The biological properties of strains of Coxiella burnetti from 9 species of wild animals and 2 species of ticks collected at Hopland, in Northern California, were compared to the properties reported for the highly infectious wildlife strains isolated in Western Montana, and the comparatively avirulent strains isolated from rodents in Utah. The Hopland strains are though to be similar to the Utah strains because they were usually more infectious for hamsters and induced higher antibody responses in this host than in guinea pigs or mice. The Hopland strains did not cause a febrile response and were difficult to transfer in guinea pigs. Although a slight splenomegaly was evident in inoculated mice and hamsters, there was no exudate around the spleen nor granuloma at the site of injection, as induced by typical virulent strains of C. burnetii. The Q fever rickettsiae isolated from deer and coyotes were the most infectious of the Hopland strains. They induced higher antibody responses in guinea pigs during the primary isolation and were more easily transferred through laboratory hosts.
Both the Utah and Hopland wildlife strains were isolated from animals collected in areas where livestock were present. It is not known whether the infectivity of certain strains of C. burnetti is influenced by host-parasite equilibrium in an animal population chronically exposed to the organism.