Dropsy of the ventricles is common in the horse in certain countries and districts, yet even there it is uncommon in cattle, sheep, swine and dogs, save as a congenital affection. It is reported as enzootic in some Alpine valleys and along the Rhone, attacking especially the mares and immature horses so that breeding becomes impossible. Mauener who reports this says that in the same localities encephalic diseases are more common in man. In America it appears to be most frequent in the rich bottom lands of the Mississippi valley and of the Southern States. Northern horses taken to the Gulf States though they may not suffer to this extent, are liable in the first year to show weakness, debility, and lack of vigor which is spoken of as the acclimatizing fever. Elsewhere the affection is one of the old horse in which the vital powers begin to fail. Common breeds of horses with lymphatic temperament, large head and narrow forehead have been found to be especially predisposed. Geldings are said to be most liable on account of the arrested development of the brain, but with the great preponderance of geldings among work horses, it is dangerous to generalize too far.
Causes. Acute encephalitis may lapse into the chronic form and then assumes the symptoms of this disease. Cases that come on slowly and imperceptibly appear at times to be hereditary, as might be expected from the fact that it usually goes with a lymphatic temperament. The conditions which cause cerebral or meningeal hyperæmia in chronic form conduce to the affection. Injuries to the cranial vault from traumatism or disease are infrequent causes. Renault records a case associated with two bony tumors, each as large as an egg, projecting inward from the frontal bone and which had produced extensive absorption of the convolutions and increase of the cerebro spinal fluid. In a case of my own with an abscess in the diplöe above the frontal sinus, and pressing inward on the brain a similar condition existed. In other cases Renault noticed that the cerebro spinal fluid was largely in excess.
Much more commonly, however, the accumulation of liquid takes place in the ventricles, and is associated with different causes: as tumors or false membranes near the base of the brain pressing on the veins returning blood from the ependyma, tumors in the ventricles (cholesteatomata, etc., of the ependyma or choroid plexus) obstructing the circulation or giving rise to local hyperæmia and chronic congestions from the other causes such as faulty conditions of fodders, or water, exposure to undue heat, overwork, etc. The enzootic prevalence of the disease in certain localities, (Alps, Rhone Valley, bottom lands) would suggest that local conditions in food or water are factors, though we cannot as yet fully explain the mode of causation. In the same way we must recognize the influence of hepatic and gastric disorders, which arise from such faulty regimen and affect the brain by nervous sympathy and by the action of toxic elements thrown into the circulation. Then again we must take sufficient account of the congestions resulting from obstructions in the lesser circulation, disease of the lungs, and of the right heart, and compression of the jugulars by a tight or badly fitting collar, or compulsory curving of the neck as set forth under vertigo and cerebral congestion.
Symptoms. Among the symptoms must be recognized the conformation with which it is usually found associated. The predisposed animals are usually low bred, common horses, with narrowness of the cranium and space between the ears and with a retreating of the head from the orbits to the poll. Other horses suffer but the majority are of this conformation, and thus the disease acquires a hereditary basis.
The expression of the face is characteristic. The eye is dull, often sunken, lacking in vivacity and life, the eyelids are semi-closed, the ears do not prick up to sounds, the muscles of the face are relaxed, so that the lips hang flaccid, and the nostrils fail to dilate freely and rhythmically. The animal is apparently unconscious of all that goes on around him, and is not aroused by the entry or exit of men or horses, by voice or slap, by food or water. His head is probably dropped and resting in the manger, and he raises it sluggishly when compelled; when moved from side to side of the stall his legs may retain a position turned outward or crossed one over the other; if energetically roused he wakes up slowly, and almost immediately relapses into his former lethargy, without accomplishing what was called for. When left with legs crossed he often remains so until wearied by the constrained position, or in danger of falling from loss of balance. Not only the legs but the head will retain for a time an abnormal position given to it,—bent, dropped, turned to one side or the other.
This same lethargy extends even to mastication, which is usually performed slowly and indifferently, and is often interrupted in the middle of the trituration of a morsel which remains in the cheek, on the tongue, or between the teeth, and perhaps hanging out of the mouth. Hence the horseman’s expression, he smokes his pipe.
His mode of drinking is no less singular. Usually the lower part of the face is dropped deeply into the water, and he will only withdraw it when it becomes necessary to breathe. He may continue to masticate while drinking.
When walked or trotted he may move a short distance all right; he may even hasten his progress for a short distance without refusing meanwhile to respond to the rein, then he may stop and for a short time longer resist all efforts with voice, whip, or spur to start him anew. In other cases he will turn to one side, getting into ditches or fences by an apparently involuntary action and in defiance of whip or reins.
One of the most striking features of the disorder is the difficulty of backing. In some cases he will back a few steps and then prove unable to back farther; in others he will show it best when heated with a journey; in other cases still he will back well enough under his own weight, but prove utterly unequal to the act if a heavy man is placed on his back, or if hitched to a loaded wagon. In backing with or without a rider the horse, pulled back by the reins, inclines backward with his hind limbs extended forward beneath the abdomen, his forefeet extended in front, and his back arched; he extends his head or turns it to one side, and when the change in the centre of gravity endangers his equilibrium, he draws back his forefeet without lifting them, each making a groove in the ground, and at the same time he makes a disorderly motion of the hind limbs to one side to restore the balance. In default of this he may drop his quarter on the ground and perform a back somersault on his rider. Even when he succeeds in balancing himself after dragging the forefeet back, the difficulty of further backing is rather increased, as the nervous irritability is enhanced by a continuance of the excitement.
When hitched in a carriage the phenomena are virtually the same; when backed he extends or flexes the head, inclines the body backward, and after a time loses his equilibrium, sometimes executes a few disorderly steps backward, or throws himself violently to one side, or turns over backward in the shafts.