59 



Soon if they remain exposed to alternating high and low temperatures. 

 These experiments show that the Psoroptes can live about the sheep 

 sheds_, yards, corrals and fences during twelve or fifteen days, at least, 

 after they have separated from the sheep. 



Although this species of parasite is but a variety of the Psoroptes 

 communis, of which the Psoroptes parasitic on horses is another variety, 

 the latter has not yet been made to grow on sheep experimentally. It 

 is not probable that either of the varieties parasitic on cattle or rabbits 

 would thrive on sheep. That is to say, so far as is now known, sheep are 

 infected with common scab from other sheep, and can not acquire it 

 from other animals. On the other hand, the ovine variety of Psoroptes 

 has not yet been found to growon other species of our domestic animals. 



Differential diagnosis. — Psoroptic, or common scab, is different from 

 Sarcoptic or head scab, in that the former chooses to live where the fleece 

 is longest, and the latter where there is little or none at all — the one on 

 the back and sides, the other on the head and occasionally on the nether 

 parts; the one is almost invisible to the naked eye, and the other is 

 plainly seen, though small. The itch due to other parasites, such as 

 sheep ticks and sheep-lice, can be easily separated, because they are 

 large and can be found in the wool. Sheep are sometimes subject to 

 an inflammation of the sebaceous glands. In this, however, there will 

 be no parasites of any kind found. 



Prognosis. — Scab, as has been said before, is one of the most dreaded 

 diseases of sheep. For the flockmaster who has but few sheep, say 

 fifty or a hundred, the task of treatment and eradication of the scab 

 from the flock is no easy affair ; but for him who owns from five to 

 twenty thousand the difficulties to be met are enormously increased. 

 Though the disease may be easily treated as far as a single sheep is con- 

 cerned, still the treatment would only be palliative and would not as- 

 sure the flockmaster that the disease would not break out again. 

 Treatment, therefore, of a flock in which scab has appeared must be 

 applied to every individual exposed and to the corral and sheep-pens 

 in which they have been lodged, and is not only a serious time-consum- 

 ing affair, but a most expensive one. 



Treatment.— This is of two kinds — preventive and curative. The 

 preventive treatment is undertaken before, during, and after the 

 curative. Indeed, if the flockmaster exercises proper care his flocks 

 will never require the curative treatment, for the disease always comes 

 from transference of the insect. 



Preventive. — An infected flock should be quarantined so that it 

 shall hot transmit the disease to other flocks, and should be kept from 

 public highways where other flocks may pass, until it can be thoroughly 

 cleansed and cured. The sheds, yards, and corrals where they have 

 been kept should also be cleansed, so that they may not transmit the 

 disease. After treatment begins the sheep should be transferred to a 

 temporary uninfected yard, so that the old yard may be thoroughly 



