44 BULLETIN 811, U. S. DEPT. OE AGRICULTURE. 



finally pass out of the body in the feces. They may be found in 

 the liver as early as 2 days after infection, in the lungs and 

 trachea as early as 3 days after infection, and in the alimentary 

 tract after their passage through the lungs as early as 6 days after 

 infection. They are common in the lungs a week to 10 days after 

 infection, becoming scarce in the liver as they become numerous in 

 the lungs. Within a little over two weeks after infection all or 

 practicall}^ all the larvae are usually eliminated, but have been found 

 still present and active in the liver, lungs, and alimentary tract as 

 late as 23 days after infection. 



In 3'oung goats and lambs the larvjB of Ascaris suum after migrat- 

 ing through the lungs settle down in the small intestine and undergo 

 development approaching maturity, these animals thus being better 

 adapted as hosts than rats, mice, guinea pigs, and rabbits. 



In pigs Ascmns larvae after migrating through the lungs settle 

 down in the small intestines and develop to maturity, and presum- 

 ably the same cycle occurs in human beings. 



Eats and mice play no part in the normal life history of Ascaris. 

 The behavior of tlie larvae in these animals and in guinea pigs and 

 rabbits may be considered simply an expression of imperfect adapta- 

 tion of the parasites to. existence in these hosts. 



In pigs and human beings Ascaris may develop to maturity within 

 two and one-half months after infection. 



In guinea pigs the larvae apparently reach a larger average size 

 than in mice in the same length of time, and still larger in rabbits. 



Migrating Ascao'^s larvae produce destructive lesions in the liver 

 and lungs, especially in the latter. 



Some larvae die during their migrations. Dead and degenerated 

 larvae have been found in the lungs as late as 86 daj^s after infection, 

 and in the liver 296 days after infection. 



The invasion of the lungs by Ascaris larvae may result in a serious 

 and sometimes fatal pneumonia, which appears in a week to 10 days 

 after ingestion of the eggs. 



Young pigs are more susceptible to Ascaris infection than older 

 animals, but may not entirely lose their susceptibility with advancing 

 age. 



No evidence has been obtained that one infection with Ascai^is ren- 

 ders animals less susceptible to later infections. 



If properly incubated Asca't'is eggs are injected beneath the skin 

 they will hatch, and the larvae will migrate to the lungs, where they 

 may be found a few days after injection of the eggs to have reached 

 the same stage of development as they would if infection had oc- 

 curred from swallowing the eggs. 



The larvae of Belascaris marginata undergo migrations in rats simi- 

 lar to those of Ascaris suum and A. lumbricoides. 



