PARASITES AFFECTING THE SKIN 29 
the vesicular and the pustular stages, and finally burst, 
allowing the escape of a sticky yellow exudate with 
an acid reaction, which soon dries into a scab. This 
scab grows in area, as the parasites now leave the centre 
and proceed to the margin, where they once more attack 
the skin, producing a fresh crop of pustules, and eventu- 
ally a fresh scab, which becomes confluent with the 
original one. The large crust thus formed has the 
appearance of dirty parchment. It drags out the 
fleece by which it is attached, and becomes raised from 
the skin. It then begins to show fissures, and is finally 
lifted off by the crop of new wool which grows beneath it. 
The surface of the body thus shows several different 
appearances. We will suppose that in a typical case some 
portions, such as the rump and the under surface of the 
body, are covered with sound normal wool. On the neck a 
recently affected patch shows matted, tangled wool and 
pustules beneath it. On the back we may see a parch- 
ment-like scab lifted off from the body, and close beside it 
an area of dry, thickened leathery skin, covered with 
desquamated epithelium, and in parts showing traces of 
a new growth of wool. 
On the sides where the sheep has been able to bite 
and scratch we may see an ulcerated patch, showing 
signs of sloughing or local necrosis. 
Diagnosis —1. There should be no difficulty in dis- 
tinguishing psoroptic mange from sarcoptic. The latter 
attacks the head, while the former seeks only parts 
covered with wool. 
The parasite in psoroptic mange is not difficult to 
discover. A portion of scab should be taken from the 
periphery, softened in diluted liq. potasse, well teased 
out, and examined microscopically. 
2. Rain-Rot.—This is really a sebaceous folliculitis, 
occurring in autumn, after excessive rain, and may 
affect many members of a flock at one time. It is non- 
