PROTOZOAL DISEASES 



877 



A. PROTOZOAL DISEASES. 



Amoebic Dysentery. — We have already seen that amoebiasis in 

 man is principally caused by Loeschia histolytica, which, escaping 

 as cysts in the faeces, is taken up by house-flies — that is to say, flies 

 belonging to the genera Musca, Fannia, Calliphora, etc. 



The cysts do not undergo development in the fly, which serves 

 merely as a carrier and protector for them, which, escaping with 

 the flies' faeces on to human food, afford a means of infection. 



This carriage is interesting, indicating that the environment of 

 the ahmentary canal of the fly is not suitable for the development 

 of this amoeba, though it is known that there are amoebae passing 

 their whole life-cycle in insects — e.g., Endamceba hlattce. 



With regard ot L. histolytica, it is believed that the sexual life- 

 cycle occurs in man, but it is not deiiniteh^ known whether gametes 

 are formed in the human intestine, or whether autogamy takes place 

 in the cyst, which is improbable, but in either case the life-cycle 

 does not require more than one host, and infection can take place 

 by direct contamination of the food, without passing through 

 the fly. 



Animals can be infected with amoebic dysentery, but there is no 

 evidence at present of any animal, other than man, acting as a 

 reservoir, but this does occur in human carriers. 



To demonstrate what we mean we give the following table: — 



Amcebic Dysentery. 



Parasite. 



Defini- 

 tive 

 Host. 



Reser- 

 voir. 



Trans- 

 mission. 



Inter- 

 mediary 

 Host. 



Infection. 



Intermediate 

 Host. 



Loeschia 

 histoly- 

 tica. 



Man in 

 carrier 

 stage. 



Man in 

 carrier 

 stage. 



Cysts. 

 Ingestive. 



Flies. 



Cysts in- Man in acute 

 gested from and chi-onic 

 contami- infections, 

 nated food ' 

 or drink. 



Flagellate Diarrhoea and Dysentery. — Whatever may be the views 

 of zoologists, medical men have little doubt that some of the flagel- 

 lates, such as Giardia intestinalis, produce diarrhoea; and Wenyon 

 and O'Connor have shown that their cysts have been found in flies, 

 and that in general the process of infection is much the same as in 

 man, but the reservoir is different. There is little doubt in our 

 minds that man is more or less accidentally infected with many of 

 these parasites, and certainly with Giardia intestinalis, which 

 appears to us to be a true parasite of the rat. 



If there is a sexual cycle, it takes place either in the cyst or in 

 the vertebrate. 



