Isolates of Borrelia turicatae, Borrelia parkeri, and the Florida canine borrelia (FCB) were examined to further phylogenetically characterize the identities of these spirochetes in the United States. DNA sequences of four chromosomal loci (the 16S rRNA gene, flaB, gyrB, and glpQ) were determined for eight isolates of B. turicatae and six isolates of B. parkeri, which grouped the spirochetes into two distinct but closely related taxa (>98% sequence identity) separate from Borrelia hermsii. The FCB was clearly separated with the group identified as B. turicatae, confirming this bacterium as a relapsing fever spirochete. Therefore, the potential for tick-borne relapsing fever in humans and other animals exists in Florida and future efforts are needed to determine the enzootic hosts and distribution of this spirochete in the southeastern United States. Analysis of plasmids demonstrated both linear and circular forms in B. turicatae but only linear plasmids in B. parkeri, which should be of interest to investigators concerned with plasmid diversity and evolution within this group of spirochetes.

Borrelia hermsii is the primary cause of tick-borne relapsing fever in North America. When its tick vector, Ornithodoros hermsi, acquires these spirochetes from the blood of an infected mammal, the bacteria switch their outer surface from one of many bloodstream variable major proteins (Vmps) to a unique protein, Vtp (Vsp33). Vtp may be critical for successful tick transmission of B. hermsii; however, the gene encoding this protein has been described previously in only one isolate. Here we identified and sequenced the vtp gene in 31 isolates of B. hermsii collected over 40 years from localities throughout much of its known geographic distribution. Seven major Vtp types were found. Little or no sequence variation existed within types, but between them significant variation was observed, similar to the pattern of diversity described for the outer surface protein C (OspC) gene in Lyme disease spirochetes. The pattern of sequence relatedness among the Vtp types was incongruent in two branches compared to two genomic groups identified among the isolates by multilocus sequence typing of the 16S rRNA, flaB, gyrB, and glpQ genes. Therefore, both horizontal transfer and recombination within and between the two genomic groups were responsible for some of the variation observed in the vtp gene. O. hermsi ticks were capable of transmitting spirochetes in the newly identified genomic group. Therefore, given the longevity of the tick vector and persistent infection of spirochetes in ticks, these arthropods rather than mammals may be the likely host where the exchange of spirochetal DNA occurs.

Rickettsia peacockii, a new species of spotted fever group rickettsiae, was identified from Rocky Mountain wood ticks (Dermacentor andersoni) collected in the Sapphire Mountain Range on the eastern side of Bitterroot Valley, Montana. DNA from R. peacockii SkalkahoT (T = type strain) in naturally infected tick tissue was amplified by a PCR assay with primer sets derived from eubacterial 16s ribosomal DNA (rDNA), rickettsial citrate synthase, and 190-kDa surface antigen (rOmpA) genes. Partial 16s rDNA and rOmpA gene sequences exhibited levels of similarity of 99.7 and 93.2%, respectively, with the sequences of the spotted fever agent Rickettsia rickettsii R. By using GimCnez staining, fluorescent antibody tests, a PCR assay, and a restriction fragment length polymorphism analysis, 76 of 115 female ticks (minimal field infection rate, 66.1%) collected between 1992 and 1995 were found to be infected. The organism is passed transstadially and transovarially (minimal vertical transmission rate, 73.3%), and infections are localized in ovarial tissues. Attempts to cultivate R. peacockii were unsuccessful.Rickettsiae are gram-negative bacteria that are maintained in nature by replication in cells of small mammals and arthropods (5, 7,23,32,47). At least 23 Rickettsia species (alpha subdivision of the Proteobacteria, family Rickettsiaceae) have been or are being described (40, 43), while undoubtedly more remain to be discovered. Of these species, Rickettsia rickettsii, the etiological agent of Rocky Mountain spotted fever (RMSF), is the most significant public health threat in the western hemisphere. Human RMSF cases were first recognized in 1873 in Bitterroot Valley in western Montana (34). Through a series of classic experiments begun in 1906, H. T. Ricketts and coworkers clearly established that the etiology of RMSF involves R. rickettsii in Rocky Mountain wood ticks (Demacentor andersoni) (39). Transmission to humans occurs principally through bites of infected ticks. Between 1981 and 1992, 4,217 confirmed human RMSF cases (minimum case/fatality ratio, 4.0%) were reported in the United States Since the pioneering work of Ricketts, several other species of rickettsiae have been recognized in Bitterroot Valley. A survey of 3,705 adult D. andersoni ticks collected in the valley during 1977 revealed four distinct serotypes of hemocyte-associated rickettsiae that were later identified as R. rickettsii, Rickettsia rhipicephali, Rickettsia montana, and Rickettsia bellii (33). Remarkably, only 2 of the 309 isolates found came from the 1,138 ticks collected on the eastern side of the valley, and both of these isolates were nonvirulent R. rhipicephali strains (33). With only one exception, spotted fever group (SFG) rickettsiae virulent for guinea pigs have never been recovered from east side wood ticks (32). These data correspond to the RMSF disease focality in the valley, where most human cases result (9).