Symptoms. Psoroptic mange attacks the parts covered with wool, so that attention is only drawn to the disease by some slight injury to the fleece, which becomes rough, matted, brittle, and liable to fall.
Scabies commences with intense pruritus. The animals scratch and bite themselves, and tear away the fleece. These symptoms become aggravated when the animals are hot, as, for instance, when travelling. If the sheep have travelled through mud, the fleece becomes matted on the neck, behind the shoulders, and at all points where they are able to scratch themselves with the hind limbs.
When a scabby sheep is touched at a diseased point, the animal shows pleasure by nibbling and moving the head up and down.
At an early stage, if the fleece is divided and the skin examined, little papules may be discovered somewhat less in size than a lentil. They are of a yellowish colour, and are distinctly visible against the reddish colour of the skin. These papules are due to the attacks of the psoroptes.
Fig. 255.—Left top figure, egg of mite which causes common sheep scab; middle top figure, six-legged stage of sheep scab mite; right top figure, young female before moulting for the last time, dorsal view; middle figure, adult male parasite of sarcoptic scabies of man (the corresponding parasite of sheep is very similar), ventral view, × 250 (after Blanchard); left bottom figure, adult female parasite of sarcoptic scabies, dorsal view, × 250 (after Blanchard); right bottom figure, same, ventral view (after Blanchard). All greatly enlarged.
They soon become more numerous and even confluent, break and discharge, become converted into pustules, and cause the formation of crusts. In a few days the diseased points are covered with a squamous, yellowish, sticky covering, under which the psoroptes lie hidden and which affords them nourishment.
The crusts steadily grow thicker and lift the individual fibres of wool, tearing them from their follicles, so that patches of skin become bare. The patches thus formed increase in diameter, for the acari leave the centre, where crust-formation is replaced by abundant desquamation of the epidermis. The skin is thickened, assumes the character of parchment, and in old-standing cases becomes wrinkled.
The disease always commences along the back, withers, loins, and the upper part of the quarters. Thence it spreads to the flanks and sides of the chest. The psoroptes are almost exclusively confined to recently affected points on the edges of the scabby patches. They are visible to the naked eye, and appear as little whitish-brown points.
Scab is specially liable to attack a flock containing lambs and yearling sheep, whose skin is thin, fine and supple, and therefore more susceptible to their attacks. If a portion of a scabby flock be shorn, the shorn animals will probably recover on account of the psoroptes transferring themselves to the animals with long fleeces.