Virulence 735 



Most cases of tuberculosis steadily advance, but a certain 

 number may recover. 



About the center of a typical tubercle there is a zone of re- 

 action in which the reparative tendency of the tissue is 

 usually but slightly outweighed by the invasive power of the 

 bacilli. If the vital condition of the individual becomes so 

 changed that the invasive activity of the bacilli is checked 

 or their death brought about, the tubercle begins to cica- 

 trize, and becomes surrounded by a zone of newly formed 

 contracting fibrillar tissue, by which it is circumscribed 

 and isolated. This constitutes recovery from tuberculosis. 

 Sometimes the process of repair is accomplished without 

 the destruction of the bacilli, which are incarcerated and 

 retained. Such a condition is called latent tuberculosis, and 

 may at a future time be the starting-point of a new in- 

 fection. 



Virulence. The virulence of tubercle bacilli varies 

 considerably according to the sources from which they are 

 obtained. Bacilli from different cases are of different 

 degrees of virulence, and bacilli from different animals vary 

 still more. Lartigau,* in an instructive paper upon " Varia- 

 tion in Virulence of the Bacillus Tuberculosis in Man," 

 found much variation among bacilli secured from the lesions 

 of human tuberculosis. The virulence was tested by em- 

 ploying cultures only for inoculation, and taking of each 

 bacillary mass exactly 5 mg. by weight, suspending it in 5 c.c. 

 of an indifferent fluid until the density was uniform and 

 the microscope showed no clumps, and injecting into rab- 

 bits and guinea-pigs, pairs of animals being injected in 

 the same manner, with the same material, at the same 

 time, and being subsequently kept under similar conditions. 

 The occurrence of tuberculosis in the inoculated animals 

 was decided by both macroscopic and microscopic tests. 



Lartigau found that human tubercle bacilli from different 

 sources produced varying degrees of tuberculosis in animals ; 

 that the injection of the same culture in different amounts 

 produces different results; that the extent and rapidity of 

 development usually corresponds to the virulence of the 

 culture; that doses of i mg. of a very virulent culture may 

 induce general tuberculosis in rabbits in a very short time; 

 that 20 mg. of a bacillus of low virulence may fail to pro- 



c "Journal of Medical Research," vol. vi, No. i; N. S., vol. i, No. i, 

 p. 156, July, 1901. 



