MICROBIAL DISEASES OF MAN AND DOMESTIC ANIMALS 749 



blood of infected animals. To germicides the Pneumococcus is very sensitive and 

 is killed in a few minutes by tbe common disinfectants in their usual strength. 



The pathogenic properties of the Pneumococcus for animals is some- 

 what variable. Natural infection is not common. To artificial infec- 

 tion mice and rabbits have been found most susceptible, while guinea 

 pigs, dogs, rats and cats are more resistant, and birds are practically 

 immune probably because of their high body temperature. Mice and 

 rabbits succumb to subcutaneous or intraperitoneal injections of viru- 

 lent cocci from cultures or in sputum with the development of a sep- 

 ticaemic condition, and in the latter case a peritonitis. By special, 

 methods lobar pneumonia has been produced in rabbits as has also 

 endocarditis. 



Variations in virulence of the Pneumococcus are very marked. The 

 virulence can be increased by passage through susceptible animals until 

 an extremely small dose will kill a mouse. Cultures obtained from man 

 may vary considerably in their virulence for animals. 



The organism gains entrance through the respiratory mucosa and 

 as a matter of fact appears to be a common inhabitant of these regions. 

 However the organism may reach the lung (the lobar distribution sug- 

 gests sowing by the blood stream), it is certainly frequent to find posi- 

 tive blood cultures during the disease — a fact which accounts for the 

 development of such complications as meningitis, endocarditis, etc. 

 The toxaemia probably arises from lysis of the organisms and it has been 

 shown that the autolysis of cultures in salt solution gives rise to a soluble 

 toxic portion and an insoluble non-toxic portion. , 



Immunity to Pneumococcus infections can be shown to exist after an 

 attack but only for a short time. 



Pneumococci may be considered as inhabiting the mucous mem- 

 branes of the respiratory tract in the majority of people and acquire 

 virulence only under some special circumstances lowering the general 

 vitality. In pneumonia and some kinds of bronchitis as above men- 

 tioned it should be remembered that sputum and mouth spray may 

 contain large numbers of virulent organisms. 



Specific therapeutic agents such as antipneumococcic serums, vac- 

 cines of dead cultures and autolysates^ as wellasleucocytic extracts, have 

 been tried and all with some promising results. The earlier failures 

 with serum therapy have been found to be due in part to the occurrence 

 of different strains. By arranging these strains into three groups with 



