SPONTANEOUS RECOVERY 131 



of the numbers of parasites in one host prevail, but 

 the production of many small schizonts is a greater 

 tax on the resources of the host. 



The life-history of all well-known pathogenic 

 parasites contains some one stage at which the 

 vitality of the organism is at its lowest, and such a 

 phase is the time at which the recuperation of the 

 host may enable it to destroy its invader. The 

 weak spot in the active life of a coccidian parasite is 

 the sporozoite stage. Should but few oocysts be 

 swallowed by the bird concerned, and the digestive 

 juices of the latter be vigorous, the delicate sporo- 

 zoites do not stand so good a chance of evading 

 digestion as they would do if the bird were younger 

 or somewhat more weakly. Hence it happens that 

 coccidiosis is especially fatal to very young birds, 

 though it is far from being unknown among older 

 ones, which may act as reservoirs of cysts. 



The onset of sporogony of Eimeria avium usually 

 means either the recovery or the death of the 

 infected bird. When the infection has not been 

 acute, the oocysts pass from the body, and provided 

 that reinfection of the bird does not occur, the 

 internal lining of the gut may be able to regenerate 

 itself, when the bird gradually becomes less anaemic 

 and begins to increase in weight. Sometimes infil- 

 tration of connective tissue into the lesions or gaps 

 formed by the parasite aids in the reconstructive 

 processes, and complete recovery may ensue. 



On the other hand, certain birds such as grouse 

 possess very long caeca. As before mentioned, 

 daughter forms produced in the duodenum may pass 



