Emergence of tick-borne diseases at northern latitudes in Europe: a comparative approach

Abstract

The factors that drive the emergence of vector-borne diseases are difficult to identify due to the complexity of the pathogen-vector-host triad. We used a novel comparative approach to analyse four long-term datasets (1995–2015) on the incidence of tick-borne diseases in humans and livestock (Lyme disease, anaplasmosis and babesiosis) over a geographic area that covered the whole of Norway. This approach allowed us to separate general (shared vector) and specific (pathogen reservoir host) limiting factors of tick-borne diseases, as well as the role of exposure (shared and non-shared pathogens in different hosts). We found broadly similar patterns of emergence across the four tick-borne diseases. Following initial increases during the first decade of the time series, the numbers of cases peaked at slightly different years and then stabilized or declined in the most recent years. Contrasting spatial patterns of disease incidence were consistent with exposure to ticks being an important factor influencing disease incidence in livestock. Uncertainty regarding the reservoir host(s) of the pathogens causing anaplasmosis and babesiosis prevented a firm conclusion regarding the role of the reservoir host-pathogen distribution. Our study shows that the emergence of tick-borne diseases at northern latitudes is linked to the shared tick vector and that variation in host-pathogen distribution and exposure causes considerable variation in emergence.

Knock‐on community impacts of a novel vector: spillover of emerging DWV‐B from Varroa‐infested honeybees to wild bumblebees

“…The risks associated with disease emergence due to novel transmission routes and the spread of disease vectors have been documented (Kilpatrick & Randolph ; Carpenter et al ; Mysterud et al ), but we lack empirical studies to understand how novel transmission routes impact on multi‐host pathogen epidemiology. The existence of multi‐host RNA viruses across pollinator species and the recent, rapid spread of a specialist viral vector, the honeybee mite Varroa destructor, provide a unique system to empirically study the impact of a novel vector on multi‐host pathogens.…”

Lyme neuroborreliosis and bird populations in northern Europe

“…All data in Norway included 4685 cases of Lyme borreliosis, of which 3254 (69%) was Lyme neuroborreliosis. As in previous analyses [26,27], we limited the data to cases for which the municipality of the tick bite was confirmed (Lyme borreliosis: n ¼ 2844, neuroborreliosis: n ¼ 1940). There may be a time lag between tick bite and disseminated infection [12], and hence, the time of registration in the disease statistics may differ from the time of the tick bite.…”

“…We used similar spatial covariates as used in previous analyses on the Lyme borreliosis incidence [26,27]. The deer population density index is based on harvest numbers of roe deer (Capreolus capreolus), red deer (Cervus elaphus) and moose (Alces alces) divided by the size of the deer habitat at the scale of municipality.…”

“…R. Soc. B 286: 20190759 model [26,27] included the region (west/east/south), the NAO winter index, the spatial deer population density, proportion of human settlement, proportion of agricultural areas, proportion of habitat greater than 200 m.a.s.l. and distance to the fjord.…”

“…Ixodid ticks are expanding in geographical distribution in elevation and towards the north in both Europe [23] and North America [24]. Lyme borreliosis is currently emerging in northern Europe [25][26][27] as well as in Canada [28], and understanding the causes is urgent. In Norway, Lyme borreliosis has a status as a 'notifiable disease', and data across the whole country for the period 1995-2017 is available [26].…”