780 PATHOGENIC MICROORGANISMS 



the long-haired Algerian sheep show a high resistance, while the 

 European variety are highly susceptible; and, similarly, the gray 

 rat is much more resistant than the white rat. Dogs, hogs, cats, 

 birds, and the cold-blooded animals are relatively insusceptible. For 

 man the bacillus is definitely pathogenic, though less so than for 

 some of the animals mentioned above. 



While separate races of anthrax bacilli may vary much in their 

 degree of virulence, a single individual strain remains fairly constant 

 in this respect if preserved, dried upon threads or kept in sealed 

 tubes, in a cold, dark place. Virulence may be reduced 15 by various 

 attenuating laboratory procedures which are of importance in that 

 they have made possible prophylactic immunization. Heating the 

 bacilli to 55 C. for ten minutes considerably reduces their virulence. 

 Similar results are obtained by prolonged cultivation at tempera- 

 tures of 42 to 43 C., or by the addition of weak disinfectants to 

 the culture fluids. 16 Once reduced, the new grade of virulence 

 remains fairly constant. Increase of virulence may be artificially 

 produced by passage through animals. 



Experimental infections in susceptible animals are most easily 

 accomplished by subcutaneous inoculations. The inoculation is fol- 

 lowed, at first, by no morbid symptoms, and some animals may 

 appear perfectly well and comfortable until within a few hours 

 or even moments before death, when they suddenly become visibly 

 very ill, rapidly go into collapse, and die. The length of the disease 

 depends to some extent, of course, upon the resistance of the infected 

 subject, being in guinea-pigs and mice from twenty-four to forty- 

 eight hours. The quantity of infectious material introduced, on the 

 other hand, has little bearing upon the final outcome, a few bacilli, 

 or even a single bacillus, often sufficing to bring about a fatal 

 infection. Although the bacilli are not demonstrable in the blood 

 until just before death, they nevertheless invade the blood and 

 lymph streams immediately after inoculation, and are conveyed 

 by these to all the organs. This has been demonstrated clearly by 

 experiments where inoculations into the tail or ear were immediately 

 followed by amputation of the inoculated parts without prevention 

 of the fatal general infection. The bacilli are probably not at first 



15 Toussaint, Comptes rend, de 1'acad. des sci., xci, 1880; Pasteur, Chamberlan 

 et Roux, Comptes rend, de 1'acad. des sci, xcii, 1881. 



16 Chamberkmd et Rowx, bid., XCVI, 1882. 



