230 PARASITES OF THE DOMESTIC ANIMALS 



certain migrations and changes of development, after which they may 

 be carried in the feces or saHva of the rats or mice to food or other 

 materials which may be ingested by human beings or pigs, thus ulti- 

 matelj^ reaching their final host. 



This conclusion is contrary to the opinion usually accepted that 

 Ascaris infects man or the pig directly through the ingestion of the eggs 

 of the parasite. In a preliminary note upon the life history of Ascaris 

 lumhricoides and related forms Ransom and Foster, of the Zoological 

 Division of the Bureau of Animal Industry, state that in a repetition 

 of Stewart's experiments in feeding rats and mice with Ascaris eggs 

 they obtained results agreeing very closely with those which he had 

 recorded, also that further investigations have shown that guinea pigs 

 as well as rats and mice may be similarly infected by feeding Ascaris 

 eggs. Their negative or uncertain results from attempts to infect pigs 

 with Ascaris by feeding the eggs agreed with the experience of Stewart 

 and other investigators, nevertheless they did not feel justified in accept- 

 ing these results as evidence against the hypothesis of a direct develop- 

 ment without an intermediate host. They note that Epstein in care- 

 fully controlled experiments with feeding eggs of Ascar'is lumbricoides 

 used very young subjects and that the positive results which he obtained 

 can scarcel}^ be explained upon any other assumption than that a direct 

 development of the parasites occurred following feeding of the eggs. 

 The failures of others to infect adult human beings and the unsuccessful 

 attempts to infect pigs several months old in the same way are considered 

 as suggesting the possibility that age is an important factor influencing 

 the susceptibility of human beings and pigs to infection with Ascaris. 

 In support of this, and in agreement with the migration of larvse which 

 occurs in rats and mice, they cite an instance of a pig about six weeks 

 old which, dying from unknown causes, revealed on examination an 

 Ascaris larva in a fragment of lung and numerous immature ascarids 

 in the intestine, the largest about two inches long. 



In order to test the possibility of infecting very young pigs these 

 investigators used two young pigs from a sow which was found by fecal 

 examination to be free from egg-producing ascarids. At the age of 

 about two weeks one of these pigs was given a large number of Ascaris 

 eggs containing motile-vermiform embryos. One week after feeding 

 the eggs this pig died; the other pig continued in good health. ''Exam- 

 ination of the dead pig," the authors state, "revealed a pneumonia, 

 with numerous petechial hemorrhages in the lung tissue. Numerous 

 ascarid larvse, varying in length from 0.7 to 1.2 mm. in length, were 

 found in the lungs, trachea, and pharynx; none in the liver, spleen, 

 esophagus, small intestine, or large intestine." As to conclusions the 

 authors are further quoted as follows: 



"Stewart's very important discoveries concerning the behavior of 



