THE ITCH OR SCAB INSECTS- ACARIASIS-ITCH -SCAB. 

 Sarcoptes, Latr. ; Psoroptes, P. Gerv.; Chorioptes, P. Gerv. 



Of all the diseases caused by external parasites those due to the scab- 

 makiug insects are of the first importance. The losses due to them are 

 very heavy, and are exceeded by those of no other external parasite, 

 and equal those of the most destructive of internal parasites. Owing 

 to the means used in preventing flocks from becoming infected, and to 

 the extermination of the pests by the proper use of eflScacious remedies, 

 these losses are annually becoming reduced. It is to be hoped that in 

 the near future, through the exercise of proper sanitary laws, this most 

 tractable of all the parasitic diseases may be completely exterminated 

 in our country. 



Scab is a disease due to the presence of minute insects which lead a 

 parasitic life on the skin of their hosts. Jt is caused by the inflamma- 

 tion they excite in penetrating the skin, that they may procure food for 

 themselves and young, and suitable conditions under which the eggs 

 may be deposited and hatched and the young matured. The disease is 

 thought by some to be aggravated by a poisonous fluid secreted by the 

 insects, which adds to the intense itching. 



The malady proceeds step by step with the growth, propagation and 

 decay of the innumerable insects which result from the acquisition of a 

 single fertile female or a few pairs of individuals, and the spread of the 

 disease, therefore, coincides with an increase in the numbers of the par- 

 asites, while the limitation of the disease follows their extermination. 



There are at least three difl'erent species of scab-forming insects, 

 parasitic on sheep, and each species is not only specifically ditferent 

 from the other, but the disease caused by each is different. This is 

 due to the variation in the anatomical structure and habits of life in the 

 several species of these pests, which causes them to attack the skin in 

 different places and by different methods, and hence gives rise to the 

 various symptoms common to each disease. 



These insects are known as Acari, and the diseases they cause as 

 acariasis. The various species parasitic on sheep are Sarcoptes scabiei, 

 de Geer, var. ovis; Psoroptes communis, Fiirst, var. ovis ; Chorioptes 

 communis, Verheyen, var. ovis. Of these the Sarcoptes causes scab of 

 the head ; Psoroptes causes common scab, and the Chorioptes foot scab. 



The life history of these parasites is in general very similar. They 

 attack the external skin of the animals in which they livx by biting it. 

 Soon after scabs are formed. Under these scabs the insects lay their 



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