Stomach Worms in Sheep. 7 



larval stage, whether active at the time or in a state of suspended 

 animation, continues its development, and in the course of 2 or 3 

 weeks reaches maturity. 



The length of life of individual worms in the stomach has not been 

 determined. Infested sheep have been kept in pens with board 

 floors, which were kept clean by sweeping and frequent scrubbing — 

 the sheep being fed from raised racks and water being supplied in a 

 trough which was frequently cleaned — for varying periods up to a 

 maximum of 19 months, and at the end of these periods were found 

 to be still infested, though the number of worms present was small. 



As the possibility of reinfection by larval worms developing from 

 eggs passed in the feces of these sheep was not entirely removed, 

 though greatly minimized, the results obtained do not necessarily 

 indicate that the worms found at the end of the period of observation 

 were all present when the experiment was begun. The experiment, 

 however, while it proves nothing as to the length of life of the adult 

 stomach worm, demonstrates the futility of attempting to rid sheep 

 entirely of stomach worms simply by keeping them away from pas- 

 ture. On the other hand, very little infection occurs among sheep 

 kept in stables if cleanly conditions are maintained. Lambs have 

 been kept with infested sheep in stables for long periods of time, the 

 only precautions against infection being the removal of manure about 

 once a week. Under such conditions they have continued in good 

 health, and acquired only a very few stomach worms and other 

 parasites. 



LENGTH OF TIME PASTURES MAY BE INFESTED. 



The maximum period during which the larval stomach worms are 

 able to survive on pastures is not definitely known, but it has been 

 found that pastures on which infested sheep had grazed were appar- 

 ently still infectious after a lapse of nearly 8 months, namely, from 

 October 25, when the infested sheep were removed, to June 16, when 

 the pastures were tested by placing in them some lambs which had 

 been raised under special precautions to avoid previous infestation. 

 In cultures made September 14, 1906, from the feces of an infested 

 sheep and kept thereafter in the laboratory, most of the larvae were 

 dead but some were still alive, though very sluggish, on June 5, 1907, 

 nearly 9 months later. Cultures in which the worms were allowed 

 to develop to the final larval stage, after they were kept in cold storage 

 at a temperature below freezing — in some cases as low as 12° F. — 

 still contained some living worms after 2 or 3 months, while in other 

 cultures eggs and newly hatched worms not yet developed to the final 

 larval stage were killed within a few hours after exposure to tempera- 

 tures below freezing. 



These experiments show that pastures may remain infected for 

 several months after sheep are removed from them, and that the 



