COCCIDIOSIS IN BIRDS AND MAMMALS 299 



of some animal with which he occasionally came in contact. This 

 needs experimental support for acceptance. 



Innnunity. The problem of individual immunity to coccidial in- 

 fections is closely bound up with the problem of host-parasite 

 specificity as is indicated in the previous section. It seems very 

 probable that a rational explanation of the mechanism of the one 

 phenomenon will help a great deal in understanding the mechanism 

 of the other. 



Conflicting reports have appeared regarding the acquisition of 

 immunity from coccidial infections in general. The author (1926a) 

 failed consistently in attempts to reinfect cats and dogs which 

 had recovered from an attack of coccidiosis. Nevertheless, cats 

 and dogs which are kept in the laboratory for a long time (a year 

 or more) have shown evidence of repeated attacks of coccidiosis 

 with intervals of apparent freedom from the disease. Whether 

 these repeated infections are newly acquired each time, or whether 

 they are the relapses of a latent infection, as in the case of bird 

 malaria, is not known. Richenow (1921) and Wetzel (1925) state 

 that no immunity is produced in rabbits and that constant rein- 

 fection is responsible for the appearance of a chronic infection. 

 Beach and C'Orl (1925) and Johnson (1927a) judging from clini- 

 cal signs believe that chicks become resistant to coccidial infection 

 as the result of previous infections. Young (1929) counting the 

 oocysts discharged showed that from a parasitological point of 

 view, subsequent infections, experimentally produced, were of 

 equal or even greater magnitude than the first. In working with 

 rabbit coccidia. Chapman (1929) has attempted to demonstrate 

 the presence of circulating antibodies with complement fixation, 

 precipitin, agglutination, lytic, and skin tests. Skin tests were prom- 

 ising, complement fixation was irregular in its results, and the 

 rest were negative. The greatest difficulty in the work was the 

 production of a pure coccidia antigen. 



The foregoing indicates some of the many possibilities in the 

 field of coccidial immunity. The following problems are still to 

 be solved conclusively, and will lead to many other questions for 

 investigation : 



I. Do coccidial infections produce in their hosts any demon- 

 strable resistance to subsequent infections — 



a. Clinically — are the percentage mortality and the symptoms 

 of the disease reduced in the latter attacks? 



