648 MANAGEMENT OF SHEEP EXTERNAL PARASITES 



hatch out naturally, overwhelm the individuals of the new 

 brood by a second dipping before they are old enough to 

 propagate their kind. If this work be perfectly done, the 

 cure is complete ; but, as will transpire later, it is not all so 

 simple as it at first appears. The mites live by biting the 

 external surface of the skin, which becomes very much 

 irritated and inflamed as a result of the action of the 

 effective hooked and pointed mandibles. Pustules form and 

 exude a serous fluid which dries into a crust or scab. This 

 gradually extends and thickens as the colony becomes more 

 numerous and the irritation and exudation greater. Under 

 the edges of these scabs, which are firmly held in position by 

 involving the bases of the wool fibres in their vicinity, the 

 females find shelter and suitable repositories for their eggs. 

 Each female when adult (fifteen days old) produces about 

 fifteen eggs, which hatch in two or three days, some two- 

 thirds of them developing into females. At first the young 

 have only six legs, but the number ultimately increases to 

 eight. The young migrate to fresh skin, and the affected 

 area grows after the fashion of a fairy ring in pastures, the 

 active part being that at the edge of the expanding 

 area. At first the development is slow, and a flock may 

 be slightly affected for weeks without much chance of the 

 disease being discovered ; but in two months or less the 

 process of the increase in numbers proceeds at a rapid 

 rate, and detection by the naked eye even at a distance 

 becomes easy. 



Such is a brief account of the common means by which 

 the affection spreads on a single sheep. A scattering of the 

 colonies takes place as the animal bites or rubs in the effort 

 to relieve the excessive itchiness with which scab is associated. 

 This same effort is a fruitful means by which the parasite 

 spreads from sheep to sheep. In the action of rubbing on 

 earth-banks, walls, rocks, and rubbing-posts, a few scab-mites 

 or their eggs are left on the " rub," to be transferred to the 

 next sheep that come along to exercise the habit natural to 

 them, although they may not be affected with scab. Scab- 

 mites do not propagate away from the skin of a living sheep, 

 but under favourable conditions the mature forms have been 

 known to exist for a period of twenty-one days on " rubs " 

 and in dry earth or manure upon which sheep have been 



