METHODS OF DIAGNOSIS 437 



are used, and injections are usually repeated on subsequent days. 

 Lustig's serum is prepared by injecting a horse with repeated and 

 increasing doses of a substance derived from the bodies of plague bacilli, 

 probably in great part nucleo-proteid. Masses of growth are obtained 

 from the surface of agar cultures, and are broken up and dissolved in a 

 1 per cent solution of caustic potash. The solution is then made slightly 

 acid by hydrochloric acid, when a bulky precipitate forms ; this is 

 collected on a filter and dried. For use a weighed amount is dissolved 

 in a weak solution of carbonate of soda and then injected. The serum 

 is obtained from the animal in the usual way. Extensive observations 

 with both of these sera show that neither of them can be considered 

 a powerful remedy in cases of plague, though in certain instances 

 distinctly favourable results have been recorded. The Indian Com- 

 mission, however, came to the conclusion " that, on the whole, a certain 

 amount of advantage accrued to the patients both in case of those 

 injected with Yersin's serum and of those injected with Lustig's serum." 

 It may also be mentioned that the Commission found, as the result of 

 experiments, that Yersin's serum modified favourably the course of the 

 disease in animals, whereas Lustig's serum had no such effect 



3. Serum Diagnosis, Specific agglutinins may appear in the blood of 

 patients suffering from plague, as also they do in the case of animals 

 immunised against the plague bacillus. It is to be noted, however, that 

 in clinical cases the reaction is not invariably present, the potency of 

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

 plicated by the natural tendency of the bacilli to cohere in clumps. For 

 the last reason the macroscopic (sedimentation) method is be preferred 

 to the microscopic (p. 111). A suspension of plague bacilli is made by 

 breaking up a young agar culture in 75 per cent sodium chloride 

 solution ; the larger Hocculi 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 micro- 

 scopically, and cultures on agar or blood serum should be made 

 by the successive stroke method. The cultural and morpho- 

 logical 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 



