Immunity and Resistance 631 



Active immunization 



Active immunization: Leishmania. Vaccination against oriental 

 sore was practiced empirically long before the causative organism was 

 discovered, and the practical value of this procedure has been confirmed 

 (22). In experimental immunization, results have varied with the host. 

 Monkeys are more readily immunized than dogs, and like man, often be- 

 come quite resistant to reinfection with L. tropica. Mice, on the other 

 hand, acquire practically no immunity. Prophylaxis with killed vaccines 

 has been generally unsuccessful, although such vaccines may have some 

 therapeutic value. 



Laboratory animals are sometimes immune after recovery from infec- 

 tion with L. donovani (123), and it is generally believed that recovery 

 also leaves man resistant to reinfection. However, no effective method of 

 vaccination has been developed. 



Active immunization: Trypanosoma. Development of acquired immu- 

 nity to a trypanosome was first demonstrated in rats recovering from in- 

 fections with T. lewisi (94). Development of such an immunity is limited 

 to rats more than 25 days old (64), and fails to occur even in adult rats 

 after hypophysectomy (59). The pathogenic trypanosomes are usually 

 lethal to laboratory animals, but sheep and goats sometimes recover from 

 chronic infections with a resulting immunity which lasts for several years 

 (65). Likewise, rats may recover spontaneously from infections with T. 

 cruzi and remain resistant to reinfection for at least five weeks (58). 



Experimental immunization has followed several methods: inoculation 

 with a living, attenuated strain; inoculation with a virulent strain, fol- 

 lowed by adequate chemotherapy; and inoculation with killed trypano- 

 somes. Living attenuated vaccines were first used in attempts to immunize 

 cattle to T. brucei (99). Although the procedure apparently was successful 

 with some animals, this interpretation has been questioned on the basis 

 that cattle sometimes recover spontaneously from infection with this 

 trypanosome. However, rats have been immunized to T. lewisi with at- 

 tenuated cultures non-infective even in massive doses (127). Ehrlich and 

 Shiga (72) showed that, after inoculation with virulent trypanosomes and 

 subsequent chemotherapy, mice remain resistant to reinfection for sev- 

 eral weeks. These observations have been confirmed in other laboratories, 

 and similar results have been obtained with rats, rabbits, and guinea 

 pigs. Obviously, the practical value of this method is dubious. Most at- 

 tempts to use killed vaccines have been unsuccessful. However, such vac- 

 cines have immunized adult rats against T. lewisi (57, 126), although 

 vaccinated nurslings may succumb as readily as controls of the same age 

 (57). A few positive results have been reported for pathogenic trypano- 

 somes (35, 138, 148). Rats have been immunized against T. equinum, 

 to the extent that vaccinated animals always outlived the controls after 



