306 VETERINARY BACTERIOLOGY 



for this organism may be found in the blood of advanced cases 

 and of convalescents, but they appear too late to be of any diag- 

 nostic value. The reaction is rather doubtfully specific. The 

 reaction occurs only in low dilutions rarely above 1 : 20. Agglu- 

 tination in much higher dilutions may be secured with immune 

 serum sometimes as high as 1 : 1000 has been observed. Pre- 

 cipitins have also been noted in laboratory experimentation. 

 Opsonins for B. pestis have been demonstrated in normal human 

 serum and in immune serum. Bactericidal substances are pres- 

 ent in the serum of artificially immunized animals. 



Active immunization of man against bubonic plague has been 

 quite extensively practised. The procedure consists in every 

 case of injection of killed or attenuated bacilli or their products 

 as a prophylactic measure. The use of the various substances has 

 proved quite successful. Haffkine's vaccine has been used in 

 India. It consists of a killed six-weeks culture of plague bacilli 

 in broth. Modifications of this method have been utilized by 

 many investigators. The immunity established is probably both 

 opsonic and bactericidal. 



Passive immunization by the injection of the serum of horses 

 hyperimmunized against B. pestis has been highly successful 

 according to some, and of no material advantage according to 

 others. Several procedures have been advocated. The following 

 is that of Kolle and Kumbein: A culture of B. pestis is passed 

 through rats to exalt its virulence. This is planted upon agar and 

 incubated forty-eight hours at 30, the growth washed off and sus- 

 pended in physiological salt solution. The suspension is killed by 

 heating to 70 for an hour and its sterility determined. The 

 horse is injected at intervals of a few days with gradually increasing 

 doses of the dead bacteria, until after seven or eight injections the 

 bacteria from six or more culture-tubes are injected at one time. 

 Injections of minute quantities of the living organism are then 

 begun and finally after repeated injections the living organisms 

 from 16 cultures are used. The interval between injections is 

 governed by the reaction of the animal. Usually it is from five 

 to eight days. The animal is then bled, and the serum preserved 

 by the addition of 0.5 per cent, phenol. 



Bacteriologic Diagnosis. The organism may be recognized 



