Fleas and Plague 155 



and therefore may carry the plague bacillus from 

 rat to rat or from rat to man. 



GROUND-SQUIRRELS AND PLAGUE 



As early as 1903 Dr. Blue, in charge of the plague 

 suppressive measures in San Francisco, became im- 

 pressed with the possibility of the common Cali- 

 fornia ground-squirrels (Otospermophilus beecheyi), 

 acting as an agent in the transmission of plague. 

 It was rumored at that time that some epidemic 

 disease was killing the squirrels in some of the 

 counties surrounding San Francisco Bay, notably 

 in Contra Costa County. None of the squirrels 

 were examined at that time, but since then many 

 thousand have been carefully studied and it has 

 been definitely shown that many of them are plague- 

 infected. Just how the plague got started among 

 them will probably never be really known. There 

 is little doubt, however, but that it was transferred 

 in some way from the rats to the squirrels. The 

 trains and the bay and river steamers running out 

 from San Francisco would afford abundant oppor- 

 tunity for the rats to go from the city to the ware- 

 houses all along the shore. Once there they would 

 use the same runways as the squirrels about the 

 warehouses and in the near-by fields. In harvest 

 time the rats migrate to the fields and make con- 



