We describe the development and preliminary characterization of a recombinant canarypox virus vectored (
ALVAC) vaccine for protective immunization of equids against African horse sickness virus (AHSV)
infection. Horses (n=8) immunized with either of two concentrations of recombinant canarypox virus vector (ALVAC-AHSV) co-expressing synthetic genes encoding the outer
capsid proteins (VP2 and VP5) of AHSV serotype 4 (AHSV-4) developed variable titres (<10-80) of virus-specific
neutralizing antibodies and were completely resistant to challenge
infection with a virulent strain of AHSV-4. In contrast, a horse immunized with a commercial recombinant canarypox virus vectored
vaccine expressing the haemagglutinin genes of two equine
influenza H3N8 viruses was seronegative to AHSV and following
infection with virulent AHSV-4 developed
pyrexia,
thrombocytopenia and marked oedema of the supraorbital fossae typical of the "dikkop" or cardiac form of
African horse sickness. AHSV was detected by virus isolation and quantitative
reverse transcriptase polymerase chain reaction in the blood of the control horse from 8 days onwards after challenge
infection whereas AHSV was not detected at any time in the blood of the ALVAC-AHSV vaccinated horses. The control horse seroconverted to AHSV by 2 weeks after challenge
infection as determined by both virus neutralization and ELISA assays, whereas six of eight of the ALVAC-AHSV vaccinated horses did not seroconvert by either assay following challenge
infection with virulent AHSV-4. These data confirm that the ALVAC-AHSV
vaccine will be useful for the protective immunization of equids against
African horse sickness, and avoids many of the problems inherent to live-attenuated AHSV
vaccines.