Virulence 



595 



The postmortem appearance of the body of a plague-infected 

 rat is as follows:* Subcutaneous hemorrhages occur in about 40 

 per cent, of the animals and are most frequently to be seen in the 

 submaxillary region. Buboes are present in the majority of cases, 

 usually in some one locality, and commonly about the neck. The 

 liver may show necrotic changes which have the appearance of an 

 excessive deposit of fat, and a condition of the greatest importance 

 in diagnosis is the occurrence of small necrotic foci scattered over 

 its surface and throughout its substance. The spleen is firm and 

 does not collapse Uke a soft normal spleen; granules or nodules may 

 be well marked in it and may be confluent. The kidneys and 

 suprarenal capsules are often congested. Hemorrhages are fairly 

 common in the lungs and visceral pleura. The presence of pleural 

 effusion is very characteristic and of great value in diagnosis. In 

 naturally infected plague rats, the most important features for 

 purposes of diagnosis are: 



1. A typical bubo — most commonly in the neck. 



2. Granular liver — not seen except in plague rats. 



3. Hemorrhages beneath the skin and in the internal organs are 

 very suggestive. 



4. Pleural effusion. 



In putrid rats, bubo, granular hver and pleural effusion may persist 

 and are of great significance. A microscopical examination of 

 scrapings from buboes and spleen and inoculation tests will clinch 

 the diagnosis (Besson). 



Virulence. — It was formerly supposed that by frequent passage 

 through animals of the same species the bacillus could be much in- 

 creased in virulence. Kolle recommended rats for this purpose, and, 

 indeed, declared that without the use of rats it is impossible to keep 

 cultures at a high grade of virulence. Yersin thought that when 

 cultivated for any length of. time upon culture-media, especially 

 agar-agar, the virulence was rapidly lost and the bacillus eventually 

 died. On the other hand, when constantly inoculated from animal 

 to animal, the virulence of the bacillus is much increased. 



Knorr, Yersin, Calmette, and Borrelf found that the bacillus 

 made virulent by frequent passage through mice is not increased in 

 virulence for rabbits. According to the researches of the Advisory 

 Committee for the study of plague in India, this is an error. The 

 virulence of plague bacilU for rats is subject to very little change. 

 Their members in investigating the question made twenty-six 

 passages from rat to rat, by subcutaneous inoculatioti, during 

 eighty-nine days, and found the original virulence of the organism 

 unchanged. 



Sanitation. — ^A disease that may be transmitted from man to 

 man byjatmospheric infection and inhalation, that can be transported 



* See "Journal of Hygiene," 1907, VII, 324. , 



t "Ann. de I'Inst. Pasteur.," July, 1895. 



