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  Discussion on The New WNV Vaccine seems to work with the old vaccine.
Author Message
Moderator:
DrO

Posted on Tuesday, Feb 22, 2005 - 9:31 am:

We have wondered if when switching from the old killed vaccine to the new live vaccine if you need to do the primary booster a month later. Here is a research project that suggests if your horse is current on the killed you may not have to do the primary booster:

Vet Ther. 2004 Winter;5(4):251-7.
The Anamnestic Serologic Response to Vaccination with a Canarypox Virus-Vectored Recombinant West Nile Virus (WNV) Vaccine in Horses Previously Vaccinated with an Inactivated WNV Vaccine.
Grosenbaugh DA, Backus CS, Karaca K, Minke JM, Nordgren RM.

Merial Limited, 115 Transtech Drive, Athens, GA, 30601, USA.

A new recombinant West Nile virus (WNV) vaccine has been licensed for use in horses. Prior to the availability of the recombinant vaccine in 2004, the only equine WNV vaccine available on the market had been an inactivated vaccine. Since the recombinant vaccine only expresses selected viral genes, the question could be posed as to whether a single dose of the recombinant vaccine would be effective in producing an anamnestic serologic response in horses previously vaccinated with an inactivated WNV vaccine. In this study we demonstrate that vaccination of horses with a canarypox-vectored recombinant vaccine, under field conditions, results in a marked anamnestic response in horses previously vaccinated with an inactivated WNV vaccine.

I am not positive this means you are adequately protected but it certainly suggests this. We are still recommending the killed vaccine based on similar efficacy and the easier handling of the killed product but continue to monitor the literature for new information.
DrO
Moderator:
DrO

Posted on Tuesday, Feb 22, 2005 - 10:21 am:

Here is another interesting piece of research that may change how you manage your vaccine program. Based on the fact that research has not shown a significant difference in protection rates at 6 and 12 months we recommend a once a year injection however this paper suggests in some cases every 6 months may be indicated:

J Am Vet Med Assoc. 2005 Jan 15;226(2):240-5.
Immunologic responses to West Nile virus in vaccinated and clinically affected horses.

Davidson AH, Traub-Dargatz JL, Rodeheaver RM, Ostlund EN, Pedersen DD, Moorhead RG, Stricklin JB, Dewell RD, Roach SD, Long RE, Albers SJ, Callan RJ, Salman MD.

Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523, USA.

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 > or = 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 > or = 1:100 and 14% had titers <1:10.> or = 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.
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