ANIMAL PARASITES AND PARASITIC DISEASES 321 



the same pasture or over-stocking is a certain way to infect stock 

 with parasites which are injurious and which may cause death. 



The classification of parasites is based upon their life history. 

 External parasites live on the skin of animals; internal parasites 

 live in the internal organs; permanent parasites spend their entire 

 life with one host; temporary parasites spend but a part of their 

 existence with one host; facultative parasites have the power to 

 change from one host to another of a different species; obligatory 

 parasites can live only on or in one species of animal. 



The kinds of parasites most commonly found attacking farm 

 animals fall into five groups or classes as follows : 



Trematodes, flat, leaf-like organisms of which the flukes are an 

 example. 



Cestodes, ribbon-like organisms of which the tapeworms are 

 an example. 



Nematodes, round, pencil-like organisms of which the round 

 worms are an example. 



Insects, organisms provided with three pairs of jointed legs, 

 such as lice and flies. 



Arachnids, spider-like organisms provided with four pairs of 

 jointed legs, such as mites and ticks. 



TREMATODES 



Liver flukes (Fasciola) are the only parasites belonging to the 

 trematodes of much economic importance. They have soft, 

 non-segmented bodies. They infest the livers of sheep and cattle 

 which have access to low marshy pastures. One stage in the 

 life of this parasite is passed in the mantle cavity of certain 

 fresh water snails, from which it migrates and crawls upon grass 

 blades. When infested grass is eaten by sheep or cattle the 

 parasites gain admission to the body and find their way up the 

 bile duct. That condition known as "liver rot" is produced. 

 No drug has been found which will rid animals of the flukes. 

 As well nourished animals are less apt to acquire flukes, it fol- 

 lows that all infested animals should be fed well. Prevention 

 consists in keeping susceptible animals from moist, low-lying 

 land. A plentiful supply of salt within easy access of sheep is 

 useful in preventing infestation. 

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