Ok, what it sounds like is going on is actually mites. With guinea pigs it is not uncommon for them to have mites. The seizure like activity that you are seeing is a kind of sensory overload. He may be so itchy that it causes this pseudo seizure response. It would be best to have him seen by your vet for a skin scraping, although it is not uncommon for it to come up negative. Treating him for mites should stop this shaking that you are seeing. They can carry these mites for a long time without causing any problems. You will also need to replace his bedding after each treatment. Depending on what your vet uses, it may be a one time treatment or several. I don't believe that this is a true seizure.
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