Evaluation of a live-attenuated foot-and-mouth disease virus as a vaccine candidate.

A variant of foot-and-mouth disease virus (FMDV) lacking the leader (L) coding region (A12-LLV2) was previously constructed and shown to be less virulent in cattle than its wild-type parent (A12-IC). In this study, cattle were tested for their clinical and immunological responses to subcutaneous inoculation with A12-LLV2 or A12-IC or to intramuscular vaccination with chemically inactivated A12-IC. Five weeks postinoculation animals were challenged by intradermal inoculation in the tongue with a virulent cattle-passaged virus. A12-LLV2-inoculated animals showed no clinical signs of disease and developed a neutralizing antibody response by 4 days postinoculation, whereas a companion control bovine did not seroconvert. After challenge, two of three inoculated animals did not develop lesions, but showed mild signs of infection. The third inoculated animal developed some lesions, but these were less severe than in the uninoculated control animal, which showed classical FMD. All animals inoculated with A12-IC developed a fever, two showed typical FMD lesions, and the companion control seroconverted, indicating that it had acquired infection by contact. The A12-IC-inoculated animals and the control were protected from challenge. Animals vaccinated with inactivated virus showed no clinical signs of disease and developed a neutralizing antibody response, and the control did not seroconvert. Upon challenge none of the vaccinated animals developed lesions, one developed a fever, and the control developed FMD. These experiments demonstrate the potential of a rationally designed live-attenuated FMDV vaccine.