PARA-SITIC DISEASES OF THE SKIN. 243 



frenzied motions of the sheep carry with them tags of wool, 

 leaving the affected surfaces denuded and sore. These become 

 again covered by scabs, which are thicker than the former, these 

 being still further increased in circumference by the outward 

 migration of the parasites. As the affected surfaces enlarge 

 their centers tend to heal, but the skin remains thick and 

 wrinkled for a long time, the growth of the fleece being re- 

 tarded and sometimes destroyed from the injury to the hair 

 follicles. 



]y[od«9 of Infection. 



Since scab is caused by insects which are being continually 

 ■distributed by the sheep rubbing on posts or dropping tags of 

 wool containing pieces of scab, which on coming in contact with 

 other sheep soon cause an outbreak on them also, it can read- 

 ily be seen that the sources of infection are many, among which 

 can be enumerated the shipping of infected sheep in railroad 

 cars as one of the commonest methods of distribution, or per- 

 mitting healthy sheep to graze on a range on which scabby sheep 

 have been kept, or the exposure in any way of healthy sheep to 

 the eggs of the parasite, which are more to be dreaded than the 

 mite itself. The parasite itself cannot live for any length of 

 time away from the body of its host, and experiments have 

 shown that it will succumb after a period of from ten to fifteen 

 days, but its eggs, being almost indestructible owing to their 

 ■ dense fibrous covering, will last indefinitely, and should the 

 sheep pick up any particle of wool or pieces of scab containing 

 these eggs it is only necessary for them to remain in contact with 

 the skin for a short period for them to hatch and become the foci 

 for new and disastrous outbreaks. Ranges when once infected 

 are not considered safe grazing grounds for the space of one year. 

 Pens when infected need to be thoroughly gone over with slaked 

 dime .wash, with some parasiticide added to it, before they are 



