Molecular and serological evidence of spotted fever group rickettsiae and Borrelia co-infection in Ixodes ricinus and humans in north-eastern Poland

Tick-borne spotted fever group (SFG) rickettsioses are considered emerging vector-borne infectious diseases with potentially severe consequences for public health, particularly in the temperate regions of Europe. The aim of the study was to determine the prevalence and the diversity of Rickettsia spp. in questing Ixodes ricinus ticks in north-eastern Poland, evaluate the serological evidence of exposure to SFG rickettsiae among adult residents of this region, as well as to determine the level of SFG rickettsiae co-infection with Borrelia spirochaetes in ticks and humans. In a total of 4090 ticks, the presence of Rickettsia spp. was detected by PCR and confirmed by sequencing. IgG antibodies against SFG Rickettsia and Borrelia were detected using ELISA. Rickettsial DNA was revealed in 11.3% of females, 11.8% of males and nymphs of 6.5% (MIR, minimum infection rate). Based on DNA sequencing, R. helvetica and R. monacensis were identified. IgG antibodies against SFG rickettsiae were confirmed in 38.7% of adult residents of north-eastern Poland. The co-infection rate of SFG rickettsiae with Borrelia spirochaetes in questing adult I. ricinus ticks was low (2.6%), compared to the co-infection rate (16.1%) observed based on the presence of immunological responses in the study group of residents from northeastern Poland. These findings provide important evidence filling the gap between the environmental detection of SFG rickettsiae in vector populations and the limited number of diagnosed clinical cases in humans.