Causation. The sole cause is the presence of Chorioptes scabiei (v. ovis). Contagion is favoured by the animals being in poor condition. The disease extends very slowly. Only 2 to 3 per cent. of the animals are affected, and the sufferers are usually those with fine skins.
German shepherds consider this disease to be due to an excessive allowance of salt, because it is most common during the winter, when the sheep are housed. Needless to say, this theory is incorrect.
Symptoms. Sometimes this form of mange attacks the limbs and develops very slowly. It commences about the pasterns, and gradually extends upwards towards the knee or hock. It really advances beyond these points. The parasites are much smaller than those of common scab, and are often overlooked. The sheep stamp their feet and scratch and bite the infected parts, sometimes transferring the disease to the lips and face, where it may persist for a time.
Fig. 262.—The left-hand figure shows the adult male parasite of chorioptic scabies of the horse (the corresponding parasite of sheep is almost identical), ventral view. The central figure shows the young female parasite of chorioptic scabies, ventral view. The right-hand figure shows the adult female parasite of chorioptic scabies with egg, ventral view. All × 100. (After Neumann.)
In very old standing cases which have been entirely neglected, it may be met with in the region of the armpit and thigh, the limbs becoming swollen so as to suggest lymphangitis. In the folds of the hock and pastern the thickening of the skin may lead to the formation of yellowish or brownish crusts, according to whether the exudation is merely of a serous character or accompanied by bleeding.
The diagnosis is easy. The parasites are found in the thickness of the crusts, and are readily recognised under the microscope.
The prognosis is not grave, because of the comparative rarity of the disease and its mildly contagious character. This disease, moreover, never attacks the head or body, but remains localised in the lower portions of the limbs.
Treatment. The crusts should be removed by washing or by standing the animals for a time in warm water. Simple cleanliness often suffices, but, to save time, some anti-parasitic may be used. The condition is easily cured even without disinfecting the folds.