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West Nile Season is Back!

Two sick horses seen at our clinic have been confirmed positive for West Nile Virus. This is an early start for the season and more cases have already been confirmed than in any other recent year. The virus is transmitted by mosquitoes from birds to horses and affect the nervous system of the animal. There is no specific treatment for the disease. Approximately one third of the horses developing clinical signs may die or have to be euthanized due to complications. Numerous surviving horses are left with residual effects, such as gait or behavioral abnormalities. It is not too late to vaccinate your horses!


For appropriate response to vaccine and therefore, optimum protection, horses must be vaccinated as follows:

Unvaccinated horses, or horses of unknown vaccination status, must receive a 2 dose series 4 to 6 weeks apart. This is essential for the horse's immune system to build a long lasting memory cell response. From then, the annual booster will act as a reminder to the immune system to keep the defenses up.

If the initial 2 dose series was not appropriately administered, long lasting immunity will not have been built. From this point, annual revaccination will not be enough to maintain proper protection.

When the vaccination history of a horse is questionable, it will do no harm to perform a two dose series 4-6 weeks apart. This will ensure proper immune protection for the disease. Annual boosters will then be adequate to maintain proper protection.

The above is true for most horse diseases we vaccinate for. Manufacturer instructions should be read for all vaccines and followed; unless advised otherwise by your veterinarian.

The optimal time of the year to vaccinate for West Nile in Western Canada is from the end of May and the month of June.

In a properly vaccinated horse, a late summer booster will provide extra protection but may not be necessary. In the face of this year's increased incidence of disease, the decision to booster a properly vaccinated horse depends on the risk tolerance of the owner as well as the health history of the horse.

Always consult your veterinarian for recommendations or if you have any questions or concerns.

It is not too late to vaccinate unvaccinated horses!

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