276 PARASITES OF THE DOMESTIC ANIMALS 



in the paleness of the body tissues, edematous swelUngs, exudate into 

 the serous cavities, and cachexia. 



Where Ostertagia ostertagi are present in the abomasum of cattle they 

 will be found both free in the stomach contents and embedded in the 

 subepithelial tissue of the mucosa in small round cysts about the size 

 of a pin-head or slightly larger. When numerous, the same lessions are 

 shown as in the attack upon the mucosa of a heavy invasion with Hcemon- 

 chus cojitortus. 



Gasteo-Intestinal Strongylosis. Development and Etiology 



The eggs of Hcemonchus contortus passed in the feces of the host will 

 hatch in a variable time according to the conditions of temperature and 

 moisture. When these are favorable it may occur in a few hours, while, 

 under more adverse conditions, it may take several days or weeks. 

 Dryness or a freezing temperature kills the embryos and newly hatched 

 larvse in a short time. Upon hatching the larva feeds upon the fecal 

 matter with which it is surrounded. Later it becomes enveloped by a 

 chitinous sheath, in this condition probably receiving nourishment 

 from food material stored within its body. At this stage the larva can 

 survive freezing and drying for long periods and is motile at temperatures 

 above 40° F., becoming more active with increase in temperature. Where 

 there is sufficient moisture, as from dew or rain, it crawls upon a blade 

 of grass or other vegetation and gradually makes its way to a position 

 well removed from the ground. In this position it is taken up by the 

 grazing ruminant host and, reaching the abomasum, becomes mature in 

 two to four weeks. If the eggs or newty hatched larvae are ingested 

 they apparently do not undergo further development. It seems, there- 

 fore, that only the ensheathed larvse are infective. 



Control. — As stated in the foregoing, moisture favors the develop- 

 ment of the embryos, while dryness kills them at their early stages. 

 High pasture ground, therefore, with good natural drainage greatly 

 reduces the chances of the larvse reaching the infective stage. Further- 

 more, larvse which have become infective are more motile in the presence 

 of moisture such as is supplied by the heavy dews and fogs occurring 

 over low land; crawling, then, out upon the wet blades of grass, the 

 worms are more likely to be taken up by the grazing animals. 



If the temperature remains constantly at about 95° F. the infective 

 larval stage is reached in three to four days after the eggs have passed 

 from the body of the host. At 70° F. one to two weeks are required, 

 while three to four weeks are necessary at about 50° F. At temperatures 

 below 40° F. the eggs are dormant and the larvse remain inactive. Under 

 the usual climatic conditions of the northern part of the United States, 

 therefore, there is little possibility of new infection from placing in- 



