204 FAMILY: AMCEBID^ 



in others the infection is of a mild nature, may be comparable with what 

 is known to occur in bacterial infections. Many individuals harbour 

 pathogenic bacilli in their throats without having symptoms of the 

 disease which may be caused by these organisms. Invasion of the tissues 

 may occur because the resistance of the host is lowered or because the 

 virulence of the organism is increased. The former seems to be the most 

 rational explanation, and in the case of E. histolytica infections it would 

 seem that the resistance of the intestine is lowered from time to time, 

 with the result that active multiplication of the amoebae with their exten- 

 sion into the tissues takes place, so that acute dysentery supervenes. On 

 the other hand, it has to be remembered that the virulence of protozoa 

 may vary considerably. In the case of trypanosomes it is well known 

 that passage of a strain from one animal to another may so change it 

 that it will bring about death in a few days instead of a few months. 

 It is possible that the virulence of E. histolytica may vary in a similar 

 manner, but there is no evidence that this occurs. By quick passage 

 of a strain from one man to another the virulence might be so increased 

 that eventually every individual infected would acquire an acute and 

 fatal amoebic dysentery. Whether this actually happens in nature is not 

 known, but judging from the results of experiments on kittens the writer 

 can find no reason to suppose that the amoebae from carrier cases with few 

 or no symptoms are less virulent than those from acute cases. Brumpt 

 (1925), however, suggests that there exist two types of amoeba included 

 under the name E. histolytica, the one infective to kittens and the other 

 not. To the latter he gives the name Entamoeba dispar, and suggests that it 

 accounts for many of the carrier cases in countries where amoebic dysentery 

 is uncommon. The writer does not believe that physiological data of this 

 kind aiiord a means of distinguishing species. 



SUSCEPTIBILITY OF ANIMALS.— Losch (1875) succeeded in infecting 

 a dog with E. histolytica, and this experiment was repeated by Hlava 

 (1887), Kruse and Pasquale (1894), Harris (1901), and Dale and Dobell 

 (1917). Young cats, however, are more easily infected, and it is with 

 them that most experimental work has been done. Hlava (1887) was 

 the first to produce infection in these animals, and he was followed by 

 Kartulis (1891), Kovacs (1892), Quincke and Roos (1893), Kruse and 

 Pasquale (1894), Marchoux (1899), the writer (1912(?), and many others. 

 Kruse and Pasquale also infected cats with the amoebae obtained from 

 liver abscess. The infection has generally been produced by injections 

 of dysenteric stools per anum, and this is the most reliable method for 

 infecting these animals. Unless cysts are present the animals cannot 

 be infected by feeding, a fact first demonstrated by Quincke and Roos 

 (1893), the first observers to describe the cysts of E. histolytica. Huber 



