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Lyme borreliosis is the most common tick-borne disease reported across Europe and North America. Intrinsically linked to the distribution of key tick vector species, it is important to better understand ecological factors that may result in changes in tick distribution and the potential impact this may have on tick-borne disease risk.
The main objective of this thesis was to better determine the spatial distribution of questing Ixodes ricinus across urban landscapes and to investigate site level and landscape level factors that have a significant influence on Lyme borreliosis risk. A multidisciplinary approach, using literature, field, and laboratory-based research, as well as geospatial datasets, was used to investigate and highlight the potential risk posed by urban Lyme borreliosis transmission cycles.
Four studies were completed, the first (Chapter 3) being a review of questing I. ricinus data published from studies in urban areas across Europe over the last 32 years. Three other studies were conducted, centred around questing tick presence, nymph density, Borrelia prevalence and the density of infected nymphs (Lyme borreliosis risk indicators). One investigated the effect of habitat in and around the city of Bath in 2015 and 2016 (Chapter 4). Another investigated local and wider habitat and connectivity factors in Bath, Bristol, and Southampton in 2017 (Chapter 5). The final study incorporated data along an urban-rural gradient in the cities of Bath and Southampton in 2018 (Chapter 6).
Key findings from this thesis were evidence of the presence of I. ricinus infected with Borrelia in a wide range of urban green spaces across Europe (Chapter 3), confirming the potential for contact and possible subsequent transmission of Lyme borreliosis to humans. Tick presence and nymph density were significantly higher in woodland compared to other urban green space habitats (Chapters 4 and 5), and better connectivity to woodland had a similar significant effect (Chapter 5). A seasonal pattern in questing I. ricinus activity was observed in urban green space, with density being significantly higher in spring and summer (Chapter 5). Woodlands across an urban-rural gradient supported similar nymph densities and Borrelia prevalence (Chapter 6), suggesting risk to public health could be similar, or perhaps elevated in urban woodland where potential for human-tick contact could be higher. This could be worsened by the dominance of Borrelia garinii detected in questing I. ricinus in urban green space (Chapters 4, 5 and 6), as this genospecies is linked to neuroborreliosis. Overall, the density of infected nymphs, a key Lyme borreliosis risk indicator, was low (Chapters 4, 5 and 6) compared to other studies in southern England, and indeed the average across Europe (Chapter 3). Key questions remain, however, around the risk of urban woodlands for Lyme borreliosis transmission due to high numbers of visitors.
This thesis presents Lyme borreliosis risk indicators for urban green space, synthesising European and country-level estimates from existing studies and generating new data for southern England. This contributes to the growing evidence of potential risk posed by Borrelia infected ticks in urban areas, which requires further consideration by scientists, policy makers and public health professionals alike. This is particularly crucial as urbanisation and urban greening accelerate, bringing members of the public into proximity with infected ticks, potentially increasing the risk of Lyme borreliosis transmission. |
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