12 PARASITES OF THE DOMESTIC ANIMALS 



are more subordinate to the mode of development of the infecting 

 species. Sheep grazing upon low, marshy land and in the vicinity of 

 ponds are more exposed to infestation with flukes, because there are 

 present conditions essential to the moUuscan intermediate host in which 

 the fluke at the stage of the miracidium must find lodgment. Infestation 

 of the pig or the ox with the larvae of the tapeworms of man is most 

 likely to occur where untreated human excrement is used as a fertilizer, 

 or where their food may otherwise be directly or indirectly contaminated 

 with such material, while invasion of the human host with the adult 

 worm only occurs after ingestion of' the tissues of the larval host. The 

 majority of ova of worms expelled by the host fail to find a new host, or 

 meet with unfavorable conditions and are lost. Some, as those of 

 ascarids, are very resistant and may find their proper host after months 

 of exposure to destructive infiuences. Migration is facilitated to some 

 extent where hatching takes place with the laying of the egg, as in the 

 strongyles of the respiratory tract and in Trichinella. 



While much remains to be determined as to the life histories of many 

 of the internal parasites, clinical experience indicates that low and wet 

 pasturage, with access to stagnant collections of water, is a strong 

 etiologic factor in helminthiasis, either as harboring possible aquatic 

 intermediate hosts of the worms, or as a vehicle which, directly or by 

 drainage, spreads infestation by dissemination of their germs. 



