PARASITIC DISEASES 1 83 



of the lung worm of hogs is not known. The ma- 

 ture worms, which are present in the bronchi, pro- 

 duce a large number of eggs and embryos. These, 

 and sometimes the adult worms, are coughed up 

 with the mucus and become scattered about the 

 pens and pastures. Moist surroundings are favor- 

 able for the development of the embryos, but such 

 conditions do not seem to be necessary for their 

 development. Pigs kept in dry lots and well cared 

 for frequently become infected, but wet surround- 

 ings are more favorable for infection, and under 

 such conditions this class of disease may take on a 

 severe form. 



Lung worm diseases cannot be produced in ani- 

 mals by feeding them the mucus from the air pas- 

 sages of diseased individuals. The only avenue of 

 infection is seemingly the air passages. It has been 

 shown that after the embryos have undergone de- 

 velopment in moist earth, they stand drying for 3. 

 short time, and when placed in moist surroundings 

 will again assume their activity. It seems possible, 

 then, for infection to occur as a result of inhaling 

 dust containing embryos. This is possible because 

 of the habit hogs have of wallowing in the mud 

 and coating themselves with dirt, which soon dries 

 and is brushed off in the hog houses and pens, and 

 helps to make their quarters dusty. When this dust 

 is inhaled, any embryonic worms contained in it 

 will find sufficient moisture in the air passages to 

 revive them. 



Symptoms and Lesions. — The disease: is largely 



