84 Parasites of Cattle and Sheep. 
pricking the skin to which it clings for a while. Considerable 
irritation is induced causing the sheep to rub and bite and so 
injure the infested part as to make it bleed, and produce the 
scab, which gives the disease its name. The lesion, or scab, 
is therefore the result of the irritation of the acarus and the 
rubbing and biting of the sheep, and its only special feature 
is the scab mite. The acari increase rapidly and invade other 
healthy parts of the skin, thus extending the original area, 
while in the process of rubbing they may be carried to distant 
parts and set up fresh centres of the disease in the animal. 
Fig. 7 represents a colony of acari in various stages of develop- 
ment as reproduced by photography from a case of actual scab. 
Symptoms . — Usually the upper parts of the body, i. e ., neck, 
shoulders, back, and sides are attacked. The sheep manifests 
its irritation by biting itself, and rubbing against anything 
which may afford the opportunity, as posts, gates, hurdles, or 
other sheep, and large portions of the fleece may be removed, 
leaving the scab in view. The wool thus removed usually 
contains the mature acari, larvae, or eggs, and is one of the 
most important means by which the disease is spread. The 
sheep is often disturbed in feeding, runs forward and makes 
nibbling movements with the lips. The diagnosis of scab, 
however, depends on the discovery of the scab mite. “ No 
parasite, no scab,” is the axiom. It is usually fairly easy for 
the expert to detect the parasite in the manner mentioned. 
The search for it is suggested by the observance of 
common symptoms : wool about the hedgerows, pastures, and 
posts, the broken matted fleece, and evidence of irritation 
shown by the sheep, alone excite suspicion ; but all these 
conditions may be brought about by other parasites than those 
of sheep scab. The scab mite lives for some time — probably at 
most a few weeks — apart from the sheep, but there is no evidence 
of its propagating elsewhere except on the skin of the sheep. 
It will therefore be seen that sheep may become affected with 
scab by contact with infected sheep, objects against which 
they have rubbed, or their wool which has been removed by 
rubbing. Infestation must of course be by transfer of eggs, 
parasites, male and female, or the impregnated female which 
lays her eggs on the skin of the invaded sheep. These are 
probably hatched out in about seven days, while about 
fourteen days are required before the young acari are mature 
and capable of laying eggs. The fleece to a large extent 
protects the acarus, which is a parasite of the skin on which 
it feeds. It exhibits no natural tendency to leave the skin of 
its host ; therefore when scabby and healthy sheep congregate 
closely, it is probable that very few acari pass from the affected 
to the healthy sheep, so that in those cases in which scab is 
