STATE HYGIENE 521 



Non-poisonous sheep dips of the phenol group may be 

 used, and these also form the subject of convenient pro- 

 prietary preparations. 



Whatever substance is employed a certain length of 

 contact is required in order to destroy the parasite, and 

 complete immersion of the body in the solution, including 

 the head, is an absolute necessity. From one to two 

 minutes' immersion, depending on the agent employed, is 

 necessary for the destruction of the parasite, though it by 

 no means follows that this is sufficient to destroy all the 

 eggs. The latter are deposited under the crusts formed by 

 the disease, where they are moderately safe from para- 

 siticides. The female acarus takes 15 days to become an 

 adult ; she lays about 15 eggs which are hatched out in 

 two or three days, and of these eggs two-thirds are females. 

 One pair of mature acari will produce a progeny of 

 1,600,000 acari in three months. 



A second dipping is therefore essential to kill off the 

 acari which as eggs escaped the first dipping, and the 

 second dipping may be repeated fifteen days after the first. 



Some other facts regarding the acarus of this disease are 

 also important to bear in mind, both from a therapeutic 

 and legislative point of view. There is no doubt of the 

 remarkable vitality of the organism apart from the body ; 

 mature acari have been known to live on rubbing places 

 for 21 days, also in dry earth and manure, while on the 

 authority of Professor Wallace it is said the eggs may 

 retain their vitality for years. 



These facts show how easy it is for a flock to get re- 

 infected, and explain, perhaps, what the lay mind finds it 

 so difficult to disbelieve, that is the apparent spontaneous 

 origin of the disease. All rubbing places, gates, posts, 

 hedges, etc., are sources of fresh infection, even the 

 pieces of wool torn out by the animal during the rubbing 

 operation are liable to reinfect. Yet in spite of these 

 sources of infection the disease, as we have seen, may be 

 eradicated, but it strongly points to the necessity for 

 regular dipping ; and this, in fact, is done in Australia in 



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