Estimating Ixodes ricinus densities on the landscape scale.
BACKGROUND
The study describes the estimation of the spatial distribution of questing nymphal tick densities by investigating Ixodes ricinus in Southwest Germany as an example. The production of high-resolution maps of questing tick densities is an important key to quantify the risk of tick-borne diseases. Previous I. ricinus maps were based on quantitative as well as semi-quantitative categorisations of the tick density observed at study sites with different vegetation types or indices, all compiled on local scales. Here, a quantitative approach on the landscape scale is introduced.


METHODS
During 2 years, 2013 and 2014, host-seeking ticks were collected each month at 25 sampling sites by flagging an area of 100 square meters. All tick stages were identified to species level to select nymphal ticks of I. ricinus, which were used to develop and calibrate Poisson regression models. The environmental variables height above sea level, temperature, relative humidity, saturation deficit and land cover classification were used as explanatory variables.


RESULTS
The number of flagged nymphal tick densities range from zero (mountain site) to more than 1,000 nymphs/100 m(2). Calibrating the Poisson regression models with these nymphal densities results in an explained variance of 72 % and a prediction error of 110 nymphs/100 m(2) in 2013. Generally, nymphal densities (maximum 374 nymphs/100 m(2)), explained variance (46 %) and prediction error (61 nymphs/100 m(2)) were lower in 2014. The models were used to compile high-resolution maps with 0.5 km(2) grid size for the study region of the German federal state Baden-Württemberg. The accuracy of the mapped tick densities was investigated by leave-one-out cross-validation resulting in root-mean-square-errors of 227 nymphs/100 m(2) for 2013 and 104 nymphs/100 m(2) for 2014.


CONCLUSIONS
The methodology introduced here may be applied to further tick species or extended to other study regions. Finally, the study is a first step towards the spatial estimation of tick-borne diseases in Central Europe.
Institute of Geography and Geoecology, Karlsruhe Institute of Technology, Reinhard-Baumeister-Platz 1, 76131, Karlsruhe, Germany. denise.boehnke@kit.edu.|Institute for Veterinary Public Health, University of Veterinary Medicine Vienna, Veterinärplatz 1, 1210, Vienna, Austria. denise.boehnke@kit.edu.
Denise
D
Boehnke
Institute for Veterinary Public Health, University of Veterinary Medicine Vienna, Veterinärplatz 1, 1210, Vienna, Austria. katharina.brugger@vetmeduni.ac.at.
Katharina
K
Brugger
Department of Ecology and Parasitology, Institute of Zoology, Karlsruhe Institute of Technology, Kornblumen Strasse 13, 76131, Karlsruhe, Germany. miripfaeffle@web.de.
Miriam
M
Pfäffle
Baden-Württemberg State Health Office, District Government Stuttgart, Nordbahnhof Strasse 135, 70191, Stuttgart, Germany. patrick_sebastian@gmx.net.
Patrick
P
Sebastian
Institute of Geography and Geoecology, Karlsruhe Institute of Technology, Reinhard-Baumeister-Platz 1, 76131, Karlsruhe, Germany. stefan.norra@kit.edu.|Institute of Applied Geosciences, Karlsruhe Institute of Technology, Adenauerring 20, 76131, Karlsruhe, Germany. stefan.norra@kit.edu.|Institute of Mineralogy, Technische Universität Bergakademie Freiberg, Brennhausgasse 14, 09596, Freiberg, Germany. stefan.norra@kit.edu.
Stefan
S
Norra
Department of Ecology and Parasitology, Institute of Zoology, Karlsruhe Institute of Technology, Kornblumen Strasse 13, 76131, Karlsruhe, Germany. trevor.petney@kit.edu.
Trevor
T
Petney
Baden-Württemberg State Health Office, District Government Stuttgart, Nordbahnhof Strasse 135, 70191, Stuttgart, Germany. rainer.oehme@rps.bwl.de.
Rainer
R
Oehme
Department of Ecology and Parasitology, Institute of Zoology, Karlsruhe Institute of Technology, Kornblumen Strasse 13, 76131, Karlsruhe, Germany. nina.littwin@kit.edu.
Nina
N
Littwin
Institute for Veterinary Public Health, University of Veterinary Medicine Vienna, Veterinärplatz 1, 1210, Vienna, Austria. karin.lebl@vetmeduni.ac.at.
Karin
K
Lebl
Institute for Veterinary Public Health, University of Veterinary Medicine Vienna, Veterinärplatz 1, 1210, Vienna, Austria. johannes.raith@vetmeduni.ac.at.
Johannes
J
Raith
Institute for Veterinary Public Health, University of Veterinary Medicine Vienna, Veterinärplatz 1, 1210, Vienna, Austria. melanie.walter@vetmeduni.ac.at.
Melanie
M
Walter
Institute of Geography and Geoecology, Karlsruhe Institute of Technology, Reinhard-Baumeister-Platz 1, 76131, Karlsruhe, Germany. reiner.gebhardt@kit.edu.
Reiner
R
Gebhardt
Institute for Veterinary Public Health, University of Veterinary Medicine Vienna, Veterinärplatz 1, 1210, Vienna, Austria. franz.rubel@vetmeduni.ac.at.
Franz
F
Rubel
D000818:Animals; D004777:Environment; D005858:Germany / Q000453:epidemiology; D007303:Insect Vectors; D018884:Ixodes / Q000254:growth & development*; D008193:Lyme Disease; D016012:Poisson Distribution; D011156:Population Density; D013984:Tick Infestations / Q000453:epidemiology*