Experimental plague in the California ground squirrel.

In experiments to determine the current susceptibility to plague, it was demonstrated that 256 Yersinia pestis (a local strain of high virulence) were required to produce a 50% infectious dose (ID50) in California ground squirrels (Spermophilus beecheyi) that had been collected in southeastern Monterey County, California; 6,070 Y. pestis were required to produce a 50% lethal dose (LD50). (The LD50 was about 24 times the ID50.) The frequency of serologic response to the specific fraction 1 antigen of Y. pestis and mortality were dose-related. Approximately half of the squirrels inoculated with six to 6,070 Y. pestis survived without seroconversion, whereas antibody to specific fraction 1 antigen was always observed in squirrels that survived challenges of greater than or equal to 60,700 Y. pestis. However, titers never exceeded 1:64. The implications of these data for enzootic and epizootic transmission of plague in resistant squirrel populations are examined.