Abstract
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.
Highlights
There are two major genetic lineages of West Nile virus (WNV): lineage 1, which is found in North America, North Africa, Europe and Australia, and lineage 2, which is endemic in central and southern Africa (Burt et al 2002)
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
Blood samples were taken from all horses on days 0, 28, 35, 42, 49, 91, 141, 182, 231, 274, 322, 364 and 413 and tested for the presence of serum neutralising antibodies against lineage 1 WNV strain NY385/99, which had been isolated from a dead bird in the USA in 1999 (Venter et al 2005), and lineage 2 WNV strain SPU93/01, which had been isolated from a human patient with encephalitis in South Africa in 2001 (Venter et al 2009)
Summary
There are two major genetic lineages of West Nile virus (WNV): lineage 1, which is found in North America, North Africa, Europe and Australia, and lineage 2, which is endemic in central and southern Africa (Burt et al 2002). WNV was not thought to be a significant pathogen of horses in South Africa as few clinical cases were reported (Guthrie et al 2003; Lanciotti et al 2002). Lineage 2 WNV infections have increasingly been associated with encephalitis in horses in South Africa (Venter et al 2009). An 18-month pilot study of neurological disease in horses identified WNV infection in 19% of cases; all cases presented with severe neurological disease, of which 85% had to be euthanased or died with signs of encephalitis (Venter et al 2009). All cases with a positive reverse transcriptase polymerase chain reaction were shown to be infected with lineage 2 WNV by DNA sequencing and phylogenetic analysis (Venter et al 2009); clinical signs resembled those of lineage 1 WNV infections in the USA, such as ataxia, weakness, recumbency, muscle fasciculation and high case fatality rates (Dauphin et al 2004; Durand, Simon & Tolou 2004; Schuler et al 2004; Venter et al 2009; Ward et al 2004)
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