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 (LD50) 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 LD50.) 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 > 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. © 1979 by The University of Chicago.