CHAPTER XXV 



ANIMAL PARASITES 



The common parasitic diseases of domestic animals are 

 caused by the followin<>' groups of worms : Flukes or trema- . 

 toides; tapeworms or Cestoides; thorn-headed worms or Acantho- 

 cephalesj and round-worms or Nematoids. Flat worms, such as 

 tapeworms and flukes, re(|,uire secondary hosts. The immature 

 and mature forms of tapeworms are parasites of vertebrate 

 animals, but an invertebrate host is necessary for the completion 



of tlie life cycle of the 

 fluke." The hog is the 

 only specie of domestic 

 animals that becomes 

 a host for the thorn- 

 headed worm. The 

 round-worm is a very 

 common parasite. 

 There are many spe- 

 cies belonging to this 

 class. 



Distoma Hepati- 

 cum (Common Liver 

 Fluke). — Sheep are the most common hosts for this parasite. It 

 is present in the gall ducts and livers, and causes a disease of the 

 liver known as liver rot. The liver fluke is flat or leaf -like and 

 from thirteen to fifteen mm. long (Fig. TO). The head portion is 

 conical. It has an oval and ventral sucker, and the body is cov- 

 ered with scaly spines. The eggs are oval and brownish in color. 

 The life history, in brief, is as follows: Each adult is capa- 

 ble of producing an immense numl)cr of eggs which are carried 

 down the bile ducts with the bile to the intestine, and are passed 

 off with the faeces. Under favorable conditions for incubation, 

 248 



FiQ. 70.— Liver flukes. 



