ANIMALS AND THEIR PROTOZOAN PARASITES 7 



periods include a prcpatcnt period which extends from the time of 

 entrance of the parasites into the host to the time when their off- 

 spring can be recovered from the host by specitied laboratory 

 methods ; a patent period, during which parasites can be found 

 in the host ; and a subpatent period, during which parasites are 

 present but not recoverable. The subpatent period may or may not 

 be followed by another patent period. The clinical periods include 

 an incubation period which extends from the entrance of the para- 

 sites into the host to the appearance of symptoms, and a period of 

 symptoms which coincides in part with the convalescent period; 

 these periods complete a primary attack. A latent period may fol- 

 low, coming to an end with the appearance of symptoms which is 

 the beginning of a period of relapse. 



Distribution of parasites zvitJiin the host. Intestinal protozoa that 

 enter the mouth in contaminated food or drink are carried through 

 the stomach and into the intestine and thus distributed without any 

 effort on their part but entirely by the activities of the host. 

 Species transmitted by contact such as the flagellate and amoeba of 

 the human mouth need no further distribution. Blood-inhabiting 

 protozoa are carried rapidly through the body by the blood stream 

 when inoculated into the host by the intermediate host. 



Primary site of infection. Each species of protozoon is more or 

 less definitely localized in the body of the host. Some of them live 

 in the mouth, others in the duodenum, colon, intestinal wall, the 

 blood plasma, red blood cells, white blood cells, muscle tissue, 

 etc. 



Secondary sites of infection. Protozoa that are ordinarily local- 

 ized in one region of the body may spread to other parts which 

 become secondary sites of infection; for example, the dysentery 

 amoeba, Endarnocba histolytica, is primarily a parasite of the large 

 intestine where it brings about dysenteric conditions. It may, how- 

 ever, be transported to the liver and other organs where it is re- 

 sponsible for the production of abscesses. 



Passive {natural) resistance of the host. Not all the stages of a 

 parasite normal to a given host are capable of living and setting 

 up an infection in that host and no doubt many species of foreign 

 parasites gain entrance to hosts in which they are destroyed. The 

 host may be said to possess a passive or natural resistance against 

 these foreign parasites. Very little is known, however, with respect 

 to the factors that make up this type of resistance. 



