196 BIOLOGY OF PNEUMOCOCCUS 



protect the horse against the subcutaneous or intravenous injec- 

 tion of living, virulent strains of the same type. Abscesses, pneu- 

 mococcemia, endocarditis, and pneumonitis, sometimes culminating 

 in fatal pneumothorax, may follow immunizing injections of living 

 cultures, even when the serum of the animal under active immuniza- 

 tion contains specific agglutinins and protective antibodies in suf- 

 ficient quantity to qualify it for therapeutic use. This paradoxical 

 phenomenon will be discussed in a later chapter. 



BIRDS 



Hens and doves have been found to be refractory to infection 

 with Pneumococcus by Fraenkel,* Gamaleia, 498 and by Kyes, 766 

 among others. Kindborg (1905) 713 apparently was the only worker 

 who claimed to have succeeded in demonstrating pathogenicity of 

 Pneumococcus for pigeons. Kyes studied the cellular reaction in 

 tissues of pigeons injected intraperitoneally with virulent pneumo- 

 cocci, and reported that the invading organisms were rapidly 

 withdrawn from the blood stream and localized in the liver and 

 spleen. Because the ultimate localization of the cocci in both of 

 these organs was within a type of fixed phagocyte — the hemo- 

 phage — common to both organs, Kyes concluded that the phago- 

 cytic destruction of pneumococci by hemophages is so extensive 

 and so rapid as actually to constitute an important, if not indeed 

 the determining, factor in the resistance of birds to pneumococcal 

 infection. The natural resistance of hens and doves may, however, 

 be lowered to the point of susceptibility through vitamin deficiency 

 or by the administration of poisons, as reported by Strouse, by 

 Guerrini, and by Corda.* 



Virulence of the Organism 



In any discussion of the pathogenicity of an organism, it should 

 be borne in mind that virulence is a purely relative term. The fac- 

 tors which enter into the determination of the invasiveness of a 



* Quoted by Neufeld and Schnitzer. 



