BACILLUS ANTHRACIS 



845 



Murata 1924). In general it requires a large dose to produce a fatal infection 

 by the mouth (Giovanardi 1931). Post mortem in cases of oral infection, in 

 addition to the enlargement of the spleen and the occurrence of septicaemia, the 

 intestinal mucosa is seen to be covered with small furuncular swellings, through 

 which the bacilli have gained access to the blood. Of the three animals the mouse 

 is the most susceptible and the rabbit the least, the guinea-pig occupying an 

 intermediate position. This difference in susceptibility is scarcely noticeable, 

 except with a strain of weakened virulence (see Chapter 66). Rats are more 

 difficult to infect than other rodents, but are said to succumb easily if fatigued 

 by continuous exercise on a revolving drum (Charrin and Roger 1890). They 

 may develop a chronic disease after subcutaneous injection, which does not 

 prove fatal for 4 or 5 weeks. Dogs may be infected by subcutaneous injection, 

 though not uniformly. Birds, with the exception of sparrows and young doves, 

 and cold-blooded animals are resistant, likewise Algerian sheep (Chauveau 1880a, 

 b). (See Davaine 1863a, b, 1864, Koch 1877, Frank and Lubarsch 1892, Sobernheim 

 1897, Oppermann 1906, Balteano 1922, Poppe 1922, Basset 1925, Katzu 1925, 

 MuUer 1925, Sanarelli 1925.) The experimental reproduction of the disease in 

 larger animals is considered on p. 1736. 



Pasteur (18816, c) found that by growing the anthrax bacillus at 42-5° C. for about a 

 month, he was able to lower its virulence to such an extent that it proved harmless to all 

 animals except new-born guinea-pigs. By successive passage through these animals, the 

 bacillus gradually regained its virulence till it was able to kill 2, 3, and 4-day, and later 

 fully grown guinea-pigs ; eventually its virulence was entirely 

 restored. From the work of Preisz (1911) it would appear that 

 this resumption of virulence is due not to a gradually increasing 

 virulence of the individual organisms, but to an alteration in 

 the proportions of virulent and avirulent baciUi in the culture. 

 He found that the effect of incubating a virulent culture at 

 42-5° C. was to cause the appearance of variants that were no 

 longer virulent to animals, so that in one and the same culture 

 both virulent and avirulent bacilli were found side by side. 

 The longer the incubation, the higher was the proportion of 

 avirulent bacilli. After a month or more the culture consisted 

 almost entirely of avirulent variants, and on injection into mice 

 proved to be harmless ; the virulent bacilli that were still present 

 were too few to cause death. But if such a culture is injected 

 into a new-born guinea-pig, these few virulent bacilli may be 

 just sufficient to overcome the very low resistance of the animal ; 

 in consequence they proliferate, and during the course of succes- 

 sive passages increase in proportion relatively to the avirulent 

 variants, till eventually the culture consists almost entirely of 

 the virulent type. The modern conception of the essential 

 heterogeneity of single strains, i.e. the presence in one and the 

 same strain of organisms showing sharp discontinuous variations 

 in virulence, not only has more evidence in its favour than the 

 older conception of the simultaneous and equal raising or lower- 

 ing of all the baciUi in the strain, but explains more easily the 

 variations in virulence that are noted consequent on altered 

 environmental conditions. 



Fig. 



182.— 5. anthra- 

 cis. 



Several observers (Hankin 1889, Martin 1890, Marmier 

 1895, Standfusz and Schnauder 1925) have shown that 



In gelatin stab cul- 

 ture, 3 days, 22° C, 

 showing inverted 

 fir-tree growth, with 

 commencing hque- 

 faction. 



