356 NODULAR DISEASE IN SHEEP 



§ 280. Etiology. Prior to Curtice's discovery, this 

 disease was generally considered to be tuberculosis. He 

 showed, however, that it was caused by the embryo of Oeso- 

 pJiagosioma Cohimb ia num. 



The life history of this parasite seems to be completely 

 known to us during its development from the immature form 

 found in the intestinal tumors to the adult stage ; but there is 

 a period from the moment when the eggs escape from the 

 intestine with the excrement to the time when they are 

 found in the intestinal tumors that remains obscure. After 

 the embryo has returned into the alimentary canal it makes 

 its way through the mucous coat of the intestine and becomes 

 encysted there. Curtice was unable to learn how it passes 

 through the mucous coat, as even on the 5'oungest specimens 

 no sign of armature is found. The very young forms found in 

 cysts show little differentiation beyond what they could have 

 attained in the shell. They are soon surrounded hy a cyst 

 which seems to belong to them and to have been created by 

 them ; but whetlier this cyst is the remains of a molt has 

 not been determined. Later in their history they become 

 surrounded by the products of the inflammation they excite in 

 the surrounding tissues and e^-entualh" break from the cyst 

 and live in the cheesy mass of the tumor. In this stage of 

 their growth the worms e.xhibit the intestine and oral cup and 

 the indistinct unicellular glands. They then molt and show 

 all these features in more distinct outline. Having attained a 

 length of from 3 to 4 mm., or less than one- sixth of an inch, 

 they break from the tumors to begin their life in the intestine. 

 In the latter thej' continue their growth and becoming sexually 

 perfect, produce eggs whicli eventually go through the same 

 cycle. 



In developing, this worm molts at least three times — once 

 in passing out of the stage in which it has no mouth or intes- 

 tines, once during the development of these parts as we find 

 them in the embryo and once while the worm changes from 

 the embryonic form to the adult form. 



§281. Morbid anatomy. The harm that these parasites 

 do the sheep is directly dependent on their numbers and life 



Digitized by Microsoft® 



