Equine Influenza: Vaccinating for Optimal Protection
In the realm of equine infectious diseases, equine influenza is one of the most contagious and costly—both in terms of finances and lost training or showing time—offenders. As such, preventing its spread in the first place is the easiest course of action.

Vaccinating horses is one of the most effective ways to prevent equine influenza . However, even that’s not fool-proof. During a Partner Sunrise Session at last year’s American Association of Equine Practitioners (AAEP) Convention, held in San Francisco, California, two veterinarians described optimal equine influenza vaccine responses and concerns regarding antigen interference.

Vaccine Efficacy

Tom Chambers, PhD, a professor at the University of Kentucky’s Gluck Equine Research Center, in Lexington, and Mark Crisman, DVM, MS, Dipl. ACVIM, equine technical services veterinarian with Zoetis and adjunct professor at Virginia-Maryland College of Veterinary Medicine, addressed whether veterinarians should give vaccines individually (either one at a time on a single day or several individual vaccines administered at separate sites on the same day) or as part of combination vaccines.

Products in the latter group, also referred to as multivalent vaccines, contain vaccines against several diseases. For example, some combine the vaccines considered by the AAEP to be core (rabies, tetanus, West Nile virus [WNV], and Eastern and Western equine encephalitis [EEE and WEE]) meaning every horse should be vaccinated against them every year, as well as the risk-based vaccines (influenza and equine herpesvirus [EHV]) in a single syringe.

“Some questions have been raised whether multivalent vaccines will induce the same level of immune response as monovalent vaccines due to antigenic (stimulating antibody production) interference,” said Crisman.

Essentially, could having multiple agents in a single syringe potentially confuse the horse’s immune system after vaccination, possibly inhibiting its ability to mount an appropriate immune response to vaccination and leaving the horse susceptible to infection?

Crisman described one study in which researchers vaccinated horses against equine influenza and EHV, plus several core vaccines (Eastern and Western encephalitis viruses, WNV, and tetanus). The research team administered these vaccines to one group of horses separately (i.e., core vaccines in one syringe and flu/EHV vaccines in a separate syringe) and to a second group of horses together in a single combination vaccine.

“The study found that immune responses were better when fewer antigens were included in a vaccine,” Crisman said. “Specifically, for a more consistent immune response, the flu/herpesvirus vaccine should be administered as a straight bivalent (containing just those two) vaccine separate from the core vaccines.”

In sum, he said, antigenic interference does occur with equine vaccines, even though, theoretically, multivalent (multidrug) vaccines should work the same as monovalent products.

“That said, there are many polyvalent vaccines that are as efficacious as monovalent vaccines,” said Crisman.

Monitoring Influenza

Influenza, Chambers said, infects horses worldwide, and he and colleagues in his lab, together with other OIE (the World Organization for Animal Health) reference labs, monitor its occurrence. Due to antigenic drift—where the flu virus essentially changes its appearance to the immune system—vaccine efficacy can vary markedly depending on how closely the vaccine matches the current circulating forms of flu virus.

“Surveillance teams look for the actual virus isolates, not just whether a horse is ‘positive’ for the flu,” he said. “We need to know antigenically how the horse’s body ‘sees’ that actual strain of the virus compared to the vaccine to create a better vaccine.”