West Nile Virus Vaccine Information

The following information is written with information from Rocky Bigbie, DVM, a representative of Fort Dodge Animal Health. Fort Dodge produces the West Nile virus vaccine.

Q: Why should we vaccinate now if adequate protection will be in force after mosquito season?

A: Complete protection is not in place until two weeks after the booster, which can be given three weeks after the initial vaccinations. Beginning vaccination now would protect horses by mid-October. Even if we have frosts before mid-October, which would kill adult mosquitoes, the eggs would not be killed. When the weather warmed up the adults produced from those eggs would be infected (there is transgenerational passage of the virus). There have been cases in North Carolina and New York through November in years past.

Q: Do you have any concern about vaccinating pregnant mares?

A: The vaccine is labeled for use in healthy horses. Studies sufficient to prove safety in pregnant mares have not been done, however five pregnant mares were included in the safety trial. None of those mares had any problems associated with the vaccine. Because the vaccine is made the same way as Fort Dodge’s Eastern and Western encephalomyelitis vaccines and because the same adjuvant (a substance enhancing the immune response to an antigen) (Metastim) is used, there is no reason to think the vaccine will not be safe for pregnant mares. In other regions, many pregnant mares have already been vaccinated in the field, and no adverse reactions have been reported.

Q: How old do foals need to be before vaccinating?

A: Technically, foals are susceptible to West Nile virus from birth. It would seem reasonable to start vaccination at three months in keeping with recommendations on other vaccines. This year, because we did not vaccinate before foaling and because we did not have the disease before foaling, there should be no passive transfer of antibodies in the colostrum. Next year, if mares have been vaccinated pre-foaling, it would seem reasonable to wait until maternal protection has waned before vaccinating foals.

Q: If titers are run post-vaccination, is it possible to differentiate between a vaccination titer and a disease exposure titer?

A: Not if titers are run by conventional means. However, it is known that after vaccination, IgM goes up for a short time (about two weeks) and then declines. IgG would remain high. After disease exposure, IgM will remain high. Thus an IgM capture Elisa test could differentiate between a disease titer and a vaccination titer.

Q: If two vaccinations are given this year, is only one vaccination due in subsequent years?

A: In regions in which mosquitoes are present year round, vaccination is recommended every six months. In Kentucky, one vaccination in the spring should be adequate (assuming primary and booster vaccinations were given the previous year).

About the Author

Tom Riddle, DVM

Tom Riddle, DVM is a founding partner of Rood and Riddle Equine Hospital in Lexington, Ky.

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