172 . DISEASES OF SWINE 



they accumulate in manure heaps, and become scat- 

 tered over the hog lots and pastures. The eggs or 

 ova containing embryonic worms are eaten by white 

 grubs, in which they encyst. The white grubs are 

 in turn eaten by the hog, the larvae of the thorn- 

 headed worm are liberated by the digestive juices, 

 and become a parasite of the small intestine, where 

 they develop into mature parasites. 



It is necessary, then, for the hog to eat a white 

 grub, possibly some other invertebrate, that is in- 

 fected with the larval form of the parasite before it 

 can become infected. ^ Hence it is in old hog lots and 

 pastures, around manure heaps and in clover fields 

 where grubs are plentiful, that hogs suffer most 

 from thorn-headed worms. \ 



Symptoms. — The worm is usually found with its 

 bead end. buried more or less deeply in the walls of 

 the intestine. The irritation to the tissues at the point 

 of attachment is severe, and, when a number of 

 worms are present, the intestines appear badly irri- 

 tated. It may not remain in one place, but drift to 

 different parts. The former places of attachment, 

 if recent, are miarked by local areas of inflamed tis- 

 sue. These red, thickened, hard areas are usually 

 albout a quarter of an inch or more across, and have 

 a central depression, which marks the point where 

 the hooked head was imbedded. The entire thick- 

 ness of the intestinal wall may be involved. 



Some writers state that perforation of the in- 

 testine sometimes occur. We should doubt the 

 truth of this statement. Whenever a hog's in- 



