussies are answered in this article on the ASHGI website.
© C.A.Sharp 2007. For reproduction guidelines and limits, go to ASHGI Legal Stuff (link will take you off-site)