148 TYPHUS IN GUINEA-PIGS 



4. INCUBATION AND DURATION OF FEVER IN GUINEA-PIGS 



In judging of the incubation period and the duration of the 

 febrile reaction it is necessary to recognize the effects of sec- 

 ondary infections. The results of many post mortems have 

 shown that unduly short incubation periods, as well as a 

 prolonged period of temperature, is due to the presence of 

 secondary infection. As a rule a very short incubation period 

 is followed by a prolonged period of fever, and the presence 

 of these two signs may be accepted as proof of a complicating 

 infection. 



The incubation period of typhus in the guinea-pig rapidly 

 becomes fixed within a definite period of days and does not 

 become changed with long continued passages. The same is 

 true of the duration of fever. Hence it may be said that the 

 virulence is not reduced or augmented by repeated passages. 



We have often noted a preliminary rise of temperature at 

 the end of the incubation period, lasting but one day and fol- 

 lowed by a remission of one day, then the period of constant 

 fever as described by Nicolle. 



The first passage from man to guinea-pig and from louse to 

 guinea-pig may be attended by exceedingly long incubation 

 periods, with the former, and exceedingly short periods with 

 the latter. Forty days, according to Nicolle (1920) is the limit 

 of incubation when injections are made from man to guinea- 

 pig. According to da Rocha-Lima (1919) the injection of louse 

 viscera may be followed by incubation periods as short as two 

 days. Our own experiences do not corroborate these state- 

 ments. 



The following table (Table XI) gives the incubation periods 

 in fourteen guinea-pigs which were proved to have acquired 

 typhus through the injection of viscera of lice fed upon typhus 

 patients. 



With one exception, that of guinea-pig 9, inoculated with 

 louse 227 which was heavily infected with rickettsia, no guinea- 

 pig with uncomplicated typhus developed a reaction under 

 seven days. The incubation period of four days in guinea-pig 9 



