BACILLUS PESTIS 559 



and guinea-pigs, in whom mere rubbing of plague bacilli into the un- 

 broken skin will often produce the disease. This method of experimen- 

 tal infection of guinea-pigs is of great service in isolating the plague 

 bacillus from material contaminated with other microorganisms. For 

 the same purpose, infection of rats subcutaneously at the root of 

 the tail may be employed. Such inoculation in rats is invariably 

 fatal. 



The studies of McCoy l upon guinea-pigs and white rats show that 

 individual plague cultures may vary considerably in virulence. The 

 size of the dose, always excepting enormous quantities such as a whole 

 agar culture, seems to make little difference in the speed with which 

 the animals die. There may be considerable variation in the suscep- 

 tibility of individual animals. Prolonged cultivation on artificial media 

 may gradually reduce the virulence of plague bacilli, though, as stated 

 above, this has not been the experience of all observers. 



In rats, spontaneous infection with plague is common and plays an 

 important role in the spread of the disease. Rats become infected from 

 the cadavers of plague victims or by gnawing the dead bodies of other 

 rats dead of the disease. The pneumonic type of the disease is common 

 in these animals and has been produced in them by inhalation experi- 

 ments. During every well-observed plague epidemic, marked mortality 

 among the domestic rats has been noticed. 



Since the examination of rats for plague is an important phase of 

 the study of epidemics, it may be well to review the typical lesions in 

 these animals as described by an experienced American student of plague, 

 George W. McCoy. 2 McCoy, agreeing with the Indian Plague Com- 

 mission, states that the naked eye is superior to the microscopical ex- 

 amination. There is engorgement of the subcutaneous vessels and a 

 pink coloration of the muscles. The bubo when present is sufficient 

 for diagnosis. Marked injection surrounds it and sometimes there is 

 hemorrhagic infiltration. The gland itself is firm but usually caseous or 

 occasionally hemorrhagic. In the liver there is apparent fatty change, 

 but this is due to necrosis. Pin-point spots give it a stippled appear- 

 ance as though it had been dusted with pepper. Pleural effusion is an 

 important sign. The spleen is large, friable, and often presents pin- 

 point granules on the surface. One or two per cent of rats may present 

 no gross lesions. Cultures should of course be made. The method 

 of examination consists in immersing the rat in any convenient antiseptic 



3? 



1 McCoy, Jour, of Inf. Dis., vi, 1909. 



2 George W. McCoy, Public Health Reports, July, 1912. 



