560 PATHOGENIC MICROORGANISMS 



to kill fleas and other ectoparasites. The rats are nailed by their feet 

 to a shingle and the skin is reflected from the whole front of the body and 

 neck so as to expose the cervical, axillary, and inguinal regions. The 

 thoracic and abdominal cavities are then opened and examined. 



Wherry,! McCoy,^ and others have found that the California groimd 

 squirrel was infected with plague, during the recent occiu-rence of 

 plague on the Pacific coast, and several cases of plague in man were 

 traced to this source. In studying these and other American ro- 

 dents McCoy found that ground squirrels as a species were highly 

 susceptible, never showing natural immunity. Field mice were but 

 moderately susceptible. Gophers were highly resistant. McCoy has 

 also described a case of spontaneous infection in a brush rat (Neo- 

 toma fuscipes). Rock squirrels were found by McCoy to be readily 

 infected. 



Wu Lien Teh (G. L. Tuck) ' has recently found that the Manchurian 

 tarbagan or marmot (Arctomys bobac), an animal trapped for its fur, 

 occasionally suffers from plague. The disease is never extensive and the 

 animal of much less importance in spreading the disease than is the rat. 



Plague is transmitted either by direct contact or inhalation, or in- 

 directly by clothing, linen, and other objects worn or handled. The 

 r61e in the transmission of the disease played by rats is probably of 

 great importance. The animals vomit, defecate, and die in cellars, 

 storerooms, etc., and thus set free vast numbers of plague bacilli for 

 indirect accidental transmission to human beings. The actual mode in 

 which this transmission takes place is by no means certain. The fact 

 that in countries where plague is prevalent many of the natives go 

 insufficiently shod or barefooted, may account for many infections. 



Simond* lays great stress upon transmission to man by means of 

 fleas, the Indian rat-flea often being parasitic upon man. His conclusions 

 are probably too far-reaching, though the possibility of such infection 

 can not be denied.^ 



It is a curious fact observed by various bacteriologists that plague 

 bacilli isolated from pneimionic cases are particularly apt to cause 

 pneumonic lesions, having, as it were, acquired a selective pathogenicity 

 for the lung. A most valuable contribution to our knowledge of pneu- 



' Wherry, Jour. Inf. Dis., v, 1908. 



2 McCoy, Jour. Inf. Dis., vi, 1909; vii, 1910. 



' Wu Lien Teh, Jour, of Hyg., xiii, 1913. 



* Simond, Ann. de I'inst. Pasteur, 1893. 



^Nuttall, Cent. f. Bakt., xxii, 1897; Nuttall, Hyg. Rund., ix, 1899. 



