Immune responses and protective efficacy in ponies immunised with an equine influenza ISCOM vaccine containing an 'American lineage' H3N8 virus

Protective responses generated by vaccination with an immuno-stimulating complex (ISCOM)-based vaccine for equine influenza (EQUIP F), containing a new 'American lineage' H3N8 virus, were studied. Seven ponies in the vaccine group received two intramuscular injections of EQUIP F given 6 weeks apart. Aerosol challenge with an A/eq/Newmarket/1/93 reference strain 4 weeks after booster vaccination resulted in clinical signs of infection and viral shedding in 7 influenza-naive control animals whereas the vaccinated ponies were significantly protected from both clinical signs and virus excretion. Influenza virus-specific IgG responses in serum following immunisation with the ISCOM vaccine were predominantly of the IgGa and IgGb sub-isotypes, a pattern similar to that generated by equine influenza virus infection. However, in contrast to the response following infection, virus-specific antibody responses in nasal washes following immunisation were characterised by the presence of IgG but not IgA.These results demonstrated that an ISCOM-based vaccine containing A/eq/Kentucky/98 provides strong protective immunity against challenge with an 'American lineage' H3N8 reference virus.