Borrelia valaisiana - a candidate human pathogen? Insights from the tick-borne diseases STING study

Borrelia (B.) valaisiana is a tick-borne spirochete within the Borrelia burgdorferi sensu lato complex. In Sweden, it occurs in Ixodes ricinus ticks, but its role as a human pathogen remains uncertain. In the Tick-Borne Diseases STING study conducted during 2007-2015, adult participants from Sweden and the Åland Islands, Finland, provided ticks, blood samples for serological analyses, and questionnaires at inclusion and at a 3-month follow-up. Medical records were reviewed for those seeking care. Ticks were characterised, feeding duration assessed, and pathogens detected by real-time PCR. During 2008-2009, B. valaisiana was identified by PCR in 1.6% (34/2,154) of the ticks. Since its significance as a human pathogen is unclear, we chose to examine this tick-bitten cohort in greater detail. Accordingly, the objective of the study was to assess whether participants bitten by B. valaisiana-positive ticks developed Borrelia-specific antibodies and/or symptoms suggestive of infection.

Participants bitten by B. valaisiana-positive ticks (VPOS, n = 33; one bitten twice) were compared with an age- and sex-matched group bitten by Borrelia-negative ticks (VNEG, n = 67). Borrelia-specific IgG was analysed in paired blood samples.

Median age in VPOS was 67 years; 58% were women. Seroconversion for Borrelia-specific IgG occurred significantly more often in VPOS than VNEG (p = 0.03). Common symptoms in VPOS included neck pain, myalgia/arthralgia, numbness, and headache, but none sought medical care. No significant difference in symptom occurrence was observed between groups.

Tick bites involving B. valaisiana were associated with higher Borrelia antibody seroconversion, without significant symptom associations or need for medical intervention. The results of this study provide new insights into the potential human impact of B. valaisiana.