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, 1 McCoy, 2 and others have found that the California ground 

 squirrel was infected with plague, during the recent occurrence 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) 3 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. 



The two principal species of rats to be considered in this connection 

 are Epimys norvegicus and Epimys rattus. The spread from rat to 

 rat, according to the Second Indian Commission, is entirely by means 

 of infected fleas. 



The ordinary spread of the disease to man, according to this same 

 commission, comes from Epimys rattus, which lives in close relation- 

 ship with man and is conveyed to man largely by the rat flea, Xenop- 

 sylla cheopis. This flea leaves the dead rat in about three days, 

 and is capable of living for three or four weeks on man's blood. 

 The plague bacilli need about three days' incubation in the body of 

 the flea. 



Summarizing the knowledge at present available about the spread 

 of the plague, it seems likely that, excepting in the case of pneumonic 

 plague, the ordinary method is by means of rat fleas. 



It is a curious fact observed by various bacteriologists that plague 

 bacilli isolated from pneumonic 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- 



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



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



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



