"Inherited epilepsy, also called
"primary" or "idiopathic" epilepsy, has long been
recognized as a canine disease. It is seen in both purebred and
mix-breed dogs, but some breeds, like the Aussie, have a much higher
incidence than that seen in dogs as a whole. This is the result of the
inbreeding inherent in the development and maintenance of any purebred
population. The founders of any given breed will have only a subset of
the genes—good and bad—present in the canine species. Selection
over generations for desired traits will limit that gene pool further
and not all the genes retained will be for desirable traits. If, at
some point in a breed's history a particular sire or line of dogs
becomes predominant, inherited problems may start to arise seemingly
out of nowhere. Attempts to concentrate the desirable genes of the
select individuals, can inadvertently bring together whatever
undesirable genes are present.
Inherited epilepsy is often called
"idiopathic," which means of unknown cause. The terminology
is no longer accurate but was developed at a time when no one realized
that genes could cause the disease. Primary epilepsy is perhaps a
better term because it indicated that the epilepsy is not caused by
some other factor.
Epilepsy presents one set of problems to a dog owner and another to a
breeder. It is a terrible disease and not something any breeder wants
to produce, but avoiding it can be difficult. We don't know exactly
how it is passed from one generation to the next. Dogs may not develop
the disease until they are old enough that they may already have
offspring. Failure by some to disclose information can make it
impossible for others with related dogs to take effective steps to
limit risk of producing epileptic dogs.
While primary epilepsy is now recognized as
inherited, the mode of inheritance is not yet known. It is possible
that there are several genetically distinct types of epilepsy, though
any one breed will likely have only a single type. In Australian
Shepherds, it is clearly not dominant or one would see it following
clear lines from parent to offspring generation after generation. A
sex-linked condition would result in many more affected males than
females, which is not the case. This leaves either recessive or
polygenic modes of inheritance. In either case, a healthy parent can
produce affected offspring and both parents must be carriers.
For Australian Shepherds, the mode of
inheritance remains a mystery. There are currently two research
projects that are looking at the disease in the breed. One is
conducted by VetGen, LLP, a commercial laboratory working in
association with the University of Michigan. The other is at the
University of Missouri. Both need DNA samples from family groups
including both affected and non-affected individuals. A
"family" includes at a minimum affected dog(s), plus parents
and siblings."