Host-Parasite Relationships 535 



reported from Endolimax nana (13), Endamoeba disparata (49), Amoeba 

 (71), and from flagellates of termites (57). 



Certain algae also parasitize Protozoa. Blue-green algae occur in the 

 testacean, Paulinella chromatophora (77), and species of Chlorella in 

 Frontonia leiicas (40) and Paramecium bursaria (63, 76, 81). Although 

 these algal-protozoan associations are often considered examples of 

 symbiosis, the experimental evidence is not entirely conclusive. 



INFECTIONS 



Upon reaching a suitable host, a parasitic protozoon which is 

 not promptly eliminated may give rise to an infection. The establishment 

 of an infection requires multiplication of the parasite at a rate rapid 

 enough to overbalance any destructive forces which may be encountered, 

 and the result is a net increase in parasite population. The ability to 

 establish an infection in a particular host, or the infectivity of the parasite 

 for that host, is somewhat variable. To what extent the apparent 

 infectivity may depend upon individual variations in the internal 

 environment or in natural resistance of the host is uncertain, although 

 it may be assumed that such factors are important. For instance, the 

 change from a normal to a high-carbohydrate diet makes the rat suscepti- 

 ble to infection with Balantidium coli, a ciliate which ordinarily shows 

 little or no infectivity for this rodent (38). In such a case, the normal 

 intestinal environment obviously is a factor limiting infectivity. In other 

 cases, infectivity may be modified by changes in the parasite. An example 

 is the loss of infectivity for kittens by two strains of Entamoeba histolytica 

 which had ceased producing cysts in cultures. Modification of the culture 

 medium so as to restore the ability to encyst was followed by the recovery 

 of infectivity (15). 



In the terminology of Justin Andrews, protozoan infections may be 

 described in terms of prepatent, patent, and subpatent periods. The 

 prepatent period precedes the appearance of parasites in numbers large 

 enough for detection by routine examinations. The apparent absence of 

 parasites is often the result of failure to find the few parasites actually 

 present in the material examined. In a malarial infection, however, the 

 parasites may be developing as exoerythrocytic stages prior to invasion of 

 the blood. 



The patent period opens with the finding of parasites in material 

 from the host. During this period, the parasite density usually continues 

 to rise for some time and the transfer of parasites by vectors is most 

 likely to be successful in this stage. Eventually, the infection may lead to 

 death of the host, the number of parasites may be sharply reduced by 

 immunological reactions of the host, or the surviving parasites may leave 

 the host upon completion of the life-cycle. The patent period passes into 

 the subpatent period when the parasites are no longer detectable. 



