A method and device for determining whether an animal is infected with West Nile Virus (WNV), or is either not infected or vaccinated with a WNV vaccine. The method includes contacting a biological sample from the subject with a first WNV polypeptide that is not an element of the WNV vaccine, and detecting whether antibodies in the sample bind to the WNV polypeptide. If the antibodies in the sample bind to the WNV polypeptide, it can be determined that the animal is naturally infected with WNV and if the antibodies do not bind to the polypeptide, it can be determined that the animal is either vaccinated or not infected.