ANIMALS AND THEIR PROTOZOAN PARASITES 5 



parasite resulting in the carrier condition, latency and relapse, 

 modifications in the resistance of the host due to biological or 

 chemical therapy, the means by which infective stages in the life- 

 cycle of the parasite escape from the body, host-parasite specificity 

 and methods of prevention and control. These subjects may now 

 be taken up briefly in the order indicated. 



Epidemiology of transmission. Intestinal protozoa usually escape 

 from the body in the fecal material and must be able to withstand 

 various factors in their new environment. Most of the intestinal 

 species produce resistant cysts which remain viable for consider- 

 able periods outside of the body, whereas trophozoites that may 

 be carried from the body of the host quickly die. In some species, 

 such as Trichomonas hominis, no cyst stage occurs in the life-cycle 

 and transmission is effected by trophozoites. Cysts appear in most 

 cases to be infective when they escape from the host. However, the 

 cysts of certain species may undergo further development after 

 they are passed, for example, those of the human coccidium. The 

 only conceivable avenue of infection for intestinal protozoa is by 

 way of the mouth. It seems probable that infective stages in most 

 cases gain entrance to a new host in contaminated food or drink. 

 This contamination may be the result of unsanitary conditions or 

 unclean habits or may be due to the distribution of the infective 

 stages by flies and other animals. Direct transmission or transmis- 

 sion by contact probably occurs in such protozoa as the flagellate 

 and amoeba that live in the human mouth and are transferred dur- 

 ing the process of kissing. 



Blood-inhabiting protozoa, such as the trypanosomes and ma- 

 larial organisms, are usually sucked out of the body of the infected 

 host by an insect, leech, or other animal, within which they pass 

 through part of their life-cycle developing into infective stages 

 which are subsequently inoculated into their vertebrate hosts. 



In rare instances, for example, in the life-cycles of the piroplasm 

 responsible for Texas fever in cattle, the protozoa gain entrance 

 to the eggs of the intermediate host and the young that hatch from 

 these eggs are thus parasitized and later become infective. This 

 so-called "hereditary" method of transmission is, however, rare. 



Clinical and parasitological periods during the course of a nat- 

 ural infection. It is convenient to distinguish between parasitolog- 

 ical and clinical periods during an infection. These periods are 

 indicated in diagrammatic form in Fig. i. The parasitological 



