176 Manual of Veterinary Microbiology. 



vived. If at the time that a fowl is inoculated with 

 a virus of a certain degree of virulence, this same 

 virus is inoculated to culture media, it there repro- 

 duces itself, communicating to its progeny the special 

 pathogenic activity which it itself possessed; the at- 

 tenuation of the germ is therefore, in this case, hered- 

 itary. A series of cultures of progressively decreas- 

 ing virulence can thus be prepared. 



The attenuation is connected with the action of the 

 ox3'gen of the air; in order to preserve the virulence 

 of cultures it suffices to exclude them from contact 

 with the air; in contact with the latter they gener- 

 ally become inoffensive at the end of two months. 



The attenuated virulence shows a series of degrees, 

 from the normal virulence up to non-virulence. 

 When the virus is enfeebled to such an extent that it 

 no more kills chickens, it develops at the point of in- 

 oculation in the pectoral muscle a local alteration, 

 ending in the formation of a sequestrum which may 

 either be eliminated or absorbed. Fowls, after recov- 

 ery, are vaccinated against the later action of a more 

 virulent or mortal virus. The immunity thus con- 

 ferred is the more efficacious the more intense the 

 vaccinal disease; its duration appears not to exceed 

 one year; the immunity from a first vaccination is 

 strengthened by inoculation of a second more virulent 

 vaccine.* 



The enfeebled virus can regain its virulence when 



* [Kitt has obtained immunity in chickens against the virus of 

 chicken cholera by injection of the blood serum of previously im- 

 munised chickens, as well as by injection of the albumen of the 

 e^gs coming from immune hens. (Centralbl. f. Bad. XIV, 25. j 

 -D.] 



