634 Immunity and Resistance 



that dogs and cats remain immune at least seven months after recovery 

 from infections with Isospora, was one of the first to demonstrate im- 

 munization against Coccidia. Similar results have been obtained with 

 other mammals (9) and with chickens (191, 192) by feeding Coccidia in 

 small doses. The severity of the infection is correlated with the degree 

 of acquired immunity, and very light infections may induce no appreci- 

 able resistance. Immunity against Coccidia is a sterile immunity which 

 prevents development of the homologous parasites. Thus, sporozoites of 

 Eimeria tenella may invade intestinal cells of immune chickens but soon 

 disintegrate (192). 



Vaccination with non-viable parasites has been unsuccessful in chick- 

 ens (191), rabbits (9), and rats (12). These unpromising results, and the 

 ineffectiveness of antiserum prophylaxis, led Becker to suggest that im- 

 munity to coccidiosis cannot be explained on the basis of a generalized 

 response of the host's tissues. Instead, resistance may involve a tissue 

 immunity which spreads from centers of infection over the remaining 

 epithelial layer (12). 



Passive immunization 



Passive immunization: Trypanosorna. In certain host-parasite com- 

 binations, antiserum from a recovered animal, or from one with a chronic 

 or subacute infection, is protective when inoculated simultaneously with 

 pathogenic trypanosomes (e.g., T. briicei, T. crirJ, T. equiperdiun). Se- 

 rum prophylaxis against T. cnizi, although not preventing infection, does 

 induce a mild type of trypanosomiasis in rats (58). Passive immunization 

 likewise is effective against T. leioisi (167) and T. duttoni (170). Lacteal 

 transfer of antibodies, from actively or passively immunized females to 

 nurslings, has been demonstrated with T. lewisi (55, 56) and T. cruzi 

 (102) in rats. During the first 24 hours, a newborn rat can receive enough 

 antibodies in milk to protect it against an inoculum of 1,000,000 T. 

 lewisi. The results with T. cruzi are usually an abortive infection and 

 survival of the nursling. Placental transfer of antibodies appears to be 

 insignificant in these cases, as indicated in the exchange of litters between 

 immunized and normal females. 



Serum therapy has been more or less beneficial in some instances. Upon 

 treatment with antiserum, goats infected with T. congolense have de- 

 veloped mild infections with recovery after three months, while controls 

 have died (144). Beneficial effects of antiserum have been obtained also 

 with T. equinum in mice (176). Such treatment of T. cruzi infections in 

 rats induces an incomplete crisis in the blood, but a relapse occurs soon 

 after the last injection of serum (58). 



Passive immunization: Plasmodiutn. Although the results are of uncer- 

 tain practical value, favorable effects of antiserum in human malaria 

 have been reported occasionally (95, 106, 163). Sera from monkeys with 



