According to a new study, owners often have their horses vaccinated following different regimes associated with the horse’s activity and veterinary recommendations, but that strategy could leave gaps in immunity affecting the equine population as a whole, said Amie Wilson, BVMS, CertAVP, AFHEA, MRCVS, an equine internal medicine resident in the Department of Equine Clinical Science at the University of Liverpool’s Institute of Veterinary and Ecological Sciences, in Neston, U.K.

“My findings show that there is a big discrepancy between the competition guidelines for vaccination and the intervals recommended by the drug manufacturer,” Wilson said. “Given the recent flu outbreaks in the U.K., I think it would be really useful to make sure horses are vaccinated as effectively as possible to provide the best protection.”

Wilson and her fellow researchers surveyed 304 veterinarians in the United Kingdom—including 74% who were exclusively involved in equine practice—online. They found that the veterinarians vaccinated their clients’ horses against equine influenza (EI) based on varying protocols arising from 25 sources of information, she said. “I found that the different guidelines gave conflicting recommendations for vaccination against equine influenza,” Wilson told The Horse.

Her team also noted greater consistency with competition guidelines than with advice from the drug manufacturer’s datasheet, she said. “It is not surprising that there is variation in vaccine practices among (veterinarians) when there is such disparity among different vaccine regulations,” the researchers stated in their report, published in Equine Veterinary Journal.

That variation includes nearly 30% of respondents vaccinating horses younger than 6 months old, which study results have shown could lead to reduced immunity compared to a first vaccination at 6 months old, she said.

Wilson said she also noted significant discrepancies in the timing of the second and third doses. This is likely related to differences cited between the Fédération Equestre Internationale (FEI) and drug manufacturer datasheets, as well as recommendations based on studies by leading EI researchers such as Anne Cullinane. Research indicates the second dose provides five-month immunity, but the FEI allows a third dose to be given up to seven months after the second dose, Wilson said. That could lead to important immunity gaps, she explained.

In addition, manufacturers state that horses reach full immunity 14 days after vaccination, but competition guidelines allow horses to enter venues earlier than that, said Wilson. Only 10% of surveyed veterinarians respected the full 14 days of waiting period before horses attended an event. “This potentially enables horses to attend events prior to onset of immunity,” her team stated.

A further problem is many adverse events related to vaccinations—such as swelling at the injection site or fever—go unreported, said Wilson. Among the 2,760 adverse events her survey respondents declared had occurred in the last 12 months, only 19.1% had been formally reported. “This is an area which requires improvement as a profession in order to provide necessary feedback to pharmaceutical companies and the veterinary medicines directorate (VMD) for drug safety,” the researchers stated. “The concern about the risk of ADR appears to be a significant contributing factor when encountering vaccine hesitancy in horse owners.”

Until guidelines are updated and safety information is improved, equine influenza can continue to spread through suboptimally protected populations, Wilson said. “There is a possibility that when a horse is vaccinated without following the drug manufacturers’ recommendations, then horses may not be as effectively protected,” she said.

With an open door to outbreaks, EI could significantly affect horses both physically and mentally, Wilson’s team explained. “Updating competition requirements to one strategy across all equestrian disciplines could reduce the number of differing guidelines and, in turn, improve equine welfare,” they reported.

The study, “Equine influenza vaccination in the UK: Current practices may leave horses with suboptimal immunity,” was published by Equine Veterinary Journal on Dec. 9, 2020.