6 Department Circular 47, 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 m 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 m the fourth stem- 
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 iscommonly 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 mfectious 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 erawl up blades of grass and other 
objects whenever the relative humidity of the airis 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 the 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, ram, or fog, the 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 
