Lyme borreliosis is transmitted among species exclusively by bites of hard-shelled Ixodes species ticks. Endemic regions have been slowly expanding, but infections remain most prevalent in specific geographic regions with temperate, moist climates. Most cases occur in the northeast and north central USA, Scandinavia and central Europe, particularly in exurban and suburban areas, where humans, reservoir hosts and larger animals (deer, bears, sheep—from which the local Ixodes sp. ticks derive their colloquial names) commingle. Four closely related Bbsl genospecies are responsible for most Lyme borreliosis. B. burgdorferi sensu stricto (Bbss) causes virtually all Lyme borreliosis in North America. In Europe B. garinii is the most likely to cause LNB, with additional cases attributable to B. afzelii, Bbss and B. bavariensis.
Infection occurs in a small percentage of individuals bitten by Ixodes ticks, even where a high percentage of ticks carry Bbsl. Longer duration tick attachment increases risk of infection. Human data from the USA indicate that at least 48 h attachment is required. Studies of the ticks and spirochetes prevalent in Europe suggest transmission may occur—both in experimental animals[10,11] and in humans[8,12]—somewhat more quickly, but infection risk similarly increases progressively the longer the attachment (>24 h).
It has long been believed that nervous system involvement is both qualitatively different and more frequent in European than US Lyme borreliosis. Expert reviews typically state that about 12% of European Lyme borreliosis patients develop LNB, with individual studies ranging from 3% in central Germany to 7% with 'moderate/severe neuro' involvement in Slovenia to 16% in southern Sweden.[16–18] Among European patients with definite LNB, radiculoneuropathy occurs in more than half, cranial neuropathy in 40%. In the USA, where Lyme borreliosis is reportable, 12.5% are diagnosed with LNB—with radiculoneuropathy in just under one-third, facial palsy in two-thirds, meningitis in ~10%. In light of this qualitative and quantitative convergence of European and US LNB, an excellent European review concluded that 'the clinical picture of Lyme neuroborreliosis in North America and Europe seems to be more similar than is often assumed'.
Brain. 2022;145(8):2635-2647. © 2022 Oxford University Press