Carlströmer Berthen et al.
The AxBioTick study: Borrelia species and tick-borne encephalitis virus in ticks, and clinical responses in tick-bitten individuals on the Aland Islands, Finland
Microorganisms. 2023;11(5):1100. doi:10.3390/microorganisms11051100

The Finnish Aland Islands is an archipelago in the Baltic Sea and is hyperendemic for TBE and Lyme borreliosis (LB). Prior to a free-of-charge vaccination program which started in 2006 for all inhabitants of the Aland Islands, the prevalence was around 6%. The vaccination program led to a TBE seroprevalence of 88% in adults in 2018. However, the incidence for TBE is still substantial.

A prospective, observational study was initiated in August 2018 in tick-bitten individuals to identify clinical responses in humans bitten by ticks and to determine the prevalence of Borrelia spp. and TBE virus in ticks.

In total, 425 ticks (all were Ixodes ricinus nymphs) have been collected from 100 volunteers. 19.7% of the ticks contained Borrelia bacteria, of which B. afzelii was the most common species (19.1%). None of the ticks contained TBE virus.

86% of the participants reported complete primary TBE vaccination. At inclusion of the study, 2% of the participants were TBE IgM positive and 60.4% were TBE IgG positive. Four participants seroconverted to TBE IgM and IgG of whom two had been boosted with vaccine. One individual with increased TBE antibody titers had been vaccinated prior to the study, while the other individual had not received any TBE vaccination; this person showed clinical symptoms such as chills, fatigue, fever, headache during the first eight weeks. The headache and fatigue persisted after four months.

14% of the bitten individuals seroconverted or had increased IgG antibody titers to Borrelia and 5% developed erythema migrans.

The ongoing study is continuing to investigate more participants and ticks for co-infections and to characterize the dermal immune response following a tick bite.

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