6 Department Circular hi, U. S. Department of Agriculture. 



affected sufficiently to show the external effects of stomach worms 

 has received a serious setback. Although it may recover and again 

 be thrifty it has lost at least a month or 6 weeks of progress toward 

 marketable weight and condition. The only safe and economical 

 way of raising sheep where stomach worms are a factor is by managing 

 the flock and pastures in a way to prevent a serious develop- 

 ment of the trouble. In most localities the methods necessary for 

 preventing stomach worms are at the same time those that need 

 to be employed for most economical production. In order to follow 

 these methods, particularly with respect to pasture rotation, the 

 shepherd needs to know just how and when infection occurs. 



HOW DO SHEEP BECOME INFESTED BY STOMACH WORMS? 



INFESTED PASTURES. 



In the adult sexual stage stomach worms are able to live and 

 carry out their reproductive functions only in the alimentary canal 

 of sheep or other ruminants, and practically only in the fourth stom- 

 ach. Each female produces thousands of eggs, of microscopic size, 

 which do not develop into adult worms in the body of the host in 

 which they are deposited, but, without hatching, pass out of the 

 intestine in the feces. In a few hours, days, or weeks, according as 

 the temperature is high or low, these eggs, if they are not killed by dry- 

 ing or freezing (either of which is commonly fatal to them), hatch and 

 the tiny embryonic stomach worms then develop to what may be 

 termed the final larval or infectious stage. This later development like- 

 wise requires days or weeks, according to the temperature, and until 

 the young worms have reached the infectious stage they appear to be 

 fully as susceptible to freezing and drying as the eggs. Having 

 reached the infectious stage, however, the worms are able to with- 

 stand long periods of dryness and severe cold, though some of them 

 succumb comparatively early. 



In the infectious stage the young worms are very active in the 

 presence of moisture, and rapidly crawl up blades of grass and other 

 objects whenever the relative humidity of the air is at a maximum, 

 provided the temperature is above 40° F. or thereabout; below that 

 temperature they are inactive. A decrease in the relative humidity, 

 with the consequent evaporation of the moisture from tho surface of 

 grass blades and other objects, stops the migrations of the worms, 

 and they become quiescent and remain in a condition of suspended 

 animation wherever they happen to be at the time. During the next 

 period of wet weather, dew, rain, or fog, tho worms again become 

 active and climb still higher on the grass, from which they are better 

 able to attain their final abode within the stomach of a sheep or cow 

 than if they remained on the ground. When swallowed by a sheep 

 or other ruminant the young stomach worm, if it has reached its final 



