Two strains of Pasteurella pestis representative of 34 strains isolated from a Peromyscus plague focus were studied and found to be morphologically and biochemically similar or identical to classical P. pestis. As compared with the virulent Alexander strain, both strains were of low virulence for guinea pigs and white mice but they were of similar virulence for the three species of Peromyscus involved in the plague focus. They were highly virulent for P. crinitus and P. truei, but only moderately virulent for P. maniculatus. Pasteurella pestis in the Peromyscus in this focus may be distinct from that normally found in squirrels and other large rodents. There is a need for increased efforts directed towards the study of atypical plague in wild rodents.
Present address: International Center for Medical Research and Training, The George Williams Hooper Foundation, University of California Medical Center, San Francisco 22, California.