LIFE HISTORY OF ASCARIS LUMBRICOIDES. 27 



considerably more susceptible to infection than oldei- pi<ji;s, but (hat 

 this susceptibility, although diminishing, does not become lost with 

 advancing age. 



Whether the lessened susceptibility of pigs to Ascaris infection as 

 they become older is the result of immunity following previous in- 

 fections or of increased resistance to infection which comes with ad- 

 vancing age is perhaps not altogether certain, but it is likely that 

 age is the determining factor rather than the establishing of an im- 

 munity in consequence of an earlier infection. If an immunity' is 

 established as a result of infection it is not immediately established, 

 inasmuch as an experiment on a kid (Experiment No. 23) has shown 

 that two infections may occur with an interval of 17 days betw^een. 

 Furthermore, natural cases have been observed in which there were 

 adults and smaller worms in the intestine and at the same time 

 larval worms in large numbers in the lungs. That age is sometimes 

 an important factor in the occurrence of parasitic infection is indi- 

 cated by experiments by the senior writer with gapeworms {Syn- 

 gamus traoheaUs) which have shown that chickens become less sus- 

 ceptible to infection as they grow older, adult chickens rarely har- 

 boring the parasites. Various observers have noted in the case of 

 other parasites that young animals are more susceptible to infection 

 than older animals, and it is evident that age often has a great deal 

 to do with susceptibility to invasions by parasites. On the other 

 hand, in the case of some parasites and some species of host animals, 

 no immunity develops with increasing age. For example, although 

 there is a great reduction in the susceptibility of chickens to infec- 

 tion with the gapeworm as they become older, there is no marked dif- 

 ference in the susceptibility of turkeys of different ages to infection 

 with the same parasite. 



The question of the production of immunity to AscaAs infection as 

 the result of earlier infections requires further investigation. Our 

 present knowledge, however, indicates that as pigs grow older they 

 become less susceptible to Ascaris infection, not because they have 

 been immunized by earlier infections, but because with advancing 

 age they become more resistant to infection. 



ASCARIS PNEUMONIA. 



As first shown by Stewart (1916a), the invasion of the lungs by 

 Ascaris larvse may cause a serious, sometimes fatal, pneumonia. Rats 

 which had been fed Ascaris eggs became ill with a pneumonia four to 

 six days afterwards and on post-mortem examination numerous 

 Ascaris larvse were found in the lungs. In later papers Stewart also 

 notes the occurrence of pneumonia in mice and pigs following the 

 administration of Ascans eggs. 



