THE SCAB. 



339 



THE ACARTTS WHICH CAUSES SCAB. 



The central figure represents the acari of their natural size on a white ground. 

 The left hand figure represents the male on its back, magnified to 366 times the 

 natural size. The right figure represents the female seen by the same magnifying 

 power. The heads or suckers of both point upward, between the inner pair of legs. 

 The less have trumpet-like appendices. Hairs are seen on them to which the young 

 ones adhere when they first escape from the pustule. 



The scab is thought often to be produced spontaneously, in 

 England, by mismanagement of various kinds, such as " bad 

 keep, starvation, hasty driving, dogging, and exposure after- 

 ward to cold and wet ;" and it spreads rapidly by contagion. 

 It is very prevalent there, and annually causes an immense 

 loss in the wool and flesh of the British flocks. In the United 

 States it is comparatively little known, and so far as I am 

 able to learn, never originates spontaneously. It is a singular 

 fact that short- wooled sheep, like the Merino, are much less 

 subject to its attacks, and this is probably one reason for its 

 little comparative prevalence in the United States. Mr. 

 Youatt observes : 



"The old and unhealthy sheep are first attacked, and 

 long- wooled sheep in preference to the short; a healthy, 

 short -wooled sheep will long bid defiance to the contagion, 

 or probably escape it altogether." 



It 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, though some considera- 

 ble time may have elapsed since the departure of the latter. 



