This is a cutaneous disease, analogous to the mange in horses and the itch in man, and is caused and propagated by a minute insect, the _acarus_. [Illustration: THE BROAD-TAILED SHEEP.] If one or more female _acari_ are placed on the wool of a sound sheep, they quickly travel to the root of it, and bury themselves in the skin, the place at which they penetrate being scarcely visible, or only distinguishable by a minute red point. On the tenth or twelfth day, a little swelling may be detected with the finger, and the skin changes its color, and has a greenish blue tint. The pustule is now rapidly formed, and about the sixteenth day breaks, when the mothers again appear, with their little ones attached to their feet, and covered by a portion of the shell of the egg from which they have just escaped. These little ones immediately set to work, penetrate the neighboring skin, bury themselves beneath it, find their proper nourishment, and grow and propagate, until the poor creature has myriads of them preying upon him. It is not wonderful that, under such circumstances, he should speedily sink. The male _acari_, when placed on the sound skin of a sheep, will likewise burrow their way and disappear for a while, the pustule rising in due time; but the itching and the scab soon disappear without the employment of any remedy. The female brings forth from eight to fifteen young at a time. In the United States, this disease is comparatively little known, and never originates spontaneously. The fact, that short-woolled sheep--like the Merino--are much less subject to its attacks, is probably one reason for this slight comparative prevalence. The disease spreads from individual to individual, and from flock to flock, not only by means of direct contact, but by the _acari_ left on posts, stones, and other substances against which diseased sheep have rubbed themselves. Healthy sheep are, therefore, liable to contract the malady, if turned on pastures previously occupied by scabby sheep, although some considerable time may have elapsed since the departure of the latter. The sheep laboring under the scab is exceedingly restless. It rubs itself with violence against trees, stones, fences, etc.; scratches itself with its feet, bites its sores, and tears off its wool with its teeth; as the pustules are broken, their matter escapes, and forms scabs, causing red, inflamed sores, which constantly extend, increasing the misery of the tortured animal; if unrelieved, he pines away, and soon perishes. The _post-mortem_ appearances are very uncertain and inconclusive. There is generally chronic inflammation of the intestines, with the presence of a great number of worms. The liver is occasionally schirrous, and the spleen enlarged; and there are frequently serous effusions in the belly, and sometimes in the chest. There has been evident sympathy between the digestive and the cutaneous systems.