BACILLUS PESTIS 411 



Pathogenesis. Animal. "Plague is primarily a disease of rodents, 

 and secondarily and accidentally 1 a disease of man." 2 The reservoir 

 of plague appears to be certain rodents; the disease exists in chronic 

 form in the marmot (Arctomys bobac) of India, and has recently been 

 discovered in the western United States as a chronic disease in the 

 ground squirrel (Citellus beechyi) by Wherry, 3 whose observations 

 have been confirmed by McCoy. McCoy 4 and Chapin 5 have found 

 that a smaller member of the squirrel family, Ammospermophilus 

 lecurus) is also susceptible to infection with Bacillus pestis. The 

 various members of the genus Mus Mus norwegicus, Mus rattus, 

 Mus alexandrinus, and probably Mus musculus, "are the producers 

 of acute outbreaks, the conduit for the carriage of the virus from its 

 perpetuating reservoir to the body of man." Guinea-pigs are some- 

 what more susceptible to infection with the plague bacillus than 

 rodents; the disease appears to have a seasonal distribution among 

 rodents 6 in India, and these epidemic periods coincide in time with 

 epidemics in man. Immediately following epidemic periods con- 

 siderable numbers of rodents appear to be relatively non-susceptible 

 to infection. Rabbits 7 and monkeys 8 are also susceptible. Dogs 

 and cats are more refractory; herbivora appear to be practically 

 immune, at least to natural infection. 



The lesions observed in rats are striking and important because 

 plague epidemics usually appear about two weeks earlier among 

 these animals than in man. Infection may take place through infected 

 fleas from other rats, from ingestion of dead plague-infected animals, 

 or by inhalation. The lesions are those of an hemorrhagic septicemia; 

 upon laying open the animal, 9 the inguinal and axillary glands are 

 usually enlarged (buboes), markedly injected and frequently hemor- 

 rhagic. The contents may be firm, or, less commonly, purulent. 

 The peritoneal and pleufal surfaces are red and injected, and there 

 is an excess of fluid in both cavities. The spleen is enlarged, engorged 

 and moderately soft, and the liver usually presents a mottled appear- 

 ance, due to punctiform hemorrhages and areas of necrosis which 



1 This statement may possibly require modification in connection with the pneumonic 

 type of plague in man (see human pathogenesis) . 



2 Rucker, Public Health Reports, July 19, 1912, p. 1130. 



3 Public Health Reports, 1908, xxiii, 1289; Jour. Inf. Dis., 1908, v, 485. 



4 Public Health Bull., April, 1911, No. 43. 



5 Ibid., January, 1912, No. 53, p. 15. 



6 See Jour. Hyg., 1908, viii, 266, for details. 



7 McCoy, Public Health Bull., April, 1911, No. 43. 



8 Wyssokowitsch and Zabolotny, Ann. Inst. Past., 1897, xi, 665. 



9 Which should be done after treatment with an insecticide to kill ecto-parasites. 



