164 PARASITES OF THE DOMESTIC ANIMALS 



of the sheep to pastures free from standuig water or overflow. Since 

 the adoption of such preventive measures the loss fi'om this source in 

 England and other European countries has been greatly reduced. 



While fascioliasis has not been as prevalent in the United States as in 

 Europe, there are a sufficient number of cases to demonstrate the pos- 

 sibility of its becoming so unless such precautions are taken as are in- 

 dicated by the life history of the fluke. Probably the freedom from such 

 destructive prevalence has been largely due to the fact that in this 

 country, more generally than in Europe, it is the practice to turn sheep 

 upon higher and dryer pasturage. 



The three species of flukes which have been mentioned infest the 

 liver, therefore the hepatic form of fascioliasis is the most important. 

 As would be concluded from the mode of infection, herbivorous animals 

 are most often affected, those which crop the grass close to the ground, 

 as sheep and goats, being for this reason especially susceptible. Horses, 

 swine, Carnivora, and even man may be invaded incidentally by flukes, 

 though in such cases they are rarely present in such numbers as to produce 

 perceptible disturbance. The giant fluke {Fasdola magna) is most often 

 found in the liver of deer or cattle in the Southwestern portion of the 

 United States. It is supposed to be a species native to wild ruminants 

 before the introduction into this country of those in domestication. 



Infection. — While infestation of sheep most frequently occurs from 

 the ingestion of plants upon which the encysted cercariae are attached, 

 water contaminated with detached cercariae may infect directly, or by 

 vegetation over which it has washed. It is probable that many cases 

 in cattle in the United States result from the introduction of the flukes 

 by the latter means. Cattle are not as close grazers as sheep, but they 

 drink more frequently, often from shallow collections of water which 

 may contain larvae derived from the excrement of sheep or rabbits. 



As many encysted cercariae survive the frosts even of the late fall, 

 the season during which infection may take place is somewhat extended, 

 some investigators claiming that it may occur at any time of the year. 

 However, warmth being highly favorable to the development of the ova, 

 it is essentially during the summer and early autumn that animals are 

 most likely to become invaded. It is obvious that the most numerous 

 and most severe cases would occur in seasons of copious rainfall, more 

 elevated pastures at such times affording by their collections of water 

 and overflow favorable conditions for the development of the parasites. 

 The flukes may be introduced into lands previously free from them 

 by new stock, or by wild herbivorous animals, such as deer and rabbits. 

 After infestation has once taken place, it will, through successive in- 

 fections, increase in degree the longer the pasture is used. 



Migrations and Pathogenesis. — It is probable that the migration 

 of the parasites from the small intestine into and along the bile ducts 



