The evolving story of Borrelia burgdorferi sensu lato transmission in Europe

Abstract

Beside mosquitoes, ticks are well-known vectors of different human pathogens. In the Northern Hemisphere, Lyme borreliosis (Eurasia, LB) or Lyme disease (North America, LD) is the most commonly occurring vector-borne infectious disease caused by bacteria of the genus Borrelia which are transmitted by hard ticks of the genus Ixodes. The reported incidence of LB in Europe is about 22.6 cases per 100,000 inhabitants annually with a broad range depending on the geographical area analyzed. However, the epidemiological data are largely incomplete, because LB is not notifiable in all European countries. Furthermore, not only differ reporting procedures between countries, there is also variation in case definitions and diagnostic procedures. Lyme borreliosis is caused by several species of the Borrelia (B.) burgdorferi sensu lato (s.l.) complex which are maintained in complex networks including ixodid ticks and different reservoir hosts. Vector and host influence each other and are affected by multiple factors including climate that have a major impact on their habitats and ecology. To classify factors that influence the risk of transmission of B. burgdorferi s.l. to their different vertebrate hosts as well as to humans, we briefly summarize the current knowledge about the pathogens including their astonishing ability to overcome various host immune responses, regarding the main vector in Europe Ixodes ricinus, and the disease caused by borreliae. The research shows, that a higher standardization of case definition, diagnostic procedures, and standardized, long-term surveillance systems across Europe is necessary to improve clinical and epidemiological data.

Keywords: Borrelia; Ixodes; Lyme borreliosis; Lyme disease; Spirochetes; Tick; Tick-borne diseases.

Figure 1

Distribution and changes in the prevalence of Ixodes ricinus, the main vector of the Borrelia burgdorferi s.l. complex among Europe. The map is a compilation of reported findings of I. ricinus in 2017 (rosé and light blue) in comparison to 2021 (others color codes) reported to the ECDC (European Centre for disease Prevention and Control). Please note that the map depicts historical and actual findings condensed on NUTS-3 level, the European socio-economic, or regional administrative level. However, no distinction is made between individual findings or stable populations. In addition, areas with “no data” should not interpreted as whether the species does or does not exist. Original maps can be accessed online: https://www.ecdc.europa.eu/en/disease-vectors/surveillance-and-disease-data/mosquito-maps. Map was created with ArcGIS 10.8.

Figure 2

Geographic distribution and vector associations of Borrelia burgdorferi s.l. adapted after Margos et al. (2012) with new included Ixodes frontalis as vector for B. turdi in Europe. Abbreviations: I., Ixodes; B. Borrelia; ^afor B. bisettiae, the vector for Europe is uncertain; ^bB. burgdorferi, B. valaisiana, and B. lusitaniae are transmitted by I. ricinus in Eastern Europe but their prevalence is low.

Figure 3

The life cycle of Ixodes ricinus. In general, the number of ticks in each stage is about one order of magnitude smaller than in the preceding stage (Randolph 1998). Approximately 200 larvae hatch from 2000 eggs, which molt after successful blood meals into 20 nymphs and then after further blood meals into 2 adults (female and male). Additionally, the Borrelia burgdorferi s.l. cycle between I. ricinus as the main vector and (reservoir) hosts are given.