Abstract
Despite the presence of several microorganisms, other than Borrelia burgdorferi sensu lato (Bbsl) and TBE virus, in Ixodes ricinus ticks from the Nordic countries, data is lacking on their pathogenic potential in humans. In this study, we wanted to investigate the aetiology and clinical manifestations of tick-transmitted infections in individuals seeking medical care following a tick-bite.The sampling frame was participants of a large-scale, prospective, multi-centre, follow-up study of tick-bitten volunteers recruited in Sweden, Finland and Norway in the years 2007–2015. Participants who sought medical care during the three-month follow-up period and from whom blood samples were collected during this healthcare visit (n = 92) were tested, using PCR, for exposure to spotted fever group (SFG) Rickettsia spp., Anaplasma phagocytophilum and Babesia spp. Moreover, 86 of these individuals had two serum samples, collected three months apart, tested serologically for six tick-borne microorganisms. The selected organisms – Bbsl, SFG rickettsiae, Anaplasma phagocytophilum, TBE virus, Babesia microti and Bartonella henselae – have all been detected in field-collected ticks from the Nordic countries. Medical records were reviewed and questionnaires were completed to determine clinical manifestations.We found Lyme borreliosis to be the most common tick-transmitted infection as seen in 46 (54%) of the 86 participants with available medical records. Among the 86 participants with paired sera, serological or molecular evidence of recent exposure to other microorganisms than Bbsl could be demonstrated in eight (9%). Five participants (6%) exhibited serological evidence of recent concomitant exposure to more than one tick-borne microorganism. Clinical presentations were mild with one exception (TBE).In conclusion, our data suggest a low risk of infection with tick-borne microorganisms, other than Bbsl, in immunocompetent tick-bitten persons from the examined regions, a low occurrence of co-infection and mostly mild or no overt clinical signs of infection in immunocompetent persons exposed to the studied agents.
Highlights
The castor bean tick (Ixodes ricinus) is widespread in the Nordic countries and is the tick species most frequently implicated in transmitting infectious agents to humans in northern Europe
Of the volunteers participating in the TickBorne Diseases (TBD) STING study (n = 4230), 10% sought medical care during the study period, suggesting a generally low risk of developing symptoms prompting a healthcare visit following a tick-bite in the studied regions (Fig. 2)
Among the examined 86 participants with paired sera, most (n = 71, 83%) had no molecular or serological evidence of recent exposure to a tick-borne microorganism. Of those with serological or molecular signs of recent exposure (n = 15), 14 seroconverted against at least one of the examined microorganisms and two exhibited microbial deoxyribonucleic acid (DNA) (N. mikurensis; case 6 and 7, Table 3). Of those seeking medical care due to a tick-transmitted infection (TTI), we found a low proportion infected with other tick-borne microorganisms than Borrelia burgdorferi sensu lato (Bbsl) with only eight participants displaying serological or molecular evidence of recent exposure to other agents (Table 3)
Summary
The castor bean tick (Ixodes ricinus) is widespread in the Nordic countries and is the tick species most frequently implicated in transmitting infectious agents to humans in northern Europe. It can carry and transmit potentially pathogenic microorganisms such as Borrelia burgdorferi sensu lato (Bbsl), spotted fever group (SFG) Rickettsia spp., Anaplasma phagocytophilum, Neoehrlichia mikurensis, Babesia spp. as well as the European subtype of tick-borne encephalitis (TBE) virus (Andersson et al, 2013; Karlsson and Andersson, 2016; Wallménius et al, 2012). The risk of such an occurrence and its significance on the clinical presentation of the human host has received little attention in the Nordic countries
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