INCUBATION AND DURATION 151 



These results practically coincide with those published by 

 Nicolle (1912, 1917) and Anderson (1914). 



The duration of the febrile period in guinea-pigs in our ex- 

 perience has not been influenced by repeated passages. Owing 

 to the fact that most of the guinea-pigs inoculated were killed 

 during the reaction period, we have data on only fifty-nine 

 which in our judgment ran uncomplicated typhus courses. 

 These are shown in the following table (Table XIV). 



TABLE XIV 



Nicolle states that the period of fever lasts from four to 

 eleven days. Anderson gives the extremes as four and eighteen 

 days. From our experience a prolonged period of fever denotes 

 secondary infections, particularly of the pseudo-tuberculosis 

 type. 



Nicolle, Anderson, da Rocha-Lima and others have noted 

 apparent instances of guinea-pigs naturally immune to typhus 

 infection. But Nicolle and Lebailly (1919) have pointed out 

 that the virus of typhus may be present after the usual in- 

 cubation period, without giving rise to fever in guinea-pigs, 

 consequently absence of temperature is not proof of absence of 

 infection. 



Our own results are as follows: 



In guinea-pig to guinea-pig inoculations, when done intra- 

 peritoneally, only seven of two hundred and four failed to 

 react. In the case of subcutaneous inoculations, fourteen out 

 of fifty failed to react. 



In inoculations from man to guinea-pig, six failed to react 

 of a total of thirty-four. One of these six was immune to a 

 subsequent inoculation. 



