36 IMMUNOLOGY 



Pathogfenicity. — The term pathogenicity is usually applied to 

 the disease-producing property of an infectious agent. 



Virulence. — The term virulence has been used by many as 

 synonymous with either the invasive power of an organism or its 

 ability to multiply in the tissues or blood stream of a host. This 

 definition seems inadequate for several reasons. In the first place, 

 the term is applied to such toxicogenic organisms as CI. tetani and 

 C. diphtheriae as an index of their ability to form tetanus toxin 

 and diphtheria toxin, respectively. 



The ''virulence test" of C. diphtheriae is essentially one for 

 toxin production. CI. tetani has no invasive power while C. 

 diphtheriae invades the mucous membrane to a slight degree only. 

 Both produce disease in suitable hosts by means of specific soluble 

 toxins. In the second place, examples have just been given of 

 infection without and also with pathological change. The host 

 must be considered as a factor. 



Definition of Virulence. — It would seem appropriate to define 

 virulence as the relative pathogenicity of an organism for a 

 particular host. In examples previously cited it will be recalled 

 that Leptospira ictcroheniorrJiafjiae is practically a virulent for the 

 wild rat but highly virulent for guinea pigs and man. The same 

 hosts exhibit similar resistance and susceptibility to diphtheria 

 toxin. 



Measurement of Virulence. — Virulence is usually expressed 

 in terms of the amount of infectious agent that will, when properly 

 administered, cause certain specified pathological changes in the 

 host within a given time. There has been no uniform standard 

 adopted for the measurement or dosage. Barber (1909), in 

 describing the virulence of the anthrax bacillus for mice, recorded 

 the actual number of organisms inoculated. Others have expressed 

 the dosage of bacteria in terms of milligrams of growth obtained 

 under standard conditions. The virulence of many bacteria has 

 been described in terms of the least volume of a 24-hour broth 

 culture grown at 37° C, or the fraction of growth obtained from 

 a blood agar or plain agar slant grown under similar conditions, 

 that will produce death or specified elianges in the test animal. 

 Some have even measured the dosage in terms of the standard 

 platinum wire loop. 



