398 A MANUAL OF BACTERIOLOGY 



by feeding on the corpses or carcases of men or animals 

 dead from the disease. 



In man the bacilli are found in large numbers in the 

 fluid in the buboes, either alone or mixed with streptococci 

 or micrococci, and in the sputum in the pneumonic form. 

 They are not usually found in any number in the blood 

 except in the septicaemic variety, or shortly before death, 

 and in stained preparations appear as short plump bacilli, 

 often in pairs, with polar staining and unstained centres 

 (Plate XIV. a and 6). If the organisms are found to be 

 free and numerous in the buboes the prognosis tends to 

 be grave, but if they are largely present within the 

 phagocytic polymorphonuclear leucocytes the prog- 

 nosis is better and the disease will probably remain 

 localised. 



Toxins. The plague bacillus forms but little toxin, the 

 minimal fatal dose of the most active filtered broth culture 

 for a mouse being about 0-02 c.c. In order to prepare a 

 vaccine or an anti-serum it is necessary, therefore, to 

 employ unfiltered cultures -4.e. the microbes themselves. 



Macfadyen obtained an endotoxin by triturating the 

 bacilli frozen with liquid air. 



Vaccines and immunity. Of the plague vaccines, that 

 of Haffldne, the Haffldne prophylactic, is the best known, 

 and has been extensively employed. It consists essen- 

 tially of a four to six weeks old butter- fat broth culture 

 of the plague bacillus, killed by heating to 65 C. for an 

 hour, with a small addition of antiseptic. As to the 

 value of Hafikine's prophylactic a mass of figures is 

 available. By its use both the incidence of, and mortality 

 from, plague are markedly diminished. Wilkinson col- 

 lected the following data of the efficiency of the vaccine : 

 Among the inoculated the case incidence was 1*8 and the 

 case mortality 23-9 per cent. ; among the uninoculated 

 the figures were 7-7 and 60-1 respectively. The immunising 



