574 PATHOGENIC MICROORGANISMS 



is the reduction in their virulence. Koch, Gaffky, and Loeffler, 1 utilizing 

 the variations in susceptibilities of different species of animals, devised a 

 method by means of which the relative attenuation of a given culture may 

 be estimated and standardized. Rabbits are less susceptible than guinea- 

 pigs, and virulent anthrax cultures, grown for two or three days under the 

 stated conditions, lose their power to kill rabbits, but are less virulent for 

 guinea-pigs. After ten to twenty days of further cultivation at 42 C. 

 the virulence for the guinea-pig disappears, but the culture is potent 

 against the still more susceptible mouse. Even the virulence for mice 

 may be entirely eliminated by further cultivation at this temperature. 



The method of active immunization first practiced by Pasteur, and 

 still used extensively, is carried out as follows: Two anthrax cultures 

 of varying degrees of attenuation are used as vaccins. The premier 

 vaccin is a culture which has lost its virulence for guinea-pigs and 

 rabbits, and is potent only against mice. The deuxieme vaccin is a cul- 

 ture which is still definitely virulent for mice and guinea-pigs, but not 

 potent for rabbits. Forty-eight-hour broth cultures of these strains, 

 grown at 37.5 C., form the vaccin actually employed. Vaccin I is 

 subcutaneously injected into cattle in doses of 0.25 c.c., sheep receiving 

 about half this quantity. After twelve days have elapsed similar quan- 

 tities of Vaccin II are injected. 



Pasteur's method has given excellent results and confers an im- 

 munity which lasts about a year. 



Chauveau 2 has modified Pasteur's method by growing the bacilli 

 in bouillon at 38 to 39 C., at a pressure of eight atmospheres. Cul- 

 tures are then made of races attenuated in this way, upon chicken 

 bouillon and allowed to develop for thirty days. Single injections of 

 0.1 c.c. each of such cultures are said to protect cattle. 



Active immunization of small laboratory animals is very difficult, 

 but can be accomplished by careful treatment with extremely attenu- 

 ated cultures. 



Passive immunization by means of the serum of actively immune 

 animals was first successfully accomplished by Sclavo. 3 



The subject of passive immunization has been especially investigated 

 and practically applied by Sobernheim. 4 The serum used is produced by 

 actively immunizing sheep. It is necessary to carry immunization to an 



1 Koch, Gaffky, und Loeffler, Mitt. a. d. kais. Gesundheitsamt, 1884. 



2 Chauveau, Comptes rend, de 1'acad, des sci., 1884. 

 s Sclavo, Cent. f. Bakt., xviii, 1895. 



a Sobernheim, Zeit. f. Hyg., xxv, 1897; xxxi, 1899. 



