METHODS OF DIAGNOSIS 501 



the serum is not of high order, and the carrying out of the test is 

 complicated by the natural tendency of the bacilli to cohere in clumps. 

 For the last reason the macroscopic (sedimentation) method is to be 

 preferred to the microscopic (p. 121). A suspension of plague bacilli is 

 made by breaking up a young agar culture in '75 per cent, sodium 

 chloride solution ; the larger flocculi of growth are allowed to settle, and 

 the fine, supernatant emulsion is employed in the usual way. According 

 to the results of the German Plague Commission and the observations of 

 Cairns, made during the Glasgow epidemic, it may be said that the 

 reaction is best obtained with dilutions of the serum of from 1 : 10 to 

 1 : 50. Cairns found that the date of its appearance is about a week 

 after the onset of illness, and that it usually increases till about the end 

 of the sixth week, thereafter fading off. It is most marked in severe 

 cases characterised by an early and favourable crisis, less marked in 

 severe cases ultimately proving fatal, whilst in very mild cases it is 

 feeble or may be absent. The method, if carefully applied, may be of 

 service under certain conditions ; but it will be seen that its use as a 

 means of diagnosis is somewhat restricted. 



Methods of Diagnosis. Where a bubo is present a little of 

 the juice may be obtained by plunging a sterile hypodermic 

 needle into the swelling. The fluid is then to be examined 

 microscopically, and cultures on agar or blood serum should be 

 made by the successive stroke method. The cultural and 

 morphological characters are then to be investigated, the most 

 important being the involution forms on salt agar and the 

 stalactite growth in bouillon, though the latter may not always 

 be obtained with the plague bacillus : the pathogenic properties 

 should also be studied, the guinea-pig being on the whole most 

 suitable for subcutaneous inoculation. In many cases a diagnosis 

 may be made by microscopic examination alone, as in no known 

 condition other than plague do bacilli with the morphological 

 characters of the plague bacillus occur in large numbers in the 

 lymphatic glands. The organism may be obtained in culture 

 from the blood in a considerable proportion of cases by with- 

 drawing a few cubic centimetres and proceeding in the usual 

 manner. On the occurrence of the first suspected case, every 

 care to exclude possibility of doubt should be used before a 

 positive opinion is given. 



In a case of suspected plague pneumonia, in addition to 

 microscopic examination of the sputum, the above cultural 

 methods along with animal inoculation with the sputum should 

 be carried out; subcutaneous injection in the guinea-pig and 

 smearing the nasal mucous membrane of the rat may be recom- 

 mended. Here a positive diagnosis should not be attempted by 

 microscopic examination alone, especially in a plague -free dis- 

 trict, as bacilli morphologically resembling the plague organism 

 may occur in the sputum in other conditions. 



