THE DISEASES OF SHEEP 331 



to two weeks, according as the temperature is high 

 or low. When the temperature is below 40 F. the 

 eggs remain dormant, and in this condition may re- 

 tain their vitality for two or three months, after- 

 wards hatching out if the weather becomes warmer. 

 Freezing or drying soon kills the unhatched eggs. 

 The tiny worm which hatches from the eggs feeds 

 upon the organic matter in the manure, and grows 

 until it is nearly one-thirtieth of an inch in length. 

 Further development then ceases until the worm is 

 swallowed by a sheep or other ruminant, after 

 which it again begins to grow, and reaches matu- 

 rity in the fourth stomach of its host in two to 

 three weeks. The chances of the young worms be- 

 ing swallowed are greatly increased by the fact that 

 they crawl up blades of grass whenever sufficient 

 moisture such as dew, rain, or fog is present, 

 provided also that the temperature is above 40 F. 

 When the temperature is below 40 F. the forms 

 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 dryness; they will 

 stand repeated freezing, and have been kept in a 

 dried condition for thirty-five days, afterwards re- 

 viving when moisture was added. At a tempera- 

 ture of about 70 F. young worms have been kept 

 alive for as long as six months, and the infection 

 in inclosures (near Washington, D. C.) which has 

 been pastured by infested sheep did not die out in 

 over seven months, including the winter, the inclo- 



