Rapidly growing tumors, like cholesteatomata, are liable to induce recurrent attacks of encephalitis in connection with periodic irritation.
Finally parasites in the cranium are sufficient causes of attacks. In the New York State Veterinary College Museum is the brain of a cat with a nematoid wound round the hypophysis. In equine subjects suffering from the strongylus armatus the larval worm or clots caused by its presence in other arteries sometimes invade the encephalic blood vessels causing disturbances of the circulation, embolism, inflammation or degeneration. (Albrecht, Von Heill). The larvæ of the œstrus has also been found in the brain substance producing inflammatory or degenerative foci (Brückmüller, Megnin, Siedamgrotzky). Their presence in the nasal sinuses at times cause encephalitis by contiguity. The cestoid worms, cœnurus in sheep and other ruminants, and cysticercus in swine, find their natural larval habitat in the brain and by their movements produce more or less congestion and inflammation. Cases of cœnurus in the horse have been described by Rousset, Frenzel, Zundel, and Schwanefeldt.
Symptoms. The symptoms of uncomplicated meningitis on the one hand and encephalitis on the other are rarely seen, the disease usually implicating more or less both brain and meninges, in a common inflammation or the symptoms of the one involving those of the other through proximity or interdependence of function. And yet in traumatic lesions of the cranial walls, the symptoms may be those of pure meningitis, and in thrombosis, embolism or parasitism of the brain, and in certain tumors they may be those of simple encephalitis. The distinction consists largely in the predominance of fever, hyperæsthesia, active delirium and convulsions in meningitis, and especially in its earlier stages; and the prominence of dullness, stupor, somnolence, muscular weakness, paralysis, anæsthesia, coma, and the clouding of special senses, with much less pronounced febrile reaction, or vascular excitement in encephalitis.
There is usually, however, a mixing of symptoms so that the benumbing or paralysis of the nervous functions alternates with periods of their exaltation, and with both conditions hyperthermia exists, though usually higher with meningitis.
The manifestations of benumbing or paresis may be continuous or interrupted, and are exhibited in stupor, coma, somnolence, lethargy, paresis or paralysis. The manifestations of excitement are not continuous but occur in paroxysms or at least exacerbations, which may show in visual or mental illusions, active, violent delirium, trembling, rigors, clonic or tonic spasms. The onset is usually abrupt, the animal passing in a few hours from apparent health, to pronounced nervous disorder. The horse seems drowsy and stupid, standing with semi-closed eyes, often drooping lower lip and ears, head pendent and resting in the manger or against the wall in front, the back arched and the limbs drawn together. When moved, it walks unsteadily and often the limbs are left out of plumb, one extending unduly forward, backward or to one side, and often crossing over its fellow. Some cannot be made to back, others back spontaneously hanging on the halter. Turning short in a circle is difficult or impossible and tends to throw the patient down. Yet some exceptional cases will turn around spontaneously to the right or left, and an animal tied to a post goes around it at the end of its halter in its effort to pass straight forward. The circling movement may be due to the irritation on the one side of the brain or to irritation of particular ganglia and nervous tracts as noticed under cerebral hyperæmia.
Appetite is usually lost, or, more properly, the animal no longer takes notice of surrounding things, not even of its food. In some cases, however, in which stupor or coma is not extreme the animal will eat a little during his quiescent intervals. In ryegrass and other dietetic poisoning, the animal may still eat and fall asleep with the month full. The digestion is impaired or suspended, the bowels costive, and fermentations with tympanies and rumbling are frequent complications. When originating from poisonous food this often contributes to these abdominal complications.
Respirations in the comatose condition are deep and slow, sometimes not more than four or five per minute. The heart usually beats strongly, often tumultuously, and the pulse varies greatly—infrequent or frequent, strong or weak, full or small. With cerebritis it is often abnormally slow.
Hyperthermia is always present to a greater or less extent, being often more marked in the more violent forms or those in which meningitis appears to predominate than in the purely cerebral forms. The temperature may vary from 101° to 106°.
The optic disc is congested.
Probably in all cases or nearly all there is a preliminary stage of excitement, in which the eye is clear, the eyelids open, the aspect alert and the whole skin affected by a marked hyperæsthesia. In some cases the symptoms of excitement are much more violent at the outset of the disease, as marked by trembling, nervous movements, pawing, pushing the head against the wall while the motions of walking or trotting are performed by the limbs, or those of plunging forward, rearing up, drawing back on the halter, etc.