Scientists recently determined that vaccination with Merial Limited’s Recombitek equine West Nile virus (WNV) vaccine would not confuse results if a horse were to be tested for recent WNV exposure or clinical infection.

At the Western Veterinary Conference in Las Vegas, Nev., in February, Eileen Ostlund DVM, PhD, head of the equine and ovine viruses section at the Diagnostic Virology Laboratory, National Veterinary Services Laboratories (NVSL), in Ames, Iowa, had said that it wasn’t clear yet if scientists would be able to differentiate between vaccinated and exposed animals when using Recombitek, which is a recombinant canarypox vectored vaccine.

In response to her statement, Merial sent pre-and postvaccination serum samples as well as post-challenge samples  to NVSL to answer that question. “We were happy to test the samples because it’s important information that people should know,” said Ostlund.

Scientists were relieved a few years ago when it was shown that the first available WNV vaccine from Fort Dodge Animal Health (which was conditionally licensed and available in August 2001 and was given the name West Nile Innovator in February 2003, when it received a full license) wouldn’t interfere with diagnostic tests designed to measure IgM, an antibody that develops early after infection. While neutralizing antibodies develop after exposure or vaccination, IgM antibodies directed against West Nile virus were not detected after vaccination. In contrast, virus exposure leads to detectable WNV IgM serum antibodies in both vaccinates and non-vaccinated horses . “It was very fortuitous from a diagnostic standpoint that you could tell a recent infection in a vaccinated animal,” explained Ostlund