Increased Mycoplasma hyopneumoniae Disease Prevalence in Domestic Hybrids Among Free-Living Wild Boar Daniel J. Goedbloed, 1 Pim van Hooft, 2 Walburga Lutz, 3 Hendrik-Jan Megens, 4 Sip E. van Wieren, 2 Ron C. Ydenberg, 2 and Herbert H.T. Prins 2 1 Department of Evolutionary Biology, Zoological Institute, Technical University Braunschweig, Mendelssohnstrasse 4, 38106 Brunswick, Germany 2 Resource Ecology Group, Wageningen UR, Droevendaalsesteeg 3a, 6708PB Wageningen, The Netherlands 3 Wildlife Research Institute, Pu ¨tzchens Chaussee 228, 53229 Bonn, Germany 4 Animal Breeding and Genomics Centre, Wageningen UR, P.O. Box 338, 6700AH Wageningen, The Netherlands Abstract: Wildlife immune genes are subject to natural selection exerted by pathogens. In contrast, domestic immune genes are largely protected from pathogen selection by veterinary care. Introgression of domestic alleles into the wild could lead to increased disease susceptibility, but observations are scarce due to low introgression rates, low disease prevalence and reduced survival of domestic hybrids. Here we report the first observation of a deleterious effect of domestic introgression on disease prevalence in a free-living large mammal. A fraction of 462 randomly sampled free-living European wild boar (Sus scrofa) was genetically identified as recent wild boar–domestic pig hybrids based on 351 SNP data. Analysis of antibody prevalence against the bacterial pathogen Mycoplasma hyopneumoniae (Mhyo) showed an increased Mhyo prevalence in wild–domestic hybrids. We argue that the most likely mechanism explaining the observed association between domestic hybrid status and Mhyo antibody prevalence would be introgression of deleterious domestic alleles. We hypothesise that large-scale use of antibiotics in the swine breeding sector may have played a role in shaping the relatively deleterious properties of domestic swine immune genes and that domestic introgression may also lead to increased wildlife disease susceptibility in the case of other species. Keywords: hybrid, porcine enzootic pneumonia, Sus scrofa, wildlife disease, wildlife–domestic interface, introgression, deleterious alleles INTRODUCTION AND PURPOSE Wildlife diseases have received much attention in recent decades (Epstein 1995; Daszak 2000). Wildlife are consid- ered to be a reservoir for potentially zoonotic diseases and an important risk factor for biodiversity conservation, livestock health and public health (Bengis et al. 2002; Bar- David et al. 2006). Most epizootic outbreaks and emerging diseases are due to spill-over from domestic stocks to wildlife, rather than vice versa (Daszak 2000; Reiner et al. 2011). There is however much public concern for the risk of spill-back from wildlife reservoirs, because of economic interests (Lipowski 2003; Kruse et al. 2004; Gortazar et al. 2007). This study concerns the relatively simple case of a single wildlife host species and a single host-specific pa- thogen. We investigated European wild boar (Sus scrofa Correspondence to: Daniel J. Goedbloed, e-mail: d.goedbloed@tu-bs.de EcoHealth DOI: 10.1007/s10393-015-1062-z Original Contribution Ó 2015 International Association for Ecology and Health Author's personal copy