One Dose of WNV Recombinant Vaccine Could Help Control Outbreaks

A recent study at Colorado State University (CSU) found that your horse might have the ability to fight off West Nile virus (WNV) less than a month after receiving a single-dose vaccination against the disease. The challenge study tested Merial's RECOMBITEK equine WNV vaccine 26 days after immunization and found that the treated horses resisted infection.

The results are particularly important for horse owners and veterinarians in areas prone to sudden natural WNV outbreaks, where early immunity could mean the difference between life and death for horses.

"If you put yourself in the place of someone in California, where West Nile suddenly appears in an area, you'd like to have a vaccine with rapid immunity--you don't want to wait two months," said study co-author Dick Bowen, DVM, PhD, associate professor in the department of biomedical sciences at CSU. "The study basically showed that the vaccine, in a single dose, elicits good immunity early on."

The study, published in the November 2004 issue of the American Journal of Veterinary Research, involved challenging nine immunized horses (which had received one dose of RECOMBITEK) with virulent WNV-infected mosquitoes. A control group of 10 horses, also challenged with the WNV-infected mosquitoes, did not receive the vaccine. None of the horses in the study had experienced prior exposure to WNV.

Study results showed that eight of the nine inoculated horses were protected from infection (viremia) within 26 days of vaccination, while eight of the 10 control horses developed viremia (clinical signs of disease were observed in one control horse which transiently had a fever). By Day 33 of the study, an antibody response was seen in the vaccinated horses.

Quick immunization can help protect horses previously unvaccinated for WNV, horses with unknown vaccination histories, and young foals during unexpected WNV outbreaks, said Stephanie Thompson, DVM, manager of veterinary technical services at Merial. "It's certainly seasonal, but we never really know when West Nile is going to start," she said. "So we're able to provide a very fast onset of protection."

Merial credits the early immune response in the test subjects to the recombinant technology used to create the company's vaccine. Recombinant vaccines are created by inserting genetic material from the antigen into a host virus. In the Merial vaccine, canarypox virus--which is harmless to horses--carries the WNV genetic material into the inoculated animal.

"We believe that the effective viral clearance and the rapid onset of immunity against WNV provided by a single administration of the live, recombinant canarypox virus-WNV vaccine in the (immunized) horses reflects strong specific priming of their immune systems," wrote the authors in the study. "The protective immunologic responses to WNV infection in horses have not been elucidated, but cell-mediated immunity may be fundamental to effective viral clearance."

For maximum protection, it is important to follow label instructions and give the horse two initial priming doses.

About the Author

Michelle N. Anderson, Digital Managing Editor

Michelle Anderson serves as The Horse's digital managing editor. In her role, she produces content for our web site and hosts our live events, including Ask the Vet Live. A lifelong horse owner, Anderson competes in dressage and enjoys trail riding. She's a Washington State University graduate (Go Cougs!) and holds a bachelor's degree in communications with a minor in business administration and extensive coursework in animal sciences. She has worked in equine publishing since 1998. She currently lives with her husband on a small horse property in Central Oregon.

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