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Bayesian modeling of factors potentially influencing the distribution of Echinococcus multilocularis in foxes Staubach, C. 1, Schwarz, S. 1, Sutor, A. 1, Hoffmann, L. 2, Tackmann, K. 1, Schmid, V. 3, Conraths, F.J. 1 1 Friedrich-Loeffler-Institut, Federal Research Institute for Animal Health, Seestraße 55, Wusterhausen, Germany 2 Thuringian State Authority for Food Safety and Consumer Protection, Tennstedter Str. 8/9, Bad Langensalza, Germany 3 Department of Statistics, Ludwig-Maximilians-University, Ludwigstr. 33, München, Germany

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Alveolar echinococcosis in humans is caused by the larval stage of Echinococcus multilocularis Obligate 2-host parasitic cycle is predominantly sylvatic definitive host: red fox (Vulpes vulpes) in Central Europe intermediate host: different rodent species Alveolar echinococcosis is considered the most dangerous autochthonous parasitic zoonosis in central Europe Alveolar echincoccosis

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Period prevalence of E. multilocularis a) Observed prevalences b) Beta-binomial model

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Digitised positions of shot foxes

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Cummulative percentage [+/-] p = Landscape proportions [%]

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Landscape proportions surrounding each shot fox an relative differences between E.m.-positive and –negative foxes (n = 3,521) Estimation based on buffer zones of 2.5 km radius Differences relative to CLC data [%] vs. p = E.m.-positive E.m.-negative Random positions vs. p = Landscape proportions (%)

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Modeling of factors potentially influencing the distribution Hypothesis –The oncosphere infectivity can be effectively reduced by elevated temperature and dessication –Microclimate and habitat can be suspected as potential risk factors Data –landscape composition per spatial unit was derived from a high-resolution land- survey vector database and supplemented by a digital elevation model –42,861 foxes were sampled in two Federal States of Germany Using Hierarchical Bayesian spatial models on municipality level to estimate the parameters of the potential influencing environmental factors Staubach et al n = 3,521

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E.m. Data Brandenburg 14,422 foxes 2000 – Location (municipality) 1,819 foxes positive for E. multilocularis - Intestinal scraping technique

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E.m. Data Thuringia 24,224 foxes 1990 – Location (municipality) 6,302 foxes positive for E. multilocularis - Intestinal scraping technique

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- landscape composition and drainage rivers per municipality derived from a high resolution survey database (map scale 1: 5,000) - digital elevation model Environmental Data Brandenburg

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Environmental Data Thuringia - landscape composition and drainage rivers per municipality derived from a high resolution survey database (map scale 1: 5,000) - digital elevation model

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Environmental Data Thuringia - landscape composition and drainage rivers per municipality derived from a high resolution survey database (map scale 1: 5,000) - digital elevation model

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1.Each individual i has an unknown probability π i that the animal is positive or negative depending on area the animal lives and one or more covariates describing the environmental conditions 2.Parameter π i is modeled using a logistic model with random intercept and structured effect per municipality 3.The structured effect displays spatial heterogeneity using a Gaussian Markov Random field (GMRF) 4.We choose uninformative prior distributions [τ ~ Ga(0.001, 001) ] for the unknown variance parameters ~ Full Bayesian Model

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Implementation of the Bayesian Model The joint distribution of the full Bayesian model is analytically intractable. Therefore, we used a Markov Chain Monte Carlo algorithm (MCMC) to estimate the parameters of the model - Burn-in:1,000 iterations - Sample: every 50 th iteration of the next 50,000 iterations - The freely available software BayesX was used for inference ( ) - And we used INLA (Integrated Nested Laplace Approximation) a new approach for Bayesian inference available as R package ( ) - very fast - accurate approximation to the marginal posterior density for the hyperparameters

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ModelDevianceDICpDEstimate (95 % BCI)Mean/St.Dev. Spatial (1.239; 2.382) Urban (-6.252; )3.805 Crop (1.164; 2.611)5.066 Pasture (0.970; 4.168)3.190 Forest (-2.431; )4.739 Drainage (8.494; )2.050 Elevation (-0.005; 0.009)0.665 Crop + Drainage (1.035; 2.511) (-99.81; ) Forest + Drainage (-2.471; ) ( ; ) Model comparison and parameter estimates regarding E.m. in foxes in Brandenburg DIC = Deviance Information Criterion; pD = Effective number of parameters; BIC = Bayesian Credible Interval

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ModelDevianceDICpDEstimate (95 % BCI)Mean/St.Dev. Spatial (0.602; 0.957) Urban (-0.104; 0.850)0.210 Crop (0.057; 0.715)2.243 Pasture (1.139; 2.484)5.315 Forest (-0.985; )3.872 Drainage ( ; )0.581 Elevation ( ; )2.639 Pasture + Elevation (1.066; 2.397) ( ; ) Forest + Elevation (-0.939; ) ( ; ) Model comparison and parameter estimates regarding E.m. in foxes in Thuringia DIC = Deviance Information Criterion; pD = Effective number of parameters; BIC = Bayesian Credible Interval

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BayesXINLA ModelDICpDDICpD Spatial Urban Crop Pasture Forest Drainage Elevation Crop + Drainage Forest + Drainage Comparison of GoF parameters using BayesX and INLA (E.m. in foxes in Brandenburg)

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BayesXINLA ModelDICpDDICpD Spatial Urban Crop Pasture Forest Drainage Elevation Pasture + Elevation Forest + Elevation Comparison of GoF parameters using BayesX and INLA (E.m. in foxes in Thuringia)

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Summary A hierarchical Bayesian model was established to analyse potential factors influencing the distribution of E. multilocularis infections in foxes using data on municipality level The study confirmed results of a previous publication, which utilized exact locations and micro-habitat data of foxes in a much smaller region and using lower resolution data The statistical inference using INLA reduces the calculation time dramatically (MCMC approx hours to below 1 minute using INLA) Furthermore, INLA provides easily accessible Goodness of fit measures (e.g. local.DIC, CPO, Marginal Predictive Likelihood) not here demonstrated Nevertheless, an easy method for the selection of variables and the best model choice is still missing

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