Immunologic responses to West Nile virus in vaccinated and clinically affected horses

Ann H. DavidsonDepartment of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.

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Josie L. Traub-DargatzDepartment of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.

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Racquel M. RodeheaverDepartment of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.

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Eileen N. OstlundCenter for Veterinary Biologics and National Veterinary Services Laboratories, Ames, IA 50010.

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Douglas D. PedersenCenter for Veterinary Biologics and National Veterinary Services Laboratories, Ames, IA 50010.

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Ron G. MoorheadAnimal Clinic and Pharmacy, Ogallala, NE, 69153.

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Joe B. StricklinDepartment of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.

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Renee D. DewellDepartment of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.

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Susan D. RoachDepartment of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.
Present address is the School of Veterinary Medicine, Ross University, Basseterre, St Kitts, West Indies.

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Rachel E. LongDepartment of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.

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Sara J. AlbersCenter for Veterinary Biologics and National Veterinary Services Laboratories, Ames, IA 50010.

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Robert J. CallanDepartment of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.

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M. D. SalmanDepartment of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.

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Abstract

Objective—To compare neutralizing antibody response between horses vaccinated against West Nile virus (WNV) and horses that survived naturally occurring infection.

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)

Abstract

Objective—To compare neutralizing antibody response between horses vaccinated against West Nile virus (WNV) and horses that survived naturally occurring infection.

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)