THE ANIMAL PARASITES OF CATTLE. 



631 



^ 



susceptible to freezing and drying. In very warm weather the em- 

 bryos complete their development, so far as they are able to develop 

 outside the bod}^, in two or three days. In cooler weather the time 

 required for this development is longer, and at temperatures below 

 70° F. 10 days to several weeks may be necessary. The embiyos are 

 then ready to be taken into 

 the body. The eggs and 

 early stages of the embryos 

 apparently do not develop if 

 swallowed, hence only this 

 latter stage seems to be in- 

 fectious. In this stage they 

 migrate up grass stalks 

 (fig. 20) or other objects, 

 showing activity whenever 

 the air is saturated with 

 moisture; that is, during 

 rains, fogs, and dews. When 

 the air becomes dry and the 

 moisture evaporates from 

 the grass the young worms 

 cease their activity, resuming 

 their migrations when the 

 air again becomes overladen 

 with moisture. Embryos 

 which have developed to the 

 infectious stage, unlike the 

 eggs and earlier embryonic 

 stages, are able to survive 

 long periods of freezing and 

 dryness. In two weeks to a 

 month after the embryos are 

 swallowed they reach ma- 

 turity and begin producing 

 eggs. 



Treatment for T^^'ISTED 

 STOMACH WORMS — Preventi ve 

 measures. — Preventive meas- 

 ures are important. As 

 moisture favors the develop- 

 ment of the embryos, high sloping ground is preferable for pas- 

 tures. If low gi'ound is used it should be properly drained. Do 

 not overstock pastures. Burning over the pasture will destroy most 

 of the young worms on the grass and on the ground, and, if possible, 

 this means of disinfection should be used at least once a year. 



a 



wisted stomach worms (HcBmonchus 

 contortus) . 



