Borrelia burgdorferi sensu lato is a complex of genetically related, spiral-shaped bacteria (spirochetes). These bacteria are the pathogens behind Lyme borreliosis (Lyme disease), the most common tick-borne infection across the Northern Hemisphere [1]. While North American Lyme disease is almost exclusively driven by a single species (Borrelia burgdorferi sensu stricto), Europe presents a much more diverse and complex picture [1, 2].

The European Genospecies Complex

In Europe, the Borrelia burgdorferi s.l. complex includes more than a dozen distinct genospecies, at least five of which routinely cause disease in humans [1]. This multi-species diversity is highly clinically significant. Different strains possess distinct tissue tropism (an affinity for specific organs or tissues), which leads to a wider variety of symptoms in European patients than what is typically seen in North America:

• Borrelia afzelii: The most prevalent species in Europe. It primarily targets the skin, often leading to a localised rash (erythema migrans) or to chronic conditions such as acrodermatitis chronica atrophicans [1, 2].
• Borrelia garinii: Notably neurotropic, making it the primary driver of Lyme neuroborreliosis (neurological complications affecting the brain, spine, and nerves) in Europe [1].
• Borrelia burgdorferi sensu stricto: Present in Europe but less dominant than its sister strains in the USA; it is most commonly linked to Lyme arthritis and joint inflammation [1].
• Borrelia spielmanii & Borrelia bavariensis: Well-documented human pathogens causing distinct clinical cases across the European continent [1, 2].

Transmission Dynamics & Rising Prevalence

These bacteria survive in nature by cycling between wildlife reservoir hosts—such as small rodents and birds—and their primary vector, the castor bean tick (Ixodes ricinus) [2, 3].

A comprehensive European meta-analysis established that, on average, roughly 12% to 14% of host-seeking I. ricinus ticks carry Borrelia spirochetes, with the highest infection rates concentrated in Central Europe [3]. However, localized risk can be drastically higher, even within heavily populated urban environments.

A prominent multi-year surveillance study conducted in popular Viennese recreational areas demonstrated just how high these urban risks can get: researchers detected pathogenic Borrelia burgdorferi s.l. DNA in 28.6% of the ticks collected in 2019, and 21.3% of those collected in 2020 [4]. Driven by shifting climate patterns and land-use changes, both tick ranges and Borrelia prevalence are steadily expanding into higher altitudes and more northern latitudes across Europe [2, 3].

References

1. Stanek, G., Wormser, G. P., Gray, J., & Strle, F. (2012). Lyme borreliosis. The Lancet, 379(9814), 461-473. https://doi.org/10.1016/S0140-6736(11)60103-7
2. Mannelli, A., Bertolotti, L., Gern, L., & Gray, J. (2012). Ecology of Borrelia burgdorferi sensu lato in Europe: transmission dynamics in multi-host systems, influence of molecular processes and effects of climate change. FEMS Microbiology Reviews, 36(4), 837-861. https://doi.org/10.1111/j.1574-6976.2011.00312.x
3. Strnad, M., Hönig, V., Růžek, D., Grubhoffer, L., & Rego, R. O. M. (2017). Europe-Wide Meta-Analysis of Borrelia burgdorferi Sensu Lato Prevalence in Questing Ixodes ricinus Ticks. Applied and Environmental Microbiology, 83(15), e00609-17. https://doi.org/10.1128/AEM.00609-17
4. Schötta, A. M., Stelzer, T., Stanek, G., Stockinger, H., & Wijnveld, M. (2022). Bacteria and protozoa with pathogenic potential in Ixodes ricinus ticks in Viennese recreational areas. Wiener klinische Wochenschrift, 135(7-8), 177-184. https://doi.org/10.1007/s00508-022-02046-7