Study: Not All WNV Vaccines Render Same Immune Response

When it comes to getting shots, a single needle prick might seem like a better idea compared to multiple pokes. But when it comes to your horses' West Nile virus (WNV) vaccinations, multiple injections might be the way to go: Researchers recently tested horses' serologic (blood) response to six WNV vaccination regimens and found some significant differences between horses' immune responses when vaccinated with a combination WNV vaccine versus a WNV vaccine administered separately but at the same time as other vaccines.

Kevin G. Hankins, DVM, MBA, a senior veterinarian in Equine Veterinary Operations with Zoetis, described the full research, results, and recommendations during a presentation at the 2013 American Association of Equine Practitioners' Convention, held Dec. 7-11 in Nashville, Tenn.

To test the immune response triggered by six different vaccine regimens, Hankins and colleagues separated 280 WNV-free Quarter Horses—aged 2 to 17 years—into seven groups. In the study they:

  • Vaccinated the horses in three treatment groups with three different commercially available vaccines that contained WNV, EEE, WEE, and tetanus antigens in a single shot;
  • Vaccinated the horses in three other treatment groups with a WNV-specific vaccine concurrently with a combination EEE, WEE, and tetanus vaccine; and
  • Treated one group with saline; these horses served as unvaccinated controls.

The team vaccinated all horses twice, 21 days apart, with their assigned treatment regimen. The researchers collected blood samples on six different occasions throughout the 42-day study to evaluate the horses' WNV antibody titer levels.

The researchers found all of the vaccinated horses showed consistently elevated WNV antibody titers compared to the control horses, meaning all of the WNV vaccines elicited an immune response. However, the researchers found that by Day 28, horses vaccinated with a combination WNV vaccine—regardless of vaccine brand—had antibody titers three- to fourfold lower when compared to titer levels of horses vaccinated with a product containing only WNV given at the same time as a combination EEE, WEE, and tetanus vaccine—also regardless of brand.

"We thought that the monovalent (protects against one disease) WNV vaccines would produce a higher titer level, but did not think it would be three- to fourfold higher than the combination WNV vaccines," Hankins said.

The researchers believe the reduction in antibody titers could be due to antigen interference, antigen load (the horses' total exposure to substances that trigger an immune response), or some other unknown factors, Hankins said. But regardless of the cause, he said, veterinarians should consider the possible implications of the combination vaccine's lower WNV antibody response when developing and implementing vaccine protocols for horse owners.

So are combination vaccines still better than no vaccines at all?

"Absolutely," Hankins said. "This study was not to show that the combination vaccines were not effective, but that the monovalent vaccines might provide a higher degree of protection. Not vaccinating your horse would not be recommended, especially when all available WNV vaccines have shown to be effective. It's just that when you have horses that are at risk for WNV infection, it might be advisable to use a monovalent WNV vaccine to provide a higher antibody titer level."

Hankins recommended that horse owners work with their veterinarian to determine the best vaccination type and schedule for their horses.

"This is very important and should be part of a complete health program the horse owner has developed with their veterinarian," he stressed.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.