582 DISEASES OF CATTLE, SHEEP, GOATS, ETC. 



provided also that the temperature is above 40 F. When the tem- 

 perature is below 40 F. the worms are inactive. 



The young worms which have reached the stage when they are 

 ready to be taken into the body are greatly resistant to cold and" dry- 

 ness ; they will stand repeated freezing, and have been kept in a dried 

 condition for thirty-five days, afterwards reviving when moisture 

 was added. At a temperature of about 70 F. young worms have, 

 been kept alive for as long as six months, and the infection in in> 

 closures (near Washington, D. C.) which had been pastured by in. 

 fested sheep did not die out in over seven months, including the 

 winter, the inclosures having been left vacant from October 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. 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 infection, 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 pasture 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 tem- 

 perature remains constantly at about 95 F. At 70 F., six to four- 

 teen days are required, and at 46 to 57 F., averaging 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 temperatures below 40 

 F., as already stated, the eggs remain dormant. 



Methods of Preventing Infection. It is evident from the fore- 

 going statements that in the northern part of the United States, 

 under usual climatic conditions, infested and noninfested sheep may 

 be placed together in clean fields the last of October or first of No- 

 vember and kept there until March or even later, according to the 

 weather, with little or no danger of the noninfested sheep becoming 

 infected. If moved then to another clean field they may remain 

 there nearly the entire month of April before there is danger of in- 

 fection. From the first of May on through the summer the pastures 

 become infectious much more quickly after infested sheep are placed 

 upon them, and during May it would be necessary to move the sheep 

 at the end of every two weeks, in June at the end of every ten days, 

 and in July and August at the end of each week, in order to prevent 

 the noninfested sheep from becoming infected from the worms pres- 

 ent in the rest of the flock. After the 1st of September the period 

 may again be lengthened. This method of preventing infection in 

 lambs would require a considerable number of small pastures or sub- 



