Emergence of Lyme Disease on Treeless Islands, Scotland, United Kingdom

Abstract

Lyme disease is usually associated with forested habitats but has recently emerged on treeless islands in the Western Isles of Scotland. The environmental and human components of Lyme disease risk in open habitats remain unknown. We quantified the environmental hazard and risk factors for human tick bite exposure among treeless islands with low and high Lyme disease incidence in the Western Isles. We found a higher prevalence of Borrelia burgdorferi sensu lato-infected ticks on high-incidence than on low-incidence islands (6.4% vs. 0.7%); we also found that residents of high-incidence islands reported increased tick bite exposure. Most tick bites (72.7%) occurred <1 km from the home, including many in home gardens. Residents of high Lyme disease incidence islands reported increasing problems with ticks; many suggested changing deer distribution as a potential driver. We highlight the benefits of an integrated approach in understanding the factors that contribute to Lyme disease emergence.

Keywords: Borrelia burgdorferi; Ixodes ricinus; Lyme disease; One Health; Scotland; United Kingdom; bacteria; bacterial zoonoses; deer; emergence; peridomestic; tick bite exposure; tick-borne infections; ticks; treeless habitats; vector-borne infections; zoonoses.

Figure 1

Tick collection sites for study on Lyme disease hazard, Western Isles, Scotland, UK, 2018–2019. Prevalence was not estimated at sites where <50 ticks were collected. Circle size indicates questing tick density. Circle color indicates Borrelia burgdorferi sensu lato prevalence. X indicates sites at which no ticks were detected.

Figure 2

Comparison of nymph density, infection prevalence, and density of infected nymphs by island, Western Isles, Scotland, UK, 2018–2019. A) Nymph density shown by 10 m² blanket drag. B) Prevalence of Borrelia burgdorferi sensu lato shown by site. C) Density of infected nymphs per 100 m² shown by site. Green indicates islands with low incidence of Lyme disease; brown indicates islands with high incidence. Data shown from grassland and moorland sites shown in Figure 1. Horizontal bars indicate means and SEs.

Figure 3

Comparison of nymph density, infection prevalence, and density of infected nymphs by habitat type in islands with high incidence of Lyme disease, Western Isles, Scotland, UK, 2018. A) Nymph density shown by 10 m² blanket drag. B) Prevalence of Borrelia burgdorferi sensu lato shown by site. C) Density of infected nymphs per 100 m² shown by site. Machair sites not shown because of low mean tick density (0.025 nymphs/10 m²; SE = 0.015). Horizontal bars indicate means and SEs.