Objective—To compare neutralizing antibody response
between horses vaccinated against West Nile virus
(WNV) and horses that survived naturally occurring
Design—Cross-sectional observational study.
Animals—187 horses vaccinated with a killed WNV vaccine
and 37 horses with confirmed clinical WNV infection.
Procedure—Serum was collected from vaccinated
horses prior to and 4 to 6 weeks after completion of
an initial vaccination series (2 doses) and 5 to 7
months later. Serum was collected from affected
horses 4 to 6 weeks after laboratory diagnosis of
infection and 5 to 7 months after the first sample was
obtained. The IgM capture ELISA, plaque reduction
neutralization test (PRNT), and microtiter virus neutralization
test were used.
Results—All affected horses had PRNT titers ≥ 1:100 at
4 to 6 weeks after onset of disease, and 90% (18/20)
maintained this titer for 5 to 7 months. After the second
vaccination, 67% of vaccinated horses had PRNT titers
≥ 1:100 and 14% had titers < 1:10. Five to 7 months
later, 33% (28/84) of vaccinated horses had PRNT titers
≥ 1:100, whereas 29% (24/84) had titers < 1:10.
Vaccinated and clinically affected horses' end point
titers had decreased by 5 to 7 months after vaccination.
Conclusions and Clinical Relevance—A portion of
horses vaccinated against WNV may respond poorly.
Vaccination every 6 months may be indicated in certain
horses and in areas of high vector activity. Other
preventative methods such as mosquito control are
warranted to prevent WNV infection in horses. (J Am
Vet Med Assoc 2005;226:240–245)