Epilepsy, eclampsia, catalepsy, and chorea (St. Vitus' dance) are all disorders of the nerve centres, and are associated with eccentric muscular action, and often with derangement of consciousness, but horses are very rarely attacked by any of them.
Epilepsy is a peculiar affection almost unknown in the horse, but not uncommon in the dog. The exact pathological conditions of the brain or spinal cord on which the intermittent attacks or epileptic fits depend are not known. It is even uncertain whether the origin of the malady is centred in the brain or spinal cord, although recent experiments lend considerable force to the view of the brain rather than the spinal cord being the seat of the derangement. Characteristic epileptic fits may be produced in dogs while under chloroform after complete disconnection of the brain from the spinal cord. Injection of a minute dose of absinthe into the circulation in an animal thus prepared is quickly followed by all the signs of epilepsy, and dogs utter maniacal cries, which, of course, are purely the result of reflex action, the dog being at the time unconscious.
The Symptoms of an epileptic fit are well known. Usually there is no marked premonitory sign of an approaching attack. An animal in a state of health, apparently, may suddenly reel and fall over on its side, in the case of a dog uttering cries which soon cease, while the whole muscular system is in a state of convulsive action. Urine and faeces are involuntarily discharged; a quantity of foam collects about the mouth. In a short time the convulsions cease and the animal regains consciousness, and is soon restored to its ordinary condition, showing no further symptoms of illness until the sudden occurrence of another fit.
Considering the great difficulty of disconnecting severe forms of megrims in the horse from epilepsy, it is not remarkable that some writers record cases of equine epilepsy. It is, however, rarely if ever the case that the horse suffers from this disease, and certainly not in that typical form in which it is seen in the dog.
Several forms of epilepsy are described by writers, for example, spontaneous, symptomatic, traumatic, and reflex epilepsy, and in all these the attack may be serious or benign. Spontaneous epilepsy is the result of functional disturbance of the brain, amounting to general irritability, which disposes the subject to an attack under trifling influences, such as fear or any kind of mental excitement. Horses are said to have suffered when alarmed by a display of fireworks, or the passing of a train, or from suddenly passing from a subdued to an intense light. Symptomatic epilepsy is the form of the disease which is associated with structural changes in the central nervous system, as thickening of the membranes of the brain, deposits of pus, or the presence of parasites. Traumatic epilepsy is due to injury, as, for example, blows upon the cranium causing compression of the nerve structures from effusion of blood or the depression of the bony boundaries of the cavities in which the brain and spinal cord are contained. Reflex epilepsy may occur in consequence of irritation affecting the terminal branches of nerves in remote parts. Such irritation depending upon pressure exercised by foreign bodies, irritation caused by parasites in the digestive organs, affections of the mouth due to the changes which occur in course of dentition. All the above forms of epilepsy, when connected with special liability to nervous excitement, may be considered as hereditary.