Abstract

Morbillivirus has now been identified as a significant cause of disease outbreaks and mortality in marine mammal species around the world. Epizootics of these viruses have been responsible for mass mortality events in marine mammals belonging to the orders Pinnipedia (seals) and Cetacea (whales, porpoises, and dolphins), and have occurred in widely dispersed locations. The most notable of these events has taken place in northwestern Europe in 1988 and 2002. The U.S. Navy Marine Mammal Program (San Diego, CA), in collaboration with industry and academic partners, has developed a DNA vaccine targeting dolphin morbillivirus (DMV). This vaccine consists of the fusion (F) and hemagglutinin (H) genes of DMV sub-cloned into the expression plasmid, pVR (Vical, Inc., San Diego, CA). Vaccine constructs (pVR-DMV-F and pVR-DMV-H) were evaluated for immunogenicity initially in vitro and then in mice. Injection protocols were designed and adapted for application in adult Atlantic bottlenose dolphins (Tursiops truncatus) to maximize and balance vaccine efficacy with clinical utility. 6 dolphins were inoculated intramuscularly with plasmid DNA in saline, 4 dolphins received both pDMV-F and pDMV-H, and 2 dolphins received a mock vaccine, which consisted of the vector alone. All animals received an initial inoculation at week 0, followed by two booster injections at week 8 and week 14. While this study is ongoing, preliminary data shows a vaccine-specific humoral response in 3 of the 4 vaccinated dolphins. Cellular assays and cytokine analysis are pending. Immunogenicity will be determined by measurement of vaccine-specific antibody levels, cell-mediated immunity, and cytokine analysis. Vaccine efficacy will be evaluated by comparison of immune responses between vaccinated and mock-vaccinated animals over time (magnitude and kinetics). The potential for this vaccine formulation to provide protective immunity will be assessed by conducting antigen-boost injections at 3-month intervals in immunized animals, to characterize improvements in immuno-kinetics, and then by a separate live-challenge study to be conducted in a non-marine mammal model. To our knowledge, this is the first report of pathogen-specific immunogenicity to a DNA vaccine in any dolphin species.