Can a horse receive too many vaccinations?

If a horse’s history is unknown, it is best to follow protocols designed for unvaccinated animals.

Q: A few months ago, my husband and I took in a rescue horse, and we are slowly doing what we can to restore his health though careful re-feeding, farriery work and veterinary care. However there’s one area of his health that has me worried: We can’t find any information on which vaccinations this horse may or may not have received in his life. How do you handle vaccinations for horses with unknown histories? Which factors, such as a horse’s age, are relevant to this question? My gelding is about 9 years old and does not seem to have health problems beyond being malnourished and having been neglected Should we act as if he hasn’t had any vaccinations and “start from scratch?” Is there any harm in that approach? Or would it be better to have his blood analyzed to see if he has immunity from previous vaccinations and fill in any gaps?

A: In general, there is no harm, other than financial, in proceeding as if a horse had never been vaccinated previously. Indeed, when a horse’s vaccination history is unknown it is best to consider it naïve and use protocols designed for unvaccinated horses. Giving additional doses of most vaccines would neither enhance nor inhibit the horse’s ability to develop immunological memory, which is the goal of vaccination.

A veterinarian giving a horse a vaccination.
In general, there is no harm in treating a horse with an unknown vaccination history as having had none at all.

The one notable exception is the strangles vaccine. In rare cases, horses who have previously contracted strangles—or have been repeatedly vaccinated against the disease—may develop a potentially serious complication known as purpura hemorrhagica.

Click here to determine if the vaccines you gave last year and still appropriate this year. 

Characterized by swelling of the blood vessels, usually of the head, legs and abdomen, purpura hemorrhagica results from an abnormal immune reaction. Although the risk of this condition is small, it is best to draw a titer to determine whether a horse carries antibodies to the causal organism (Streptococcus equi) before scheduling a strangles vaccination.

David W. Horohov, PhD
Director, Maxwell H. Gluck
Equine Research CenterUniversity of Kentucky
Lexington, Kentucky

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