Fig. 264.—Ixodes ricinus. (After Railliet.) A, Natural size; B, ventral surface; C, dorsal surface.
(The cattle ticks of America are of especial importance in relation to the disease known as Texas fever. Those who wish to study the entomology of this subject are referred to the masterly account and fine coloured illustrations of Salmon and Stiles, “Cattle Ticks of the United States,” Ann. Rep. U.S.A. Bureau of Agriculture, 1900, p. 380.)
HYPODERMOSIS IN THE OX (WARBLES).
Causation. This is a parasitic disease characterised by subcutaneous swellings due to the presence of larvæ of the Hypoderma bovis. The larva is met with throughout Europe. It attains the perfect stage during the summer, from the middle of June to the commencement of September.
The female deposits her eggs on animals with fine skins. These eggs are elliptical, and provided with a kind of tail of a brownish colour. They soon become converted into larvæ, provided with rows of little spines.
The manner in which the eggs are laid is not exactly understood, nor are we better informed regarding the hatching of the young larvæ. Until recently it was believed that the larva perforated the skin as soon as it quitted the egg, and then penetrated as far as the subcutaneous connective tissue. Recent observations, however, have upset this view. It is probable that this larva, like other gastrophili, is swallowed by animals of the bovine species, and passes through the intestine into the surrounding tissues by a path which is yet unknown, possibly by the blood-vessels, whence it makes its way after a longer or shorter interval into the subcutaneous connective tissue.
Fig. 265.—A, Hypoderma bovis, natural size. B, larva of the hypoderma escaping from a “warble.” (After Railliet.)
Certain recent observations seem to support the latter view, which is also corroborated by known facts regarding hypodermic myiosis in man. The eggs are laid in summer, and the swellings indicating the presence of the larvæ only appear during the winter. Henrichsen found young larvæ in the fatty tissue situated between the periosteum and spinal dura mater, between the period from December to March.
Symptoms. Whatever the mode of development of the larvæ, cutaneous swellings appear between the months of February and March on the back, lumbar region, quarters, shoulders and ribs, and, less frequently, over the chest, belly and thighs.