21 



embryo apparently spending long seasons of suspended life functions 

 on the ground. The fimbriated tape-worm develops more slowly, con- 

 suming the greater portion of the year ; its embryos may exist on the 

 ground for indefinite periods. The cystic tape-worms pass indefinite 

 periods as cysts in sheep, depending on their resistance to the vital 

 forces of th-e organs infested and upon the date of their liberation from 

 Imprisonment. The life cycle of the liver flukes seems to be completed 

 iu a few months. The majority of the round worms seem capable of 

 withstanding the elements while scattered over the pastures for indefi- 

 nite periods, either as ova or partially and completely developed 

 embryos. Their cycle of life in sheep is of variable periods, depending 

 on the species. (Esopliagostoma, some of whose embryos invade the 

 intestinal wall, offers a retarded development lasting through months. 

 Other species develop more rapidly. The exact cycle for each species 

 has not been determined, but most of them become adult in less than 

 six months, some in less time than three. 



The seasonal appearance of each species depends on its life cycle, 

 the average temperature and the humidity of the season, and the age 

 of the lambs. Spring and fall seem to offer the most outbreaks of dis- 

 ease produced by parasites. Summer and winter also have their spe- 

 cial parasitical diseases. Sheep-ticks, lice, and scab are more prevalent 

 in winter when the sheep are closely herded in yards or barns, and when 

 they are covered with heavy fleeces. The gad-fly occurs most in June 

 and July, but in milder climates it evidently flies the greater part of 

 the year. The disease it develops is more prevalent in older sheep, 

 yearlings being the yo^^ngest that show distinct signs. The broad 

 tape- worm infests young lambs early and causes their disease in a very 

 few months. March lambs harbor adult worms in May and June, and 

 May lambs in August. The fimbriated tape-worm also infests Iambs 

 early, but does not i)rodnce its worst effects until late fall and winter. 

 The liver flukes generally appear first in summer and fall. The round 

 worms appear in young sheep of three months and upwards. The ma- 

 jority of those that produce disease develop it as they grow adult. The 

 thread lung- worms [Strongylus filaria) infest lambs, and epidemics due 

 to them usually occur from spring to fall. The hair lung- worm, on the 

 other hand, develops slowly, and while their presence can be detected 

 in the lungs of young lambs it is the lungs of old sheep which show the 

 greatest amount of changes due to their invasion. As a rule warm, 

 moist seasons are most favorable to their development. The climate of 

 the tJnited States so varies from North to South and East to West that 

 no exact seasonal appearances of the various species can be given. Most 

 of the species seem to be present in sheep in greater or less numbers 

 the year round. The most important factors in the time of outbreak of 

 difierent diseases seein to be the age of the hosts and the cycle of life 

 of ttie parasite. 



