Rickettsia spp. in finnish ixodid ticks

Rickettsia spp. are intracellular, arthropod-borne bacteria pathogenic to humans and animals. The clinical manifestations of rickettsial infections range from mild to severe, but diagnosis is sometimes missed owing to mild symptoms or empirical antibiotic treatment for suspected tick-borne diseases. This study aimed to determine the prevalence of Rickettsia spp. in ticks across Finland, explore possible regional variations, identify the infecting Rickettsia species, and assess rickettsial exposure of certain human patients.

We analysed 5101 ticks from 20 locations, collected by flagging, crowdsourcing, or removal from pet cats and dogs between 2003 and 2021 in three study sets. Tick species were determined by quantitative PCR (qPCR), Sanger sequencing, or morphology. Rickettsial prevalences were determined by qPCR, with species confirmed by Sanger sequencing. Indirect immunofluorescence assay was used for detecting Rickettsial antibodies in human samples.

Ixodes ricinus predominated in southern Finland, while Ixodes persulcatus was found in central Finland and along the western coast up to southern Lapland. Occasional I. ricinus-persulcatus hybrids were observed in central southern Finland where both species co-occurred. Rickettsia DNA was more frequently detected in ticks collected from regions dominated by I. ricinus compared to those dominated by I. persulcatus. However, variations in sampling and processing may limit the comparability of these results. Ticks collected from dogs and cats contained Rickettsia spp. more frequently than those collected by flagging. Rickettsia helvetica was the primary species identified, with sporadic Candidatus Rickettsia tarasevichiae found exclusively in I. persulcatus. Among 226 sera from patients suspected of tick-borne encephalitis, 5.8% showed IgG reactivity against Rickettsial antigens, but titres were below the 1:512 threshold for confirmed infection.

This study highlights the need to better understand environmental or host-linked factors influencing Rickettsial prevalence and emphasizes the importance of monitoring in areas prone to tick expansion due to climate change. The findings underscore the potential for Rickettsial diseases, necessitating enhanced diagnostic frameworks to address disease burden and improve surveillance.