Figure 7. Multiplication of myxoma and fibroma viruses in primary rabbit kidney 

 cells at various temperatures. *Log rabbit skin infectious units per ml produced in 

 48 hours. 



36^ C. We have recently been looking at this more directly by com- 

 paring the multiplication of Bl Coxsackie virus in primary cultures 

 of adult and infant mouse tissues at various temperatures in order to 

 determine the optimum temperature for multiplication in the tissues 

 usually affected in the mouse. Our preliminary results suggest that 

 the virus can multiply in adult tissues if the temperature of incuba- 

 tion is reduced to 35° C and that it is inhibited at higher tempera- 

 tures .whereas in infant tissues it multiplies to high titer at 37° C to 

 38° C as well as at lower temperatures. Our experiments have not 

 proceeded far enough, however, to provide really reliable data. 



I have already indicated that in his study of myxoma virus in- 

 fections in rabbits, Marshall (19 59) found a pattern similar to the one 

 I have described for Coxsackie virus; that is, an ameliorating effect 

 of high temperature as well as enhancement of the infection by cold. 

 Similar protective effects of elevated ambienttemperature had pre- 

 viously been reported by Thompson (1938). Both Thompson and Mar- 

 shall found that rabbits at testtemperatures had shifts of their rec- 

 tal temperatures of only about 0.5° C downward in the cold and 

 and 0.5° C to 1° C upward in the hot room, but their skin tempera- 

 tures changed 4° C to 5° C. The possibility that these temperature 

 changes may have been important in controlling the course of the in- 

 fection is supported by other evidence indicating that multiplication 

 of myxoma and fibroma viruses is easily affected by change of tem- 

 perature. The myxoma and fibroma viruses and their various vari- 



330 



