Not only is the disease very circumscribed in its distribution as a whole, but locally in the places mentioned infected and non-infected spots are pointed out. These may be quite contiguous. The flocks in the North Tyne district feed up and down the hills in limited “cuts,” and it is one of the features of the illness that certain “cuts” are very liable to it, while others, even on the same farm, are just as free. In many cases the nature of the pasture is such as to suggest to an experienced man the probability of its being subject to the disease. A dry and foggy pasture seems best suited for harbouring the cause of the malady. These infected places have remained wonderfully constant, but a peculiar feature about them is that some may be very bad for louping-ill one year, and others bad another year. Of two adjoining farms, one may be badly attacked and the other mildly, while in the following year the conditions may be reversed. Districts may present the same peculiarities. Thus, though the disease is essentially endemic, it is not absolutely constant in its recurrence. There seem to be certain circumstances capable of favouring or retarding it.
Fig. 198.—Larva of the grass tick.
Length, ¹⁄₄₀th of 1 inch.
Fig. 199.—Pupa of the grass tick.
Length, ¹⁄₁₈th of 1 inch.
Lesions. The chief lesions are localised in the membranes of the brain and spinal cord, which are congested or inflamed, and contain an increased amount of cerebro-spinal fluid or a jelly-like, sometimes blood-stained exudation. Softening and hardening of the spinal cord have both been observed. Inflammation of the pleura and pericardium, with fluid or jelly-like exudation, are common; lobar congestion of the lungs, endocarditis, gastritis, and enteritis have all been described; some observers have mentioned congestion of the kidneys and liver and swelling of the spleen. Lesions of the nerve-centres are the most constant and reliable.
Etiology. Depressing and weakening influences of all kinds have been blamed for producing the disease, but the general consensus of opinion points in the direction of infection with microorganisms carried and introduced into the sheep’s system by the common sheep tick or “grass tick” (Ixodes redurius). The following remarks on, and illustrations of, this parasite are from an article by Mr. Wheeler, of Alnwick (Veterinarian, Vol. LXXIII., No. 867, p. 141).
Life History of the Grass Tick. Sheep ticks (which must not be confused with the sheep-ked, or keb, a wingless six-legged fly, universal on sheep everywhere) are allied to the spiders. They pass through four stages of existence: the egg—the six-legged larva—the eight-legged pupa—and, finally, the eight-legged adult male or female.
Fig. 200.—Adult male of the grass tick. Length, ⅑th of 1 inch.