LIFE HISTORY OF ASCAKTS LTTMBRICOIDES. 6 



Foster, 1917) also indicated that the tlieory tliat rats and mice act 

 as intermediate hosts is not tenable. In fact it has become quite evi- 

 dent that human beings and pigs become infected with Ascaris as a 

 result of swallowing the eggs, the behavior of the parasites in rats 

 and mice being simply the expression of an abortive development in 

 animals imperfectly adapted as hosts. Our investigations have 

 shown that the parasites migrate through the lungs in the guinea 

 pig, rabbit, sheep, goat, pig, and presumably man, as well as in the 

 rat and mouse. Very definite evidence has been obtained showing 

 that the animals in which the parasites reach maturity become in- 

 fected by swallowing the eggs, that the larvse after hatching migrate 

 out of the intestine into the lungs and back to the intestine, under- 

 *♦ going a development similar to that which occurs in rats, mice, and 

 ;, other unsuitable hosts. Having returned to the intestine, following 

 this migration, some of the young worms may be eliminated in the 

 feces and perish, l)ut others may establish themselves and complete 

 their development to the adult stage. 



Stewart's very striking discoveries therefore have not upset our 

 former views of the life history of Ascaris so far as concerns the 

 spread of the parasite from infested human beings and pigs to others, 

 but they have added some highly important facts to our former im- 

 perfect knowledge. Stewai^t, furthermore, has shown that the young 

 worms not only migrate through the lungs, but in so doing may set 

 up a serious pneumonia that in experimentally infected rats and 

 inice is liable to be fatal. The present writers, in a preliminary 

 note already referred to (Eansom and Foster, 1917), noted the occur- 

 rence of pneumonia in the pig as well as in smaller animals as a 

 result of the invasion of the lungs by Ascaris larvae, and Stewart in 

 some of his later papers (1917, 1918) also reported Ascaris pneu- 

 monia in pigs. It is evident that Ascaris vaf\.j likewise affect the 

 lungs of human beings, and it is an interesting fact that Mosler 

 (according to Leuckart, 1867) and Lutz (1888) in experiments in 

 feeding Asca^ris eggs to human subjects obserA-ed certain lung symp- 

 toms undoubtedly to be attributed to Ascaris infection, though of 

 course! the significance of the symptoms was not appreciated at the 

 time. It is not unlikely that many cases of lung disease of obscure 

 nature may have as an etiological factor the invasion of the lungs 

 by Ascaris larvae. Lung- affections in children especially should be 

 studied with reference to this possibility. In the case of pigs the 

 frequent and serious condition commonly known as " thumps " is un- 

 doubtedly often the result of Ascaris infection. Stewart's contribu- 

 tions to our knowledge of the life history of Ascaris may accordingly 

 prove the starting point from which an important advance can be 

 made along the line of disease prevention. 



