The rabies vaccine we use in horses might be an oldie, but it’s still a goodie—and it could be even better than we thought, said David Wilson, BVMS, MS, Hon Dipl. ACVIM, a professor in the Department of Medicine and Epidemiology at the University of California, Davis, School of Veterinary Medicine. In recent research he’s found that it might protect horse from the fatal infection for longer than previously thought.

Wilson presented the results of his study at the 2016 American Association of Equine Practitioners (AAEP) Convention, held Dec. 3-7 in Orlando, Florida.

The AAEP classifies the rabies vaccine as one its core vaccines, recommending that every horse in the United States be inoculated with it annually. That same rabies vaccine (IMRAB 3 by Merial, which can be used in six species: dogs, cats, ferrets, horses, cattle, and sheep) only needs to be administered once every three years in dogs and cats.

“Studies in dogs and cats demonstrated that the antibodies produced in response to the IMRAB 3 rabies vaccine exceed protective levels for a prolonged period, allowing the vaccine to be labeled for three years instead of one,” said Wilson. “Similar studies in horses have not yet been performed,” he added. “Until we document a prolonged persistence of rabies virus antibodies in horses following vaccination, it is impossible to recommend vaccinating horses every three years like we do for dogs and cats.”

To that end, Wilson and colleagues conducted a study on 48 healthy adult horses with unknown vaccine histories. Based on the horses’ ages and histories, the study authors noted which horses they believed might have been vaccinated previously and which likely had not.

The team measured rabies virus neutralizing antibody (RVNA) levels, which are the infection-fighting proteins that protect a horse, should he ever be exposed to the rabies virus, immediately prior to giving each horse a dose of the IMRAB 3 vaccine. Then they repeated the measurement, three to seven weeks after vaccination, and at six-month intervals for two to three years thereafter.

Wilson and colleagues found that:

  • RVNA levels exceeded protective levels (>0.5 IU/mL) for the duration of the study in all horses that the study authors suspected had been vaccinated previously;
  • There was no significant difference in response to vaccination between horses 20 years and older and horses younger than 20 years; and
  • Protective RVNA levels persisted for more than one year following vaccination in only one of the seven horses (14%) suspected of not being vaccinated previously.

“These results suggest that a revaccination interval greater than one year in horses previously vaccinated against the rabies virus may be indicated; however … horses that have not previously been vaccinated require revaccination within one year following their first rabies vaccine,” said Wilson.

At this point, the tested rabies vaccine is still USDA-licensed for annual administration in horses, and Wilson believes this is unlikely to change in the near future.

“To change licensing of a vaccine, the manufacturer needs to conduct USDA-approved challenge studies to prove persistence of protection for a prolonged period,” he said. “These studies … would be difficult to justify economically and would require nonvaccinated control horses to be challenged with virulent rabies virus and likely succumb to infection."

Wilson did suggest, however, that knowing the rabies vaccine “lasts” more than a year helps veterinarians advise owners of horses that have reacted adversely to the vaccine in the past.

“Clients can have their veterinarian collect a blood sample and send it to the Kansas State University Rabies Diagnostic Laboratory to measure neutralizing antibody titer,” he said. If the horse has a protective antibody titer, the veterinarian might recommend postponing revaccination, he said; the need for a booster vaccine would be based on the measured titer rather than being dictated by the calendar.