HOST-PARASITE RELATIONS I INTESTINAL PROTOZOA 



the coccidium, Isospora hominis, diarrheic symptoms 

 appear before oocysts are recoverable in the feces. As 

 indicated in Fig. 19, the number of parasites usually 

 increases considerably before symptoms become evident. 

 The curves as given probably do not represent actual 

 conditions in any specific protozoan infection but are 

 meant to indicate that in general the increase in parasite 

 number precedes the appearance of symptoms and that 

 increases and decreases in the severity of the symptoms 

 follow the rise and fall in parasite number. 



(b) The Period of Symptoms begins when the incuba- 

 tion period ends and ends of course with the cessation 

 of symptoms. 



(c) The Convalescent Period is represented as begin- 

 ning at the point of maximum symptoms. It ends, not 

 when the symptoms disappear but later with the recovery 

 of the host. 



(d) In diseases characterized by relapses one or more 

 Latent Periods may be present. During these periods the 

 causative organisms are too few in number to bring about 

 symptoms, but, after intervals of indefinite length, some 

 change occurs in parasite or host or in both that results 

 in an increase in parasite number and a reappearance 

 of symptoms. 



(e) The reappearance of symptoms following a latent 

 period is known as the Period of Relapse. 



3. DISTRIBUTION AND LOCALIZATION OF PARASITES 

 WITHIN THE HOST 



( I ) Distribution. As already noted intestinal proto- 

 zoan parasites may gain entrance to their natural hosts 



26 



