Treatment of Metastatic Equine Melanoma with a Plasmid DNA Vaccine Encoding Streptococcus Pyogenes EMM55 Protein

A 19-year-old castrated male Arab/Quarter horse presented with an extensive history of cutaneous metastatic melanoma. Over a period of 8 months, a total of 8 doses of plasmid DNA vaccine expressing the Streptococcus pyogenes emm55 gene (pAc/emm55) were administered intratumorally at 300 μg/dose via a needless injector. Upon completion of the vaccination protocol, the size of the injected lesions, on average, were reduced by 40.3% from the initial size measurements. Lesions that were not injected were reduced by 47.6%. The overall reduction in total tumor burden was 42.3%. Tumor regression was also associated with the augmentation of antimelanoma IgG antibody response, thus implying that an induction of an effective antimelanoma response would be of great advantage in the management of equine melanoma.