West Nile Virus vaccination:
questions & answers


Q: What month is my horse most at risk?

ANSWER:

  • In general, the highest risk of infection coincides with the months during which mosquito activity is highest, typically July, August, September, and October in most states, including California. However, cases have occurred in all months of the year in areas with year–round mosquito activity.

Q: Can I do my own vaccination for WNV?

ANSWER:

  • It is recommended that you consult with your veterinarian to formulate a vaccination schedule best suited for your horse based on challenge and climate. Protection only results when the vaccine has been stored and handled properly.

Q: What does proper storage and handling of the vaccine consist of?

ANSWER:

  • The vaccine should be stored at 35–45 degrees F. Direct sunlight can cause vaccine inactivation. Vaccines should be used according to the manufacturer’s instructions and outdated product should be discarded.

Q: What are the side effects of WNV vaccine?

ANSWER:

  • Side effects that may occur include transient local reactions at the injection site.

Q: Should I vaccinate my horse that was previously infected or recovered?

ANSWER:

  • You should consult your veterinarian. In general, vaccination should be resumed no later than one year after infection.

Q: Will vaccination protect my horse 100%?

ANSWER:

  • Vaccines have been shown to reduce the risk of infection with West Nile Virus, although clinical disease may not be fully prevented. In challenge models, both vaccines have been proven to significantly reduce the magnitude of viremia (amount of virus circulating in the blood) in experimentally infected vaccinated horses when compared to non–vaccinated control horses for up to 12 months after primary vaccination with two doses of vaccine. Both vaccines have met USDA requirements for safety testing.

Q: Is my horse protected after one vaccination from the initial series?

ANSWER:

  • Directions for both vaccines include administration of 2 doses of vaccine 3 to 6 weeks apart (consult the specific label). Optimal protection cannot be expected until 2 weeks after administration of the second dose.

Q: Do I need to revaccinate my horse?

ANSWER:

  • Re–vaccination of previously vaccinated horses is recommended by the vaccine manufacturers on an annual basis or more frequently when local conditions are conducive to a prolonged period of exposure to infected mosquitoes.

Q: When should I revaccinate my horse?

ANSWER:

  • Annual revaccination is best completed in the Spring, prior to the onset of peak insect vector season.

Q: Should I vaccinate my pregnant mare?

ANSWER:

  • Neither of the licensed vaccines carry label recommendations for administration to pregnant mares. It is well recognized that pregnant mares are at risk of acquiring infection from infected mosquitoes. Therefore, it has become an accepted practice by many veterinarians to administer vaccines to pregnant mares based on the assumption that the risk of adverse consequences of WNV infection exceeds any vaccine associated risk. Booster vaccination of pregnant mares 4 to 6 weeks before foaling may provide foals with passive colostral protection that lasts up to 4 months.

Q: Should I vaccinate my foal from a vaccinated mare?

ANSWER:

  • You should consult your veterinarian because data on this subject is incomplete at this time. It is recommended that primary vaccination of foals from vaccinated mares be started with the first dose of vaccine at 3 to 4 months of age, followed by a second dose approximately 1 month later, and a third dose, 6 to 8 weeks after the second dose. This third dose increases the likelihood that foals with high levels of maternal antibodies, will be protected. A booster should be administered during the Spring of the yearling year, after which the recommendations for vaccination of adult horses should be followed.

Q: Should I vaccinate my foal from a non–vaccinated mare?

ANSWER:

  • You should consult your veterinarian. The vaccination schedule may include the first dose as early as one month.

For more information on WNV Vaccination:

Return to West Nile Virus Home Page

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