Whatever disturbs or undermines the general health lays the system open to the disease and many flockmasters in tick-infested districts have succeeded in greatly reducing the mortality by feeding sound hay and oats in winter. Sudden changes of weather have long been noticed to coincide with outbreaks of the disease. A change to cold and wet is especially dangerous as causing a chill and robbing the system of its tone and vigor. But a sudden access of warm spring weather, especially if at the same time moist, may have a decidedly predisposing effect by lowering the general tone. A fatal paresis common in the flocks of New York, in the absence of ticks, shows a similar tendency to select the atonic animal. This occurs mainly in spring, when the sheep have been shut up in close confinement for weeks or months, with flaccid muscles and fatty livers, and above all if they are in advanced pregnancy with twin lambs, and if their fleeces are extra heavy. In both affections the majority of the flock escape, while those that are specially predisposed succumb. Lambs suffer most, doubtless because of relative weakness, and on account of their innate and unexhausted susceptibility.
Symptoms. After an incubation varying from ten days to thirty some impairment or disorder of the innervation is shown, varying widely, however, in different cases.
The two names “trembling” and “louping-ill” long used by shepherds as characteristic of the disease indicate spasmodic disorder of a clonic kind, the paresis which is essentially passive having been very naturally overlooked, or held to be subordinate. There is hyperthermia the temperature rising at times to 105° or higher, and often marked hyperæsthesia and excitability at the outset. On approaching the patient it is very much frightened, and when caught, struggles and twitches in a remarkable manner or trembles violently. If merely raised or disturbed the trembling or clonic spasms are very marked, the nose is jerked forward and upward from the contraction of the muscles attached to the occiput; the legs may be lifted jerkingly as in stringhalt; when raised they are moved stiffly or sway uncertainly before the foot is once more planted; or the sheep loses its balance, falls to the ground and struggles convulsively in its efforts to get up and escape. As the result, it will in certain cases jump to its feet, rising meanwhile to an undue height in the air. In other cases there is squinting or rolling of the eyes, and movements of the jaws with frothing from the mouth. Or the spasms may be tonic affecting especially the muscles of the back and loins, and causing extreme stiffness or rigidity or even opisthotonos. Lambs are unable to suck.
But whether the early spastic symptoms are well marked or not, paresis and even paralysis set in sooner or later. This usually begins as paraplegia, or exceptionally one limb only may be affected at first, causing the animal to walk on three legs. For a time the fore limbs may be free, and the patient attempts to move by dragging the hind limbs, which are extended backward. When the fore limbs become involved the animal remains down helpless and after awhile apathetic. Temperature and sensibility are both greatly lowered in the paralyzed limbs. Sometimes the spasms are lateral and the head may be drawn to one side.
In the animals that survive the early attack, there is likely to remain some lasting deformity, such as wry neck, arched back, stiff or swollen joints. Abscesses, which appeared in the inoculated rabbits after a lapse of four months, are a not uncommon sequel in sheep, the pus collecting in the neighborhood of a joint, or of the lymph glands of the axilla, inguinal region, breast or shoulder.
The succession of symptoms are in the main such as are observed in other cases of myelo-meningitis, first exalted function and later depressed and abolished.
Diagnosis. From other forms of myelo-meningitis it may be distinguished by its enzoötic occurrence on tick-infested pastures, which already have a reputation for causing this malady, by its appearance only in the season of the development of the tick, by the presence of the tick or of its sores on the skin, and by its entire absence from adjacent fields which are free from ticks. From paralytic rabies it is differentiated by the same conditions, and as a rule by the absence of rabies from the district and of any evidence of a bite. In tetanus the tonic persistent nature of the spasms, the absence of paralysis, the marked spasms of the muscles of the eye, and the usually isolated condition of the case should prevent any confusion. Braxy is to be distinguished by its emphysematous swellings near the surface of the body, and by the comparative absence of hyperæsthesia, spasm, or paralysis. The carcass in braxy undergoes much more rapid decomposition. Anthrax is more rapidly fatal, shows no such marked nervous disorder, has a dark, nonoxygenated and often incoagulable blood, an enlarged sanguineous spleen, and the characteristic anthrax bacilli. It attacks the larger herbivora as readily as the small.
Lesions. The most constant and striking lesions are found in the nerve centres. In many cases there have been found cerebral meningitis, involving the choroid plexus (Fair, Hamilton, Klein, Murray, McFadyean) with an encrease of the ventricular and subarachnoid fluid (Murray, Hamilton, Williams). The exudate may be yellow or rosy from contained blood globules (Klein). The meninges are thickened and the seat of ramified redness. In the region of the spine inflammation is found not only in the meninges but also in the cord, which may be blood-stained, softened (Mathewson, Goodwin, Robertson), or in older cases indurated (Robertson, Hamilton, Williams, Young). In this last condition there is a sclerotic condition of the neuroglia, and it may be a distinct atrophy. The exudate is usually abundant and more or less coagulated into a soft, diffluent jelly.
In many cases there is inflammation of the serous membranes of the chest (pleura, pericardium, endocardium), and even of the lungs (Fair, Hamilton, Klein). In some instances there has been inflammation of the stomach and intestines (McFadyean), liver and kidneys (Klein), and enlargement of the spleen has been noted. The most constant lesions appear to be those of the nerve centres, but the wide variety of organs involved in different cases sufficiently accounts for the variability of symptoms.
Prevention. As the ticks are the chief media of infection, the disease may be eradicated by their destruction. The burning of all withered grass and brush during the winter months will do much in this direction. Their destruction is rendered even more complete by ploughing and putting the land under a series of cultivated crops. By this means not only is the winter shelter of the tick removed, but the animal host which it requires for its complete development is denied it, and it must perish before the land is again seeded to grass. When the land is unsuited to cultivation, the same end may be in some measure secured by fencing off half the pasture, and leaving it unpastured for a season, meanwhile burning the dry grass or temporarily suppressing it by a liberal application of salt. The following year the pasture so treated may be restored to pasturage, and the other half subjected to the same course of treatment. In the absence of such thorough treatment, a liberal application of lime to a virgin soil will often bring a growth so fresh and appetizing that the stock keeps it closely cropped and thus removes the shelter for the offensive ticks. Finally, the ticks may be prevented from attacking the animals, by repeated use during April, May and June of a dip in which tar oil, cade oil, heavy petroleum or other odorous insecticide forms a component part (see the dip of Meek and Greig-Smith).