Lineage 2 West Nile virus (WNV) strains are endemic in South Africa and cause severe neurological disease in horses. An inactivated lineage 1 vaccine, Duvaxyn WNV, protects mice against challenge with a neuroinvasive South African lineage 2 strain of WNV. To evaluate the potential of Duvaxyn WNV to protect horses against lineage 2 strains of WNV, serum neutralising antibody responses of horses against lineage 1 WNV strain NY385/99 and lineage 2 WNV strain SPU93/01, isolated from a human with meningo-encephalitis in South Africa, were compared following vaccination with two doses of Duvaxyn WNV, 28 days apart, and a third dose one year later. Twenty-two seronegative horses were randomly assigned to two treatment groups: 16 to a vaccinated group and six retained as unvaccinated controls. Blood samples were taken from all horses on study days 0, 28, 35, 42, 49, 91, 141, 182,
231, 274, 322, 364 and 413. Primovaccination with Duvaxyn WNV resulted in high titres of serum neutralising antibodies against both strains. Following a single dose of Duvaxyn WNV on day 399, one year after primovaccination, there was a strong anamnestic response with a
log25-fold rise in the titres of neutralising antibodies against strains NY385/99 and SPU93/01.
These results provide further evidence that Duvaxyn WNV is likely to protect horses against infection with lineage 2 strains of WNV and that a single annual booster may be sufficient to maintain immunity against lineage 2 WNV infection in horses.