56 THE AMOEBAE LIVING IN MAN 



the stages intervening between ingestion and the establishment of the 

 amoebae in the tissues of the large intestine require further investigation. 



Walker {vide Walker and Sellards, 1913) fed 20 men on E. histolytica, 

 sometimes encysted, and sometimes free — a special technique beinc 

 employed in the latter case to enable the amoebae to pass through the 

 stomach. He succeeded in infecting 18 out of the 20 men fed by these 

 methods. The infections were established — as determined by the 

 appearance of cysts in the stools — in times varying from i to 44 days, 

 the average period being 9 days. The time which elapsed between 

 the infective feed and the attack of dysentery — in four cases which 

 developed dysentery — was 20, 57, 87, and 95 days (average 64-8 days). 

 This last figure is clearly of doubtful value, since it ignores all the cases 

 which did not have dysentery — any of which might subsequently 

 develop amoebic dysentery at any time, and so change the " average 

 incubation period " within indeterminate limits. It may be noted that 

 the "incubation period" in the cat— which, if infected at all, invariably 

 suffers from dysentery, and does not become a carrier — is usually 

 about a fortnight, if the infection is con\eyed to it by cysts per os. 

 With intrarectal injection of active amoebae it is much shorter as a 

 rule, infection being sometimes established within twenty-four hours.* 



Since it is impossible to observe the earliest stages in the develop- 

 ment of E. histolytica in human beings — their normal host — it appears 

 probable that recourse must be had to animal experiments if further 

 information on this subject is to be obtained. The cat at once suggests 

 itself as a suitable subject for investigation ; but unfortunately, it is not 

 so suitable as one might suppose. In the course of work undertaken 

 with Dr. H. H. Dale in 1916, attempts were several times made to infect 

 kittens with cysts administered per os ; and then, by killing them later, 

 and examining the contents of their intestines, to discover the earlier 

 stages in the development of the amoebae in this animal. Unfortunately 

 all these attempts failed. The only definite results which we obtained 

 showed clearly that cats are not easily infected by this method ; and 

 that as a rule the majority of the ingested cysts die in the cat's intestine, 

 though a few will pass through unchanged. We made altogether 17 

 attempts to infect kittens by these means, using cysts from 7 different 

 human infections, but we succeeded in infecting a kitten only once. 

 It thus seemed to me probable that it would be necessary to make a 

 very large number of experiments, and a great sacrifice of kittens, to 

 obtain conclusions of any value in this way. The attempt was therefore 

 abandoned after a few preliminary failures. 



Chatton (19176) appears to have been more successful with this 

 method. He found that the cysts of £. histolytica, when swallowed by a 

 cat, passed through its stomach without undergoing any change save the 

 "digestion "f of their chromatoid bodies. In the small intestine, 



made the suggestion on analogy with their results with cats. It should be remem- 

 bered, however, that Grass! .md Calandruccio had previously shown that man acquires 

 infection with E. colt by swallowing the cysts of this species. 



" With a strain of amoebae which was passed through 106 kittens by rectal injection, 

 the average incubation period was slightly over 2 days (Dale and Dobell, 1917). 



t Chatton does not say whether the chromatoids are " digested " in the stomach by the 

 host or the parasite. I take it that actually he merely observed no chromatoids ; and 

 that he interprets their absence as an indication that the amoeba has assimilated them 



