g Department Circular 47, U.S. Department of Agriculture. 
infection is not destroyed by cold weather. They show, however, 
that during a winter with more or less freezing weather there is 
likely to be little or no increase in the amount of infection in pastures 
occupied by infested sheep. The eggs passed in the feces of the sheep 
will either be killed at once by freezing, or, on account of low tem- 
peratures above freezing, will remain dormant or develop so slowly 
that they are killed later by frost before they have reached the final 
larval stage, which is resistant to cold. At the same time, while the 
infestation of pastures may not be increased during the winter, the 
infestation of the sheep may be added to by their picking up from 
time to time larval worms which, prior to the beginning of cold 
weather, had developed already to the stage in which they are able 
to w ithstand freezing. 
If sheep, goats, and cattle are kept off a pasture for a year, it is 
fair to assume, upon the basis of our present knowledge, that all, or 
practically all, larval stomach worms will have died within that time. 
There is also little doubt that the period required for the practical 
disinfection of a pasture may be shortened considerably by plowing 
it and placing it under cultivation. 
Thus there are two ways by which a pasture may be freed of infesta- 
tion, one by excluding sheep or other ruminants for at least a year, and 
the other by turning it into a cultivated field. In view of the fact 
that sheep placed on disinfected fields or pastures probably will not 
be entirely free from infestation, it is not of much consequence 
whether every larval stomach worm in the pastures is dead or not. 
The approximation to this point, which is attained by vacating pas- 
tures for a year or by plowing them up, is sufficient for practical 
purposes. 
WHAT METHODS CAN BE EMPLOYED TO PREVENT LOSS FROM 
STOMACH WORMS? 
It is barely possible that some means of artificially producing in 
lambs an immunity against the evil effects of stomach worms may 
be devised, but at the present time it is only a matter for speculation 
and experimental research. Our present knowledge of the stomach 
worm leads us to direct our efforts toward bringing about freedom 
from infestation or, as the next best thing, reducing the amount of 
infestation to a minimum and keeping it there. 
EARLY DEVELOPMENT OF LAMBS. 
One great step toward evading the stomach worm is found in the 
plan of having lambs dropped early and feeding to develop them as 
much as possible before they go to pasture. 
Where sheep graze during winter and spring there is little danger 
of infestation in freezing weather, as the eggs or young larve are killed 
