In this study, ticks of the generalist species I. ricinus and I. persulcatus, as well as nidicolous I. trianguliceps, attached to small mammals, were analyzed for the presence of vector-borne Rickettsia spp. including a species not previously reported in Europe.
Currently there are many studies regarding circulating Rickettsia species as in their vectors, tick and fleas, as well as in vector associated mammals, and possible Rickettsia-reservoir presence [18, 20, 25, 26, 27]. In the course of screenings of vector arthropods and their hosts, more and more new "Candidatus" Rickettsia species are being found [7, 13]. At the moment, little has been studied about the connection between mammals and rickettsiae. Xenodiagnosis studies show negative results for R. helvetica [20]. Furthermore, percentage of gaining Rickettsia spp. from the positive host is low, as described by Tomassone et al. [21]. Additional studies are required to determine the relationship between rodents and rickettsiae, the bacteremia duration, the distribution and natural cycle of Rickettsia spp. and their association with different arthropod vectors. To add more, further researches are necessary to find potential reservoir hosts and how Rickettsia maintains in nature.
To our knowledge, this study is the first report on the detection of a newly described species, Ca. R. uralica, in Europe. In this study the genospecies was detected only in I. trianguliceps ticks removed from voles, which is in agreement with the initial Ca. R. uralica report from Siberia designating the specificity of Ca. R. uralica to I. trianguliceps [7]. The authors claim that the same Rickettsia variant was previously detected in Myodes rutilus (northern red-backed voles) and S. araneus, which are also present in Estonia. Together with I. trianguliceps ticks these small mammals might play a role in the circulation of this Rickettsia species in nature. Despite the genetic clustering of this newly-described Rickettsia within the spotted-fever group, the pathogenic potential of Ca. R. uralica for domestic and wild mammals, pets or humans remains to be studied.
Although spotted fever rickettsioses are known to be emerging diseases spreading across the globe, human case reports due to R. helvetica infections are scarce. Serological or molecular tools have been used to detect R. helvetica infection in samples from patients with suspected Lyme neuroborreliosis in the Netherlands [10], with unexplained fever following a tick bite in France and Italy [11] and with rash, febrile illness and meningitis in Sweden [8, 9]. Rickettsia helvetica, a tick-borne rickettsiae species, is also frequently detected in Europe and Asia [2, 28, 29], being reported to be the prevalent Rickettsia species in some of the regions, e.g. in Germany [30], Slovakia [31] and Sakhalin Island in Russia [29]. Estonia belongs to one of the predominant region as well as over 95% of all Rickettsia species detected in questing [12] and rodent-attached ticks in the present study were R. helvetica. While there are no clinical reports of illness caused by R. helvetica in Estonia to date, the detection of this tick-borne pathogens (TBP) at positivity rates within ticks population similar to rates of 23.3% for Borrelia burgdorferi s. l. (I. Golovljova and J. Geller, personal communications), suggests that R. helvetica should be considered during surveillance for tick-borne diseases in Lyme borreliosis-endemic regions.
Rickettsial DNA was detected in 8.7% of all investigated attached ticks, and in 10,0% of I. ricinus, compared to 3,4% in I. trianguliceps. High rates of detection of rickettsial DNA in rodent-attached I. ricinus were also recently reported from Lithuania [26] where 22.6% of individually tested larvae (maximum likelihood estimation, MLE = 26.5%) were positive for Rickettsia spp.
There have been reports of the detection of several TBP, such as Anaplasma phagocytophilum [32], Neoehrlichia mikurensis and Babesia microti [33], Francisella tularensis [34] in nidicolous rodent-specialists I. trianguliceps ticks removed from small mammals. As reported by Igolkina et al. [7], SFG Rickettsia was found in 41.2% (14/34) of analyzed I. trianguliceps ticks feeding on voles in Western Siberia, which is significantly higher than the results of the current study (3.4%, 4/117). Nevertheless, the role of I. trianguliceps in the circulation and maintenance of TBPs is still largely unknown as is its importance and participation in the transmission of pathogens between ticks and rodent hosts.
The absence of rickettsial DNA in rodent-attached I. persulcatus larvae (0/64) and nymphs (0/12) could be explained by the relatively small number of I. persulcatus covered in the current study. However, several Rickettsia species, such as Ca. R. tarasevichae (1/530, 0.2%) and R. helvetica (8/530, 1.5%) were previously reported in unfed questing I. persulcatus ticks in Estonia [12].
We found rickettsial DNA in ticks removed mainly from My. glareolus and A. flavicollis, but also from several S. araneus. Although there are reports on the detection of R. helvetica in various small- to large-sized wild mammal samples from Lithuania [35], the Netherlands and Germany [18, 30, 36] and also in Erithacus rubecula (European robins) and Prunella modularis (dunnocks) from Hungary [37], the significance of these animals in the transmission and maintenance cycle of Rickettsia is still debatable [20]. The Rickettsia spp. infection rates in ticks, removed from the same animal, varied from 4.8% to 100%, most likely indicating that the ectoparasites might acquire these pathogens not only during blood meals on these animals, but also through previous infected by transstadial, transovarial or horizontal transmission [38]. However, as there were no animal samples tested for the presence of rickettsial DNA in the current study, there is no strict evidence of whether ticks of this study could have acquired the detected Rickettsia through feeding.
Surprisingly, 42.7% (44/103) of all Rickettsia-positive ticks were removed from rodents caught in Pärnumaa county. Although this region was not covered in the previous study on Rickettsia spp. in questing ticks in Estonia [12], a high rate (28%) of Rickettsia DNA was also detected in questing ticks in Pärnumaa (M. Vikentjeva, J. Geller, I. Golovljova, unpublished observations). Interestingly, this region has previously not shown such high infection rates with any TBP [39, 40, 41]. However, our longitudinal observations on ticks indicate that the local environment and climate of western coastal Estonia may provide favorable conditions for tick population maintenance and survival, as ticks have always been abundant in these areas (I. Golovljova, unpublished observations).