THE ANIMAL PARASITES OF CATTLE. 



507 



development. The embryos which hatch from the eggs are micro- 

 scopic in size, and like the eggs are susceptible to freezing and drying. 

 In very warm weather the embryos complete their development, so 

 far as they are able to develop outside the body, in two or three days. 

 In cooler weather the time required for this development is longer, 

 and at temperatures below 70° P. ten days to several weeks may be. 

 necessary. The embryos 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 infectious. 

 In this starp they migrate up grass stalks (fig. 17) 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 mi- 

 grations 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 swal- 

 lowed they reach maturity and begin producing 

 eggs. 



Treatment for twisted stomach worms — 

 Preventive measures. — Preventive measures are 

 important. As moisture favors the develop- 

 ment of the embryos, high sloping ground is 

 preferable for pastures. If low ground is uesd 

 it should be properly drained. Do not over- 

 stock 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. Change the herd to fresh pasture as 

 often as possible. Cattle should be supplied 

 with water from wells, springs, or flowing 

 streams, preferably in tanks or troughs raised above the ground. 

 To a slight degree, salt serves to protect cattle against infection 

 with internal parasites, and plenty of it should therefore be kept 

 accessible. 



Medicinal treatment.— Among the remedies used to remove stomach 

 worms may be mentioned coal-tar creosote, bluestone, and gasoline. 

 It is advisable to treat not only the animals which are seriously affected, 

 but the rest of the herd as well, since the parasites with which they 

 are infested will remain as a source of reinfection to the others. The 

 cattle should be removed to fresh pasture after treatment, if possible. 



Fig. 17.— Embryo of twisted 

 stomach worm (Hmmon- 

 chus contortus) coiled on 

 tip of grass blade. 



