Viruses 2019, 11, 1109; doi:10.3390/v11121109 www.mdpi.com/journal/viruses Article Development and Validation of a S1 Protein-Based ELISA for the Specific Detection of Antibodies against Equine Coronavirus Shan Zhao 1, Constance Smits 2, Nancy Schuurman 1, Samantha Barnum 3, Nicola Pusterla 3, Frank van Kuppeveld 1, Berend-Jan Bosch 1, Kees van Maanen 2,*,† and Herman Egberink 1,*,† 1 Virology Division, Department of Infectious Diseases & Immunology, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 1, 3584CL Utrecht, The Netherlands;