TREMBLING IN SHEEP. LOUPING-ILL. INFECTIVE MYELO-MENINGITIS. IXODIC TOXÆMIA.
Definition: infective, tick-borne disease, characterized by meningo-myelitis. Animals susceptible: sheep, and possibly swine (Meek, Greig-Smith) and cattle (Williams). Known in North Britain only in spring (in Skye also in autumn), on rough pasture with much brush, and wood ticks, (Ixodes ricinus, erinaceous, marginatus or other). Experimental infection. Bacteriology: Bacterium fluorescens β and γ found in exudate and infecting. Accessory causes: dried grass of previous year, brush, low condition, cold, youth. Symptoms: incubation 10 to 30 days; impaired innervation, hyperæsthesia, timidity, excitability, trembling, jerking, lack of coördination and balance, falling, convulsive struggling, jumping, rolling of eyes, stiffness, opisthotonos, paresis, paralysis of hind—later of fore limbs, apathy. Wry neck, arched back, stiff joints. Diagnosis from myelo-meningitis by its enzoötic appearance, in spring, on tick infested ground: from paralytic rabies, also by absence of that disease locally; from tetanus by its general prevalence, the absence of tonic spasm, and presence of palsy; from braxy by the lack of emphysematous swellings, and of speedy sepsis; from anthrax by usually healthy spleen and its confinement to sheep. Lesions: cerebral meningitis with encrease of subarachnoid fluid, of myelon, reddened, softened, also of other serosæ, stomach, bowels, liver and kidneys. Prevention: destroy ticks in winter by burning grass and brush, by ploughing and cropping; or fence off ½ the pasture one year and the other half next; or lime soil; or dip repeatedly in April, May and June to keep off ticks; avoid moving sheep in these months: Give liberal feeding. Mortality 10 to 20 per cent.
Definition. All infective disease of sheep, inoculated by ticks, and producing a meningo-myelitis, with drowsiness, hyperæsthesia, irritability, paresis and other nervous disorders.
Animals susceptible. This is almost exclusively a disease of sheep, yet Meek and Greig-Smith claim to have seen it in swine that have eaten the raw carcasses of louping-ill sheep, or that have ranged the tick infested pastures, and in rabbits inoculated with the microbe from the wound caused by the tick. W. Williams claims to have seen well marked cases in cattle, and heard of cases in horses and swine. He speaks, however, rather obscurely of “the tick disease” and seemingly includes in this all affections inoculated by ticks.
Geographical Distribution. This disease has been hitherto described as existing in the northern part of Great Britain only, but given the presence of the tick, and of the infection which it carries and the malady might easily be extended indefinitely. It is known to prevail in Northumberland north of the Tyne, in Kirkcudbright, Dumfries, Ayrshire, Lanark, Peebles, Roxburgh, Berwick, Argyle, Inverness, and Ross. In Berwick it is less prevalent than in the other countries named, while in the Western Isles it is not only widely spread rising on the hills 2000 feet or as high as the sheep range, but in Skye there are two distinct outbreaks, in spring and autumn respectively, apparently coinciding with the appearance of two successive generations of ticks. This may be due to the prevalence of warm winds from the Gulf Stream. Further investigation will doubtless show a much wider distribution—the author has seen an affection bearing the same general characters on the spurs of the Lammermoors in East Lothian, and the supposed adaptation of the Norse word hloupa (staggering) suggests that it is probably not unknown in Scandinavia. W. Williams notes its prevalence on the Silurian formation, but ticks confine themselves to no geologic stratum, and the tick is the main agent in carrying infection.
Causes. 1st, Sheep Ticks. The sheep tick is not the ked (Melophagus ovinus) which is common on long wooled sheep everywhere and is an example of a wingless, degraded dipterous insect.
The sheep ticks, on the other hand, are true ixodes, and of the same family with our common wood tick and of the cattle tick (boöphilus bovis) of the Southern States and the West Indies. The ticks collected from the sheep by W. Williams were identified by Mr. Moore, of the British Museum, as ixodes ricinus, ixodes erinaceous and ixodes marginatus. Those obtained by Meek and Greig-Smith showed the following characters: The male is 2.48 mm. long, by 1.30 mm. broad; the female is 5 to 5.5 mm. long, by 3 mm. broad, or when gorged with blood, 10 mm. long, by 7 mm. broad. The fasting female is yellowish green, and when full of blood, blue. Following the rule of their genus, the mature tick has eight legs, the larva but six. The fasting larva has head, legs and dorsal plate (scutellum) brown, the remainder of the body yellow. Scattered hairs appear on the body, legs and maxillæ.
In all ticks the rostrum is a characteristic feature. It consists centrally and interiorly of a dart covered below and on each side by rows of teeth turned backward, which, when imbedded in the skin, hold so firmly that the parasite may be pulled in two in any attempt to pull it out. Above this dart and on the two sides lie the cheliferæ (horns), each furnished with three or four teeth turned outwardly and more or less recurved, by which the dart is worked into the skin. Finally, on the lateral sides of this central apparatus, are the two maxillary palpi, which are not inserted into the skin, but applied against it and operate as feelers prior to and during the insertion of the dart and cheliferæ. The maxillary palpi are club-shaped and soft.
Ticks pass through three moultings before they attain to the sexually mature eight-legged form, and though the hexapod larvæ attach themselves to animals and irritate the skin by their bites, it is only the mature, impregnated egg-bearing female that lives exclusively on blood and sucks this to excess.
While given species of ticks show a preference for particular genera of animals, yet ticks generally in their vagabond life will leave the long grass and brush where they have been hatched to become temporarily the guests of any passing animal. It is, therefore, premature to seek to identify any single species of tick as the only bearer of the infection, and it is quite possible that any one of several species may contribute to its propagation.
Experimental Infection by the Tick. W. Williams muzzled four sheep from a healthy district and turned them for several hours a day on a tick-infested field, and two sickened—one on the eighth day and one on the sixteenth. Twelve ticks sent out of the district and put on a healthy sheep caused illness on the tenth day. In a second experiment with ten sheep, during a colder spring, when there were fewer ticks, no deaths occurred.
Meek and Greig-Smith turned twenty sheep on a tick-infested louping-ill pasture, six of the number wearing muzzles to prevent grazing, seven having been dressed with a mixture of sweet oil, 2 quarts; castor oil, 1 quart; train oil, 1 quart; pitch oil, 3 gills, and cade oil, 1 gill, while the remaining seven were unmuzzled and undressed. The muzzled sheep were regularly taken out and fed on food from a healthy locality. At the end of a fortnight oils were reapplied on the second lot (seven sheep). No ticks appeared on these sheep, while many were found on the undressed ones. On the fifteenth one of the unmuzzled and untreated sheep sickened and died with lesions of louping-ill. On the twenty-second day one of the muzzled sheep sickened, and died of louping-ill next morning. On the thirty-first day another of the muzzled sheep took ill and on the thirty-eighth was killed. It showed characteristic symptoms and lesions. The seven salved sheep, on which no ticks could at any time be found, remained healthy throughout.
Bacteriology. W. Williams describes a bacillus which he figures as forming filaments of very uneven breadth, with frequent branching (contrary to the habit of bacilli), and forming spores in clusters. These were obtained from the coagulum of the cerebro-spinal fluid and Meek and Greig-Smith conclude that the alleged mycelium was but the filaments of fibrin.
McFadyean found pus microbes.
Meek and Greig-Smith (Veterinarian, 1896–7) found in the black bloody swellings under the skin, where the ticks had inserted their proboscides, a variety of microbes which in pure cultures did not provoke louping-ill. These included staphylococcus cereus albus, sarcina lutea, bacterium putridum, bacterium coli commune, micrococcus sulphurous, micrococcus bicolor, and micrococcus caudicans. He also found penicillium glaucum. Two organisms allied to the bacterium fluorescens and designated as β and γ (G) respectively, were found in these sores and produced in rabbits and sheep nervous disorders and lesions which could be fairly identified with louping-ill. These are about 1.5 to 1.7μ long by 0.7μ broad, and chromogenic with a special fluorescent appearance. Microbe β inoculated on a rabbit caused on the second day rhythmic movements of the head downward and to one side, the eyelids closing as the head dropped, as if the animal were constantly falling asleep. In another rabbit it caused stiffness of the legs only and in a third it had no effect. Microbe γ when cultivated ærobically was harmless to rabbits, but when grown anærobically on mutton bouillon to which a drop of blood had been added, it caused on the second day general paralysis of the neck and limbs, spasmodic twitching of the muscles, dyspnœa and feeble heart action. Two rabbits which survived the early effects developed large axillary and inguinal abscesses four months later.
A lamb inoculated with the bouillon culture of γ showed after thirteen days, lameness of one hind limb, with paresis, and a disposition to fall to one side or the other. Two months later, when it had greatly improved, a second inoculation of microbe γ grown anærobically in bouillon and blood, caused, on the third day, a severe aggravation of the lameness.
Accessory Causes. Much depends on the abundance of old dried grasses of the previous year in which the larval ticks may hibernate. Land that has been burnt over in winter, that is cleared of brush, or in which the aftermath has been killed by free salting or liming will be largely cleared of the ticks.
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).
A very obvious precaution is to avoid the movement of sheep during April, May and June, from tick-infested pastures to others which furnish rank grass, brush or other suitable shelter for the preservation of the parasite.
It has been noticed that sheep indigenous to the tick-infested and louping-ill pastures are less susceptible than those that have been introduced from outside, but, as yet, no attempt appears to have been made to secure immunity by the use of sterilized products of the microbe, nor therapy by the resort to antitoxin. A liberal and tonic diet is an important element in prevention. Grain and hay should therefore be allowed whenever necessary to bring the sheep to early summer in good condition.
Treatment can hardly be said to have been attempted, though mild cases are allowed by the shepherds to recover. According to Meek, the deaths often average 10 to 20 per cent. of the flock.