496 IOWA DEPARTMENT OF AGRICULTURE. 



to four weeks ai'e necessary for the eggs to hatch and the young worms 

 to develop to the infectious stage. At temperatures below 40" F., as al- 

 ready stated, the eggs remain dormant. 



METHODS OF PREVENTING INFECTION. 



It is evident from the foregoing statements that in the northern part 

 of the United States, under usual climatic conditions, infested and non- 

 infested sheep may be placed together in clean fields the last of October 

 or first of November and kept there until^ March or even later, accord- 

 ing to the weather, with little or no danger of the non-infested sheep 

 becoming infected. If moved then to another clean field they may re- 

 main there nearly the entire month of April before there is danger of 

 infection. From the 1st of May on through the summer the pastures be- 

 come infectious much more quickly after infested sheep are placed upon 

 them, and during May it would be necessary to move the sheep at the 

 end of every two weeks, in June at the end of every ten days, and in 

 July and August at the end of each week, in order to prevent the non- 

 infested sheep from becoming infected from the worms present in the 

 rest of the flock. After the 1st of September the period may again be 

 lengthened. This method of preventing infection in lambs would re- 

 quire a considerable number of small pastures or subdivisions of large 

 pastures, and in many instances could not be profitably employed, but in 

 cases where it could be used it would undoubtedly prove very effective. 

 By the time the next lamb crop appeared the pastures used the year be- 

 fore would have remained vacant long enough for the infection to have 

 disappeared, and would consequently again be ready for use. By con- 

 tinuing this rotation from year to year, not only would each crop of 

 lambs be protected from infection, but as reinfection of the infested 

 ewe flock is prevented at the same time, the parasite would in a few 

 years be entirely eradicated from the flock and pastures. 



If such frequent rotation is not possible or practicable, a smaller 

 number of pastures may be utilized, after the ewe flock has been treated 

 with vermifuges. The treatment may be given either before or after the 

 birth of the lambs. If before, the ewes should be treated before preg- 

 nancy is too far advanced, in order to avoid possible bad results from 

 the handling necessary in treatment. Probably the best time for treat- 

 ment is late in the fall or early in the winter. The treated sheep should 

 be placed immediately on clean pasture in order to avoid reinfection. 

 The object of treating the ewes is to get rid of the worms with which they 

 are infested, and thus remove the source from which the pasture be- 

 comes contaminated. If it were possible by treatment to free the old 

 sheep entirely from stomach worms, it is evident that the lambs would 

 remain free from infection, provided, of course, that the flock were after- 

 wards kept on clean pasture. Unfortunately, there is no vermifuge known 

 which can always be depended upon to remove all of the worms, but it is 

 possible to get rid of most of them, and thus greatly reduce the amount of 

 infection to which the lambs will be exposed. 



Two other methods may be suggested by which lambs can be kept free 

 from infection with stomach worms. 



