Surveillance of tick-borne pathogens in ticks collected from Swiss residents

Tick-borne diseases (TBDs) represent an increasing public health threat globally. Climate change has facilitated tick range expansion and extended active seasons, contributing to rising TBD incidence rates. In Switzerland, TBDs represent a major health concern. This study aims to characterise patterns in spatio-temporal distribution of tick-borne pathogens (TBPs) in ticks removed from humans across Switzerland, examine associations between tick developmental stages and TBP infection prevalence, and analyse co-infection patterns amongst different TBPs.

We employed the "Tick Prevention" citizen science app to collect spatial and temporal data on tick bite incidents and to obtain tick specimens for pathogen screening throughout Switzerland during 2018-2020. Specimens underwent DNA extraction for TBP detection. Quantitative PCR targeted different TBPs at genus and species levels. Data analysis examined TBP infection prevalence in submitted ticks across geographic regions, seasons, and tick developmental stages, including co-infection patterns in ticks.

Of 1056 tick specimens, 352 (33.3%) tested positive for at least one TBP, with Borrelia spp. (16.3%) and Rickettsia spp. (12.69%) showing higher infection prevalence than other TBPs, including Neoehrlichia mikurensis (5%), Anaplasma phagocytophilum (1.8%), Chlamydiales (1.8%), and Babesia spp. (1.7%). Co-infections occurred in 59 specimens (5.6%), predominantly dual infections (5.2%), with Borrelia spp. and N. mikurensis representing the most common co-infection pattern. Among 58 larvae, 898 nymphs, and 96 adult ticks examined, tick infection prevalence increased with developmental stage, rising from larvae (18.9%) to nymphs (32.6%) to adults (48.9%), consistent with pathogen acquisition through successive blood meals during tick development. Spatially, TBPs were detected across 70 of 76 Swiss administrative regions, with most TBPs displaying uniform distribution. Temporally, tick-human encounters peaked during May-June (59.7% of announced events), with TBP detection rates remaining steady (28-37%) across the tick-active months from April to September. One-third of examined ticks harboured at least one TBP, with weighted models indicating infection prevalence in submitted ticks could reach 45% in certain Swiss Plateau regions.

These findings emphasise the importance of continued tick and TBP surveillance programmes to inform public health interventions and prevention.