Rocky Mountain spotted fever occurs sporadically in man in western California, a region where the classical vector, Dermacentor andersoni, is absent and the vector-pathogen-host interrelationships are poorly known. Therefore, from 1974–1979 we surveyed the potential mammalian hosts and tick vectors of spotted fever group (SFG) rickettsiae in northwestern California. Significant complement-fixing (CF) antibody titers of 1:16 or greater were detected in 45 of 887 sera (5 of 16 species examined). These comprised 37 of 135 blacktailed jack rabbits (Lepus californicus), 2 of 3 brush rabbits (Sylvilagus bachmani), 3 of 12 kangaroo rats (Dipodomys californicus), 1 of 37 brush mice (Peromyscus boylii), and 2 of 72 deer mice (Peromyscus maniculatus). Indirect fluorescent antibody (IFA) titers of ≥1:8 for SFG rickettsiae also were present in 30 of 84 L. californicus sera. Neither D. californicus nor P. boylii has been implicated previously as a host species for SFG rickettsiae. Lower CF antibody titers (1:4–1:10), suggestive of past infection but of less certain significance, were found in 46 additional sera from nine species. A total of 4,487 ticks (418 pools) comprising eight species were tested for the presence of rickettsiae. SFG antibodies detected by the IFA method were produced in adult mice inoculated with tick pool suspensions as follows: Dermacentor occidentalis, 23 of 65 pools (35.38%); Haemaphysalis leporispalustris, 5 of 182 pools (2.75%); and Ixodes pacificus, 2 of 46 pools (4.35%). The positive pools of D. occidentalis were collected from deer or vegetation, those of H. leporispalustris from jack rabbits, and those of I. pacificus from jack rabbit and raccoon. This is the first time that I. pacificus from California has been found associated with SFG rickettsiae. Reisolation attempts in guinea pigs, suckling mice, and/or VERO cell cultures subsequently yielded SFG rickettsiae from 4 of 22 of the tick pool suspensions found positive by the initial adult mouse-IFA screening method. Three of the isolates were recovered from D. occidentalis adults flagged from vegetation, and the fourth was from H. leporispalustris adults removed from a jack rabbit. This study supports earlier indications that in western California a mammal-tick cycle for SFG rickettsiae, different from the usual mammal-Dermacentor andersoni cycle for Rickettsia rickettsii in the western United States, exists. Further studies of the natural cycle and its implications for human disease are indicated.
Present address: Department of Entomological Sciences, University of California, Berkeley, California 94720.