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. 2016 Nov 15;11(11):e0166653.
doi: 10.1371/journal.pone.0166653. eCollection 2016.

Prevalence of Rickettsia Spp. In Ticks and Serological and Clinical Outcomes in Tick-Bitten Individuals in Sweden and on the Åland Islands

Free PMC article

Prevalence of Rickettsia Spp. In Ticks and Serological and Clinical Outcomes in Tick-Bitten Individuals in Sweden and on the Åland Islands

Anders Lindblom et al. PLoS One. .
Free PMC article


Tick-transmitted diseases are an emerging health problem, and the hard tick Ixodes ricinus is the main vector for Borrelia spp., tick-borne encephalitis virus and most of the spotted fever Rickettsiae in Europe. The aim of the present study was to examine the incidence of rickettsial infection in the southernmost and south central parts of Sweden and the Åland Islands in Finland, the risk of infection in humans and its correlation with a bite of a Rickettsia-infected tick, the self-reported symptoms of rickettsial disease, and the prevalence of co-infection between Rickettsia spp. and Borrelia spp. Persons with a recent tick bite were enrolled through public media and asked to answer a questionnaire, provide a blood sample and bring detached ticks at enlistment and at follow-up three months later. Blood samples were previously analysed for Borrelia spp. antibodies and, for this report, analysed for antibodies to Rickettsia spp. by immunofluorescence and in 16 cases also using Western Blot. Ninety-six (44.0%) of the 218 participants were seropositive for IgG antibodies to Rickettsia spp. Forty (18.3%) of the seropositive participants had increased titres at the follow-up, indicating recent/current infection, while four (1.8%) had titres indicating probable recent/current infection (≥1:256). Of 472 ticks, 39 (8.3%) were Rickettsia sp. positive. Five (31.3%) of 16 participants bitten by a Rickettsia-infected tick seroconverted. Experience of the self-reported symptoms nausea (p = 0.006) and radiating pain (p = 0.041) was more common among those with recent, current or probable infection compared to those who did not seroconvert. Participants who showed seroreactivity or seroconversion to Rickettsia spp. had more symptoms than those who were seronegative. Seven (3.2%) participants showed seroconversion to Borrelia spp., and three (1.4%) of these showed seroconversion to both Rickettsia spp. and Borrelia spp., in accordance with previous studies in Sweden. Symptoms of rickettsial disease were in most of the cases vague and general that were difficult to differentiate from other tick-borne diseases.

Conflict of interest statement

The authors have declared that no competing interests exist.


Fig 1
Fig 1. Map showing the areas where participants were recruited.
V = Västra Götaland; Ö = Östergötland; J = Jönköping County; K = Kalmar County; Å = Åland Islands.
Fig 2
Fig 2. Western Blot analysis of IgG antibodies against R. helvetica whole cell antigen.
Lane A-P demonstrates the lipopolysaccaride ladders and specific reactions against R. helvetica proteins in the 110-150-kDa region for serum 2 for patients (Lane) V16(A), V43(B), V46 (C)(Area V); S71(D), S72(E), S75(F), V6(G) (Area Ö); K7(H), K9(I), K14(J), K46(K) K56(L) (Area K); Å13(M), Å16(N), Å23(O), Å35(P) (Area Å) in titres 1:200. Lane P(h) demonstrates specific proteins and the lipopolysaccharide (LPS) ladders reacting with a human antiserum from a patient diagnosed with rickettsial infection and N(h) a healthy negative blood donor. Mw = molecular weight marker. “Fig 2” is compiled of four figure panels representing the groups of lanes that originated from different gels/blots (Gel A-D). The short vertical lines of “Fig 2” divide the individual non-adjacent lanes in the gels. The original analyses are presented in S1–S4 Figs with Gels A-D as Supporting Information.

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Grant support

The study was financially supported by grants from the Swedish Research Council Branch of Medicine (grant no. K2008-58X-14631-06-3)(PF); the EU Interreg IV A project ScandTick (grant no. 167226)(PEL); the County Council of Östergötland (grant no. LIO-56191)(PF); Stiftelsen Olle Engqvist Byggmästare (11877)(KN); Uppsala-Örebro-Regional Research 324 Council (25021)(KN); the Center for Clinical Research Dalarna (9028)(KN). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.