This invention relates to Equine Herpes Viruses (EHV) wherein the protein gM is essentially absent or modified and non-functional with respect to its immunomodulatory capacity. Further aspects of the invention relate to nucleic acids coding said viruses, pharmaceutical compositions comprising these viruses or nucleic acids and uses thereof. The invention also relates to methods for improving the immune response induced by an EHV vaccine against wild type EHV infections, methods for the prophylaxis and treatment of EHV infections and methods for distinguishing wild type EHV infected animals from animals treated with EHV's according to the invention.