***Horse Report coverage of this story***

It’s extremely rare for a horse to contract rabies. Of the nine million horses in the United States, only 25 of them were reported to have contracted the disease in 2014. Perhaps that success rate is due to the strict annual rabies vaccination recommended by the American Association of Equine Practitioners (AAEP).

Most other animals recommended to receive rabies vaccinations get them every two to three years, so is it really necessary for horses to be vaccinated every year? Thanks to a donation to the Center for Equine Health (CEH), UC Davis researchers are taking a hard look at that question.

How does a horse contract rabies? In California and the Midwest, infection typically occurs when being bit by a rabid skunk. In the East, it’s from raccoons, and in parts of Texas and Arizona, it’s mostly from foxes. Rabies is zoonotic, meaning the disease can pass from animals to humans. Therefore, protecting horses from rabies also protects the humans who interact with them. While this research is ultimately aimed at finding the best vaccination guidelines for horses, there is a human health benefit as well.

To be fully protected, AAEP recommends that vaccine-induced rabies virus neutralizing antibodies (RVNA) titer should be 0.5 IU/mL. UC Davis studied titer levels in 48 horses – 28 younger than 20-years-old, and 20 older or equal to 20 years, since age may be a determinant. Serum samples were collected at eight time points (pre-vaccination, approximately three weeks later, and then every six months for three years).

The study found that most of the horses held the recommended 0.5 IU/mL level for up to three years. The lowest percentage of horses in either group above the threshold, at any point during the three years, was 82% (the younger horses at 12 months). The highest rate (excluding the 100% of younger horses at three weeks) was 91% of the older horses at 30 months.

“There is an increasing body of evidence that protective antibody titers persist for three years for many pathogens included in vaccines,” said Dr. David Wilson, professor emeritus and former director of the UC Davis veterinary hospital and its Large Animal Clinic.

The researchers’ results were published as “Duration of serum antibody response to rabies vaccination in horses,” in the Journal of the American Veterinary Medical Association and presented at the 62nd Annual AAEP Convention in 2016.

While this study is by no means definitive, and there remains minimal published data on the correlation between RVNA levels and protection of horses against rabies, Wilson feels that arguments could be made for decreasing the time between vaccinations. He also studied a separate group of 11 CEH mares and found that all of them maintained recommended titer levels for more than nine years.

“An administration of a second dose about one year later creates a robust and persistent response in almost all horses,” said Wilson. “Prolonged persistence of antibody above the 0.5 IU/mL protective level suggests that a vaccination of primed horses every three years is probably sufficient.”

Horse owners are encouraged to consult with their veterinarians about the titer levels and best vaccination recommendations of their specific animals.

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