Abstract
Summary
1. Eighty to 90% of the guinea pigs infected intraperitoneally with a strain of low virulence of R. rickettsii were protected against a simultaneous injection of a highly virulent strain of spotted fever, or boutonneuse fever, provided the low virulent strain was given in about 10 to 30 times the concentration of the virulent strain. 2. Infection with rickettsiae of Q fever, scrub typhus, endemic typhus, and epidemic typhus protected guinea pigs against a virulent strain of spotted fever under the above conditions. 3. Columbian ground squirrels, field mice (Microtus), and cottontail rabbits were infected by exposure to many D. andersoni containing a low virulent strain of Rocky Mountain spotted fever and a few D. andersoni containing a highly virulent strain. Uninfected ticks fed upon these animals were found to contain only the low virulent strain.
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