EEE, WNV Vaccinations Do Not Interfere With Disease Testing

EEE, WNV Vaccinations Do Not Interfere With Disease Testing

If a vaccine stimulates production of high enough levels of antibodies, diagnostic tests can’t distinguish between pre-existing infection and an adequately vaccinated horse. This is called test interference.

Photo: Erica Larson, News Editor

Vaccinations save millions of horses from suffering clinical disease or even loss of life; however, vaccines are not 100% effective at preventing disease. Instead, vaccines minimize severity and shorten the duration of illness. In cases of neurologic disease, such as those caused by the Eastern equine encephalitis virus (EEEV) or West Nile virus (WNV), a vaccinated horse might show mild clinical signs if exposed to the virus, but he won’t develop severe disease.

“Some horses might develop clinical neurologic signs after vaccination and, thus, laboratory testing is important to determine whether or not a horse was infected with the virus before the vaccine was given,” said Frank Andrews, DVM, MS, Dipl. ACVIM, who spoke on the topic at the 2016 American Association of Equine Practitioners Convention, held Dec. 3-7 in Orlando, Florida. Andrews is a professor of equine medicine and director of the Equine Health Studies Program at Louisiana State University’s School of Veterinary Medicine.

One problem associated with vaccinations pertains to “test interference.” Essentially, if a vaccine stimulates production of high enough levels of antibodies—proteins that fight disease—diagnostic tests can’t distinguish between pre-existing infection and an adequately vaccinated horse.

Early in the natural infection with EEEV and WNV (within seven days), the horse’s IgM antibody level increases rapidly and is used as a marker for true disease. Andrews explained that veterinarians consider a blood antibody titer of 1:400 or higher plus clinical signs consistent with EEEV or WNV infection indicators of true disease.

To determine if vaccines against EEEV and WNV resulted in an IgM titer of 1:400 or higher, Andrews and colleagues conducted an experiment. They administered a multivalent vaccine (one that contains multiple antigens in a single dose) containing EEEV and WNV antigens to six healthy, regularly vaccinated adult horses and six naive pony foals (their dams had been vaccinated regularly, but not during the study period).

The researchers took blood samples from all animals before the start of the study and seven, 14, and 21 days post-vaccination (recall dams were not vaccinated) to measure IgM titers. The foals received a booster 3 to 4 weeks later, and the team collected additional blood samples to measure IgM titers seven, 14, and 21 days after the booster.

“None of the tested samples had IgM antibody titers close to the 1:400 value cutoff for true disease,” Andrews said. “This means the tested vaccine for EEEV and WNV does not interfere with the currently available blood test to diagnose EEEV and WNV.”

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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