The application discloses therapeutic vaccines based upon the “pING” DNA plasmid vector expressing the gene encoding the rat Her2 protein. Vaccines according to the instant disclosure are used as an adjunct treatment for surgery, radiation and/or chemotherapy for dogs and cats with cancers that over express the Her2 antigen, and prolong the post-surgical disease free interval and/or survival time. Also included are therapeutically effective methods of immunization using said vaccines.