SEVENTH ANNUAL YEAR BOOK— PART X. 495 



LIFE HISTOHY OF THE STOMACH WORM.* 



The worms in the stomach produce eggs of microscopic size, which 

 pass out of the body in the droppings and are thus scattered broadcast 

 over the pasture. If the temperature Is above 40° to 50° F. the eggs 

 hatch out, requiring from a few hours 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 retain their vitality for 

 two or three months, afterwards hatching out if the weather becomes 

 warmer. Freezing or drying soon kills the unhatched eggs. The tiny 

 worm which hatches from the egg 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 

 maturity in the fourth stomach of its host in two or three weeks. The 

 chances of the young worms being 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 tempera- 

 ture is above 40° F. "When the temperature 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 dryness; 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 inclosure (near Washington, 

 D. C.) which had been pastured by infested sheep did not die out in over 

 seven months, including the winter, the inclosures having been left 

 vacant from October 2.5th to June 16th. 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 infection, and fields which have had no sheep or other ruminants 

 upon them following cultivation may also be safely used. The time re- 

 quired 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 drop- 

 pings of the sheep have hatched out and the young worms have de- 

 veloped to the final larval stage, and the rapidity of this development 

 depends upon the temperature. It may be stated here that neither the 

 eggs nor the newly hatched worms are infectious, and only those worm3 

 which have reached the final larval stage are able to continue their de- 

 velopment 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 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 



* Some of the details in the life history of this parasite are treated more at length 

 in Circular No. 93 of the Bureau of Animal Industry. 



