True seizures are rare in guinea pigs. If your guinea pig cries out, falls to one side, and starts twitching, it is most likely infected with a skin parasite known as guinea pig mange (Trixicara cavis). The seizure-like behavior is due to the intense itching sensation caused by the mites burrowing through the skin. Some guinea pigs may have mites and a normal coat of hair. Other guinea pigs will develop patches of hair loss. In the span of a few weeks, a guinea pig may become bald over most of its body. Read here for more information on guinea pig skin parasites.
In Arizona, true seizures are frequently caused by a guinea pig getting too hot and developing brain damage. Other common causes are ear infection, a head injury (getting dropped onto the floor), not enough vitamin C in the diet, infection, tumors, and poisons. Many guinea pigs may never have a cause found and this is termed "idiopathic epilepsy". If your guinea pig has seizured, do not delat getting to a veterinarian! An exam including bloodwork and other diagnostic tests should be done as soon as possible. If you suspect your guinea pig got too hot, this is a life-threatening emergency! Cool its feet with rubbing alcohol mixed with water and place it on a damp washcloth until you are able to get to the veterinarian.