74 VARIATIONS IN VIRULENCE [ch. vi 



{a) Pastetir showed 30 years ago that B. anthrads, 

 if grown at a temperature of 43'5°C.,.lost its virulence in 3 

 or 4 weeks. Hewlett and Knight (1897) destroyed the viru- 

 lence of a strain of diphtheria bacilli by subjecting it for 17 

 hours to a temperature of 45° C. Muir and Ritchie state that 

 a broth culture of the diphtheria bacillus if exposed for only 

 one hour to a temperature of 65° 0. is rendered much less 

 toxic while a culture of the tetanus bacillus under the same 

 conditions is deprived altogether of toxicity. The bacillus of 

 " blackleg " can likewise be rendered innocuous by exposure 

 to a high temperature (Mohler and Washburn, 1906). 



It has been thought that recovery from infectious diseases, 

 such as the exanthemata, might be due to the effect produced 

 in this way on the infecting organisms by the continued fever 

 which their presence provokes. That the increase in tempera- 

 ture may be a protective measure on the part of the body is 

 suggested by the experiments of Lowey and Richter (1897), 

 in which the resistance of rabbits to infection by the pneumo- 

 coccus, the diphtheria bacillus and the hog cholera bacillus 

 was artificially increased by injury to the corpus striatum 

 and a consequent rise in temperature, before inoculation. 



These observations do not prove that the rise in tempera- 

 ture lessens the virulence of the organisms. Indeed this 

 opinion has been proved to be erroneous, in some cases at 

 least, by the work of Leutscher (1911), who tested the 

 virulence of pneumococci isolated from the affected area of 

 the lung at different stages of an acute lobar pneumonia. He 

 found that the virulence of the organisms isolated at the 

 period immediately preceding the crisis was even greater 

 than that of those isolated in the early stages of the disease. 



Other observers, working along different lines, have shown 

 that a comparatively high temperature is not necessarily 

 inimical to virulence. Eyre, Leatham and Washbourn (1906) 

 quote the observations of Kruse and Pansini, which their 

 own work confirms, to the effect that the virulence of the 

 parasitic pneumococcus is often associated with inability to 

 grow at a temperature much below that of the body — 37° C. 

 A slightly virulent strain which would grow readily at 20° C. 



