Dogs definitely have a wide variation in seizure activity but it is hard to say whether seizures which might not cause recognizable clinical signs occur because it is so hard to judge the mental state of the dog. I assume that almost any type of seizure possible in a human probably occurs in dogs as well, though.
The Difference between Dog Seizures and Convulsions
Most people use the terms seizure and convulsion interchangeably but it is probably more accurate to say that the seizure is the brain activity that leads to the physical symptom of convulsing.
Convulsions can occur as the result of toxins and there have been reports of them occurring due to allergies in people. I am not sure if this has been documented in dogs. If there is a problem with food or with the stuff your dog eats on her runs, allergy to a plant or a particular ingredient of the dog food is the most likely problem. In this case, changing foods will only help if the offending ingredient (like beef, chicken, food coloring, etc.) is not found in the new food.
Causes of Primary Epilepsy in Dogs
It isn't unusual at all for primary epilepsy (seizures for no discernible cause) to begin as late as 3 years of age or even later. However, it is always good to maintain a high degree of suspicion that there is a discoverable cause of the seizure activity. To help in your thought processes, here is a list of some causes of seizures:
trauma -- even if it happened a long time ago, primary epilepsy, infectious diseases, shunts in the liver circulatory system, low blood calcium levels, low blood sugar levels, high blood sugar levels (diabetes), kidney damage, toxic substances (antifreeze, lead, insecticides and strychnine are the ones we have seen causing seizure activity), liver failure and possibly hormonal disorders such as Cushing's disease and hypothyroidism.
There are also things that people sometimes mistake for seizures. These are heart disease causing fainting, sleep disorders (narcolepsy), peripheral vestibular syndrome, inapparent sources of pain leading to odd behaviors, muscle tremor disorders and obsessive/compulsive disorders that lead to repeated behavior patterns.