Epilepsy is a term used to describe seizures that can not be explained by identifying a source of the seizure activity. So it is a pretty broad term really. It covers any unidentifiable brain disorder that leads to seizure activity. Congenital epilepsy normally shows up in by the time a dog is two or three years of age.

Brain tumours are possible in almost any case of seizuring but brain tumours often have some other identifiable clinical symptoms since they often damage nerves exiting the brain or the centres that control these nerves. So gait abnormalities, facial paralysis, vestibular disorders, blindness, or other signs of nervous system damage may occur with a brain tumour, helping to identify it. Viral illness, fungal diseases, trauma, vascular disease or other problems can also lead to seizures by direct effects on the brain. Liver disease, kidney disease, and hormonal disorders cause seizures by more indirect effects.
When nothing is found we fall back on the term acquired epilepsy to define the problem, even though it really just means we can't identify the problem. Seizure control is usually possible. When to start is a issue of some debate among veterinarians. The standard rule of thumb has been to use seizure control medications (usually phenobarbital) when seizures occur more than once a month. Some veterinarians feel that it is wiser to start sooner than this because it appears that "mirroring" and "kindling" of seizures can occur in dogs. Mirroring is when a seizure focus occurs in one side of the brain and then an identical site occurs in the other side of the brain after several seizures. Kindling is when the seizure focus in the brain develops strong enough pathways that it makes it easier for the seizure to occur -- almost as if the brain "learns" to seizure. It is possible that by controlling seizures quickly through the use of medications that these effects could be stopped and that may lessen the amount or duration of need for seizure control medications.

We try to decide on an individual basis what the potential for all of these risks are, whether the pet owner can administer seizure control medications on a set schedule and the risks of the medications themselves. Then we decide when to start attempting to control seizure activity. If a dog has a really violent seizure we may start immediately. If there is a mild seizure and then subsequent seizures occur at long time intervals we may never attempt to control them. This decision just has to be made on a patient by patient basis.