A horse down the road was just diagnosed with equine influenza virus (EIV). But you don’t have to worry because your horse is vaccinated against the virus, right? Wrong. Equine infectious disease experts have recently determined that influenza vaccination breakdowns are common, making even vaccinated horses susceptible to the disease.

“Overall, equine vaccines commercially available in the United States are both safe and efficacious; however, suboptimal protection of vaccines can and do occur,” said Nicola Pusterla, DVM, PhD, Dipl. ACVIM, professor of equine internal medicine, chief of large animal medicine, and section head of equine medicine and dentistry at the University of California, Davis, School of Veterinary Medicine.

Vaccine breakdowns are particularly common with viruses (rather than bacteria) that are capable of mutating, because the body essentially sees them as a new disease-causing organism. As a result, the horse’s immune system is not “primed” against the new version of the virus, making them susceptible to infection (which can produce clinical signs including high fever lethargy, nasal discharge, and coughing).

Pusterla and colleagues discovered these widespread occurrences of vaccine breakdown in horses vaccinated against EIV after collecting data on 2,605 horses with signs of respiratory disease from 38 states over a 45-month period. Of those, 239 tested positive for EIV, 84 of which had been vaccinated against EIV (the time of vaccine administration ranged from less than 6 months to more than a year prior)