Rickettsia peacockii sp. nov., a New Species Infecting Wood Ticks, Dermacentor andersoni, in Western Montana

Abstract

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).

Rickettsia peacockii, an endosymbiont of Dermacentor andersoni, does not elicit or inhibit humoral immune responses from immunocompetent D. andersoni or Ixodes scapularis cell lines

“…It is not known how Wolbachia avoids eliciting immune responses, but living inside cells may be an important adaptation that hides Wolbachia from arthropod immune systems (29). In nature, R. peacockii inhabits the ovaries of female D. andersoni and is rarely found in hemolymph (16). The ovaries of D. variablis, a tick closely related to D. andersoni, contain little lysozyme transcript relative to the abundant lysozyme transcript found in D. variablis hemocytes (9), suggesting R. peacockii inhabits lysozyme-poor tissues as part of a strategy of avoiding contact with more immunoresponsive tick tissues.…”

“…Immune interactions between ticks and endosymbionts are difficult to assess in vivo, and it is not known whether endosymbionts elicit immune responses similar to those induced by Escherichia coli or Micrococcus luteus. Rickettsia peacockii is a rickettsial endosymbiont of D. andersoni, and is found within the interstitial cells of the ovaries, and developing oocytes (16). It is not known why R. peacockii appears to be restricted to the female reproductive tissues, but host immune responses may limit rickettsial distribution.…”

Rickettsia felis: molecular characterization of a new member of the spotted fever group.

“…Since R. felis has thus far resisted attempts at cultivation, thereby limiting the standard rickettsial typing by serological means, the organism was further characterized through the utilization of molecular means as has been employed to describe other uncultivated rickettsial organisms Neibylski et al, 1997). The rompA gene from cat fleas containing R. felis was amplified by PCR using different primers and amplification conditions and sequencing data for the 17 kDa protein gene were obtained.…”

Coinfection of Dermacentor silvarum Olenev (Acari: Ixodidae) by Coxiella-Like, Arsenophonus-Like, and Rickettsia-Like Symbionts

“…To date, a wide range of symbionts, such as Coxiella-like (13), Francisella-like (14), Wolbachia-like (15), Rickettsia-like (16), Arsenophonus-like (17), "Candidatus Midichloria mitochondrii" (18), and Rickettsia peacockii (19) symbionts, have been detected in several tick species. However, little attention has been given to coinfection with multiple symbionts of ticks.…”