386 TYPHOID FEVER 



pigs and other laboratory animals, but, in these, characteristic 

 changes in the intestine are often awanting. Shiga, however, 

 obtained such effects by introducing the organism into the 

 stomach of young cats and dogs, and confirmatory results were 

 obtained by Flexner. Such attempts have been specially suc- 

 cessful when the virulence of the organism has been previously 

 exalted by intraperitoneal passage. In two cases, apparently 

 well authenticated, a dysenteric condition has followed in 

 the human subject from ingestion of pure cultures of the 

 organism. 



In the action of the bacillus a toxin may be concerned. If 

 the organism be grown for two or three weeks in an alkaline 

 bouillon, there appears in the culture medium, probably by 

 autolysis of the bacteria, a toxin separable by filtration in the 

 ordinary way. The optimum alkalinity is achieved by adding - 3 

 per cent, of soda to bouillon neutral to litmus, the resulting 

 precipitate not being removed ; free access of oxygen is permitted 

 during growth. Apparently, the Shiga-Kruse strains yield the 

 most toxic filtrates, and with the Flexner strain, the results of 

 most observers show that soluble toxins cannot be obtained. 

 The poison is very toxic to animals, especially rabbits, and 

 however introduced into the body it causes after an incubation 

 period hemorrhagic enteritis with a diphtheritic-like exudate 

 on the surface of the mucous membrane. Toxins isolated from 

 different strains differ as regards the animals for which they are 

 most toxic. The toxin is fairly resistant to heat, standing 

 temperatures up to 70° C. without being injured. 



It may be said that an aggressive reaction {vide p. 187) has 

 also been described in the case of the dysentery bacillus. 



Immunisation Experiments. — Both large and small animals 

 have been immunised against the bacillus and also against its 

 toxic filtrates. In the former case the immunisation has been 

 commenced either with non-lethal doses of living cultures, or with 

 cultures killed by heat. The nature of the immunisation is 

 probably complex. When cultures have been used, a bactericidal 

 serum, in which immune bodies and complements (vide Immunity) 

 are concerned, is developed. When the toxin is used for 

 immunisation, a serum protecting against the toxin is produced. 

 According to some results, animals immunised with cultures are 

 immune against the toxin, and vice versa. All races of animals 

 do not lend themselves to immunisation. 



Considerable work has been done in immunising large 

 animals (horses, goats) against the soluble toxins of the 

 dysentery bacillus with a view to obtaining therapeutic sera. 



