In this study, ticks of the generalist species I. ricinus and I. persulcatus, as well as nidicolous I. trianguliceps, and attached to small mammals were analyzed for the presence of vector-borne Rickettsia spp. including a new species, previously unreported in Europe.
Rickettsial DNA was detected in 8,7% of all investigated attached ticks, and in 10,0% of I. ricinus; rates which are significantly higher than those previously found in questing ticks in Estonia -overall prevalence 5,1%, and 6,7% for I. ricinus, respectively. (P value = 0,0002 for overall and P = 0,0068 for I. ricinus positivity and prevalence rates, respectively) [11]. This may be due to differences in study design and sampling: the rodent-attached ticks analyzed in this study were dominantly larvae, while previous studies were performed in questing ticks and those results reflected the positivity in unfed nymphs and adults, which might have already had 1 - 2 blood-meals, undergone diapause and 1 - 2 moltings, thus resulting in a possible dilution of Rickettsia. As I. ricinus larvae often quest in groups, which might originate from the same hatch, a single animal could harbor a group of larvae having already acquired Rickettsia transovarially, prior their first blood-meal [19]. This might also explain why there is no difference in infection rates between rodent-attached larvae and nymphs, as shown in this study.
Up to date, I. ricinus generalist ticks represent the main vector and the natural reservoir host of tick-borne SFG Rickettsia in Europe [5]. In this study, a higher positivity rate of rickettsial DNA was found in I. ricinus ticks compared to I. trianguliceps (10,0% vs 3,4%, respectively). High levels of rickettsial DNA detection rates in rodent-attached I. ricinus were also recently reported from Lithuania [20] where 22,6% of individually tested larvae (maximum likelihood estimation, MLE = 26,5%) were positive for Rickettsia spp.
There have been reports on the detection of several tick-borne pathogens (TBP), such as Anaplasma phagocytophilum [21], Candidatus Neoehrlichia mikurensis and Babesia microti [22], Francisella tularensis [23] in nidicolous rodent-specialists I. trianguliceps ticks. As reported by Igolkina et al. [17], SFG Rickettsia was found in 41,2% (14/34) of analyzed I. trianguliceps ticks 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 relatively small number of I. persulcatus covered in the current study. However, several Rickettsia species were previously reported in unfed questing I. persulcatus ticks in Estonia [11] although at significantly lower positivity rates than in I. ricinus.
We found rickettsial DNA in ticks removed mainly from M. glareolus, A. flavicollis, but also from several S. araneus. Although there are reports on the detection of R. helvetica in various wild small- to large-sized animal samples from Lithuania [24], the Netherlands and Germany [12, 25, 26] and also in Erithacus rubecula (European robins) and Prunella modularis (dunnocks) from Hungary [27], the significance of these animals in the transmission and maintenance cycle of Rickettsia is still debatable [28]. Although animal samples were not analyzed in this study, 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 previously infected by transstadial, transovarial or horizontal transmission [29]. Similar data on varied Rickettsia spp. infection rates in ticks removed from the same animal have been reported from Lithuania [20].
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 [11], our unpublished data also showed the high rate of rickettsial DNA detection in questing ticks in Pärnumaa (28%). Interestingly, this region has previously not shown such high infection rates with any TBP [30, 31, 32], and our longitudinal observations on ticks indicate that the local environment and climate of western coastal Estonia may provide favorable conditions for tick population abundance and survival.
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 [9], with unexplained fever following a tick bite in France and Italy [10] and with rash, febrile illness and meningitis in Sweden [7, 8]. Rickettsia helvetica is also the most prevalent tick-borne Rickettsiae species as elsewhere in Europe and Asia [5, 33] as well as in the Estonian tick population, comprising over 95% of all Rickettsia species detected in questing [11] as well as in rodent-attached ticks as our current study results show. While there are no clinical reports to date of illness caused by R. helvetica in Estonia, the detection of this TBP at positivity rates compatible to those for Borellia burgdorferi s. l. [14], R. helvetica should be considered in the surveillance of tick-borne diseases in Lyme endemic regions.
To our knowledge, this study is the first report on the detection of a newly described species, Ca. R. uralica, in Europe. In our study this genospecies was detected only in I. trianguliceps ticks, and only in those removed from voles, which is in correspondence with the initial Ca. R. uralica report from Siberia, which designates the host specificity of Ca. R. uralica to I. trianguliceps [17]. Siberian Ca. R. uralica has also been identified in I. trianguliceps removed from voles, but rickettsial DNA was not found in the mammals. 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 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.