A 15-year monitoring of Rickettsiales (Anaplasma phagocytophilum and Rickettsia spp.) in questing ticks in the city of Hanover, Germany

Rickettsiales (Anaplasma phagocytophilum and Rickettsia spp.) are regarded as potentially emerging tick-borne pathogens and may change in abundance in response to global climate change. However, continuous monitoring on their prevalence in questing ticks is only available for the northern German city of Hanover. In the presented follow-up of this long-term study, 2100 questing ticks of the Ixodes ricinus/Ixodes inopinatus-complex collected from April to October 2020 at ten different recreation sites in Hanover were individually analysed for Rickettsia and A. phagocytophilum infection by quantitative real-time PCR. Together with previous results from years 2005, 2010 and 2015, the current study allows to assess potential changes in tick infection rates with Rickettsiales over a 15-year monitoring period. In 2020, 3.0% (63/2100) of ticks were infected with A. phagocytophilum, 36.0% (756/2100) with Rickettsia spp. and 1.2% (26/2100) with both pathogens. Regarding the different developmental tick stages, nymphs showed a significantly lower A. phagocytophilum prevalence of 0.5% (5/1050) than adult ticks (5.5% [58/1050]) as well as compared to females (5.4% [38/700]) and males (5.7% [20/350]). For Rickettsia spp., nymphs also showed a lower prevalence of 33.2% (349/1050) with a significant difference to adult ticks (38.8% [407/1050]) and female ticks (40.7% [285/700]), while males had a Rickettsia infection rate of 34.9% (122/350). Comparison with previous years indicated a stable A. phagocytophilum prevalence over the 15-year monitoring period. In contrast, fluctuating Rickettsia prevalences were observed, with a peak in 2015 in all developmental stages, but similar infection rates in 2005 and 2020. Therefore, epidemiological changes in response to climate change are not (yet) evident. Nevertheless, the long-term monitoring study will be continued in the future, as climatic impacts on tick and reservoir host populations may have a delayed effect on pathogen prevalence and, consequently, transmission to humans and domestic animals.