Latent Infection in Experimental Spirochetosis. 83 

 49 (1509) 



Latent infection in experimental spirochaetosis. 

 By John L. Todd. 



[From McGill University, Montreal.] 



The following observations were made while studying im- 

 munity to SpirochcBta recurrentis in white rats. Since it had 

 been suggested that an immunity might be maintained by per- 

 sistent latent infection, it was necessary to make careful examin- 

 ations for spirochetes in all animals exhibiting resistance to in- 

 fection. 



The work was done with strains of spirochetes which usually 

 produced well-marked infection with one or more relapses. During 

 the observation of several hundred animals it was noticed that 

 parasites occasionally reappeared in the peripheral circulation 

 after unusually long absences. It often happened that spiro- 

 chetes were found in blood films, on one or two days, after they 

 had been absent for from ten to fifteen days ; once spirochetes 

 were seen after an absence of twenty-one days. Blood, taken 

 from the tail, was examined for spirochetes in thick dehemoglobin- 

 ized and stained films. Examinations, throughout the work, 

 were made daily. 



It often happens that a normal infection follows when a rat is 

 inoculated with blood from an animal, once infected, although 

 repeated examinations have failed to find spirochetes in it. 

 In many instances, infection resulted when rats were inoculated 

 with blood taken from other rats which were killed from fifteen 

 to twenty days, in one instance twenty-seven days, after spiro- 

 chetes were last found in them by microscopical examination. 



Although the inoculation of blood, and other material, obtained 

 by killing an experimental animal, is sometimes a more efficient 

 method of detecting spirochetal infection, it is not always so 

 and, since it involves the destruction of the animal supplying the 

 material inoculated, it is a method which cannot be repeated. 

 Inoculation of material in which living spirochetes have been 

 seen occasionally fails to infect one of two rats inoculated at the 



