1. Department of Equine SciencesFaculty of Veterinary Medicine, Utrecht University Utrecht The Netherlands
2. Department of Infectious Diseases and ImmunityFaculty of Veterinary Medicine, Utrecht University Utrecht The Netherlands
3. Department of Medicine and EpidemiologySchool of Veterinary Medicine, University of California Davis California
4. Department of Pathology, Microbiology & ImmunologySchool of Veterinary Medicine, University of California Davis California
5. Department of Population Health and ReproductionSchool of Veterinary Medicine, University of California Davis California
6. Centre for Infectious Disease Control (CIb)National Institute for Public Health and the Environment (RIVM) Bilthoven The Netherlands