EXPERIMENTAL REPRODUCTION OF PLAQUE IN ANIMALS 115 



strain, Past, aviseptica, for example, will protect mice against infection with a 

 strain from swine plague or pleuropneumonia of calves. Likewise an immune 

 serum prepared against one strain is said to protect against all other strains. 

 Schirop (1908), however, obtained evidence of varietal differences in the species ; 

 according to him protection can be realized with certainty only by the use of 

 monovalent sera (see also Priestley 19366.) 



Pathogenicity. — Past, pestis and Past, pseudotuberculosis cause disease in rodents ; 

 Past, pestis is also pathogenic for man. The hsemorrhagic septicaemia group is 

 pathogenic for a large number of animals and birds, but only very exceptionally 

 for man (see Chapter 73). 



The virulence of all three organisms is subject to considerable variation, and 

 appears to be determined, at least in part, by the particular variant that has 

 gained the ascendancy. The D variant of Past, lepiseptica is highly virulent, the 

 G variant comparatively avirulent. There is evidence that the smooth type of 

 Past, pseudotuberculosis is more virulent for guinea-pigs than the rough type 

 (Zlatogorofif and Moghilewskaja 1928a), though, according to Boquet (1937), this 

 relationship is fortuitous. There are several reports on variations in virulence 

 of the plague bacillus occurring under natural and experimental conditions. (See 

 Yersin, Calmette, and Borrel 1895, Keport 1906, McCoy 1911, Rowland 19146, 

 Eberson 1917, Pirie 1929, Burgess 1930.) Many of the statements are conflicting, 

 rendering it impossible at the moment to draw any definite conclusions on the 

 relation of virulence to colonial appearance or antigenic structure. Further study 

 of this problem by careful quantitative methods is required. For maintaining 

 the virulence of the plague bacillus, Sokhey (19396) recommends growing the 

 organisms on 5 per cent, rabbit blood agar for 4 days at 26°-32° C, sealing the 

 tubes in the flame, and storing them at 4° C. 



Experimental Reproduction o£ Plague in Animals. 



Bubonic plague can be reproduced in rodents and monkeys by experimental 

 inoculation. Dogs, cats, pigs, cattle, sheep, goats, and horses are difficult to 

 infect ; birds, with the exception of sparrows, are completely resistant. The 

 disease is said to occur naturally in camels ; but these animals are refractory to 

 experimental inoculation (Zabolotny 1923). Even the rodents show great variation 

 in susceptibility to infection. The less resistant members succumb rapidly, whereas 

 the more resistant ones either fail to develop the disease, or else develop a sub- 

 acute or chronic type. Spencer (1921) found in America that about 30 per 

 cent, of the rats from a plague-free district were resistant to subcutaneous inocu- 

 lation of Past, pestis. According to Sokhey (1939a), the white mouse is the most 

 susceptible laboratory animal, but Otten (1938) on the contrary maintains that 

 the guinea-pig, in spite of its greater weight, is more susceptible than the mouse. 



Pneumonic plague has been produced in rats and also in marmots {Spermo- 

 philus citellus) by causing them to inhale cultures of Past, pestis (Eberson and 

 Wu Lien Teh 1917, Wu Lien Teh and Eberson 1917). 



Rats. — Subculuneuus injection of a very small number of vii'ulent plague bacilli leads 

 to death in 2 to 8 days. Post mortem, there is necrosis and oedema at the site of inocula- 

 tion ; the regional lymph glands are swollen and surrounded by a hsemorrhagic infiltration 

 of the subcutaneous tissue. Glands in other parts of the body are often congested and 

 swollen ; the spleen may be enlarged and dark red ; the liver and lungs are hypersemic, 

 and sometimes a pleural exudate is seen. Bacilli are found in large numbers in the local 



