332 SHEEP FARMING IN AMERICA 



sures having been left vacant from Oct. 25 to 

 June 16. It is uncertain whether infection in fields 

 from which sheep have been removed will die out 

 more rapidly during warm weather or during cold 

 weather ; experiments on this point are under way, 

 but have not been sufficiently completed for definite 

 statements to be made. It is, however, safe to say 

 that a field which has had no sheep, cattle, or goats 

 upon it for a year will be practically free from in- 

 fection, and fields which have had no sheep or other 

 ruminants upon them following cultivation may also 

 be safely used. The time required for a clean pas- 

 ture to become infectious after infested sheep are 

 placed upon it depends upon the temperature ; that 

 is, the field does not become infectious until the 

 eggs of the parasites contained in the droppings 

 of the sheep have hatched out and the young worms 

 have developed to the final larval stage, and the 

 rapidity of this development depends upon the tem- 

 perature. It may be stated here that neither the 

 eggs nor the newly hatched worms are infectious 

 and only those worms which have reached the final 

 larval stage are able to continue their development 

 when swallowed. This final larval stage is reached 

 in three to four days after the eggs have passed 

 out of the body of the host if the temperature re- 

 mains constantly at about 95 F. At 70 F., six 

 to fourteen days are required, and at 46 to 57 

 F., aggregating about 50 F., three to four weeks 

 are necessary for the eggs to hatch and the young 

 worms to develop to the infectious stage. At tern- 



