SHEEP SCAB. 3 



vats should be kept in repair, and for several years to come each 

 sheep owner will probably find it wise to dip his sheep at least once 

 a year as a precautionary measure. 



THE PARASITE WHICH CAUSES SHEEP SCAB. 



The mites which cause common sheep scab are small insectlike 

 parasites known t^^chnically as Psoioptes communif< oris or Psoroptvs 

 oris, the male measuring when fully grown only about one-fiftieth 

 and the female one- fortieth 

 of an inch in length. They 

 may be seen with the naked 

 eye, particularly if they are 

 placed on a dark back- 

 ground. They occur on any 

 portion of the body covered 

 by wool, but are most com- 

 mon where the wool is 

 thickest; they are the sole 

 cause of the disease. (See 

 figs. 1 and 2.) Their de- 

 struction is followed by re- 

 covery. Avhereas any treat- 

 ment which does not destroy 

 them fails to cure scab. 



LIFE HISTORY OF THE SCAB MITE. 



The various stages in the 

 life historj" of the scab mite 

 are all passed on the body of 

 the sheep. After the mat- 

 ing of the sexes the females 

 deposit their eggs in clumps 

 on the skin at the base of the wool fibers. Each female may deposit 

 at least as many as 15 eggs, which hatch after 3 or 4 days' incubation ; 

 the young mites grow to maturity in 7 or 8 days and in 3 or 4 days 

 more mate and deposit their eggs. The females apparently live but 

 a short time after they have laid their eggs. 



According to the figures above, which are those given by Gerlach, 

 the entire life cycle is completed in 12 to 15 days. Other observers 

 state that the period of incubation of the eggs may be as long as 

 10 days, but this is probably exceptional, and Tdays may be assumed 

 to be the usual maximum limit of incubation. Assuming that each 

 female has 15 offspring and taking 15 days as the period required 

 for each brood of eggs to hatch and the young mites to develop 



Fig. 1. — Sheep-scab mite (Psoroptcs ovis). Male. 

 Dorsal view, sri'^i^tly enlarged. (After Salmon 

 and stiles. 189S.) 



