Efficacy of a commercial vaccine for preventing disease caused by influenza virus infection in horses.

This study examined the effectiveness of a commercial vaccine in preventing equine influenza, a respiratory disease in horses. Despite the use of the vaccine, no significant decrease was detected in the risk of developing respiratory disease during an influenza outbreak, and in turn, vaccine’s overall benefit to protecting horses from the disease was found to be questionable.

Methodology

• The study was a double-masked, randomized, controlled field trial involving 462 horses at a Thoroughbred racetrack.
• Horses were given either the vaccine or a saline solution placebo. This was done four times over regular 6-week intervals.
• The vaccine contained three strains of deactivated influenza virus as well as a deactivated strain of the equine herpesvirus type 4.
• A minimum of one or two doses of the vaccine or placebo was administered prior to the onset of a natural influenza epidemic. Horses in the study were closely monitored, with check-ups 5 days a week to identify any cases of respiratory disease, while antibody levels in the blood serum were measured and virus detection was performed.

Results

• The study found that vaccination prior to the influenza outbreak did not significantly decrease the chance of the horses developing respiratory tract disease.
• The severity of the disease observed in vaccinated horses was no different from those that hadn’t been vaccinated. However, the duration of the disease was found to be three days shorter on average for the vaccinated horses.
• Horses that fell ill during the epidemic showed lower antibody levels against the H3N8 influenza viruses prior to the initial vaccination. Their antibody levels did not increase following the vaccination.
• Upon arrival at the racetrack, younger horses were found to have lower antibody concentrations than older horses and did not respond as well to the vaccination.

Conclusions

• The vaccine’s benefit in overall disease prevention was questionable based on the study results.
• It was highlighted that for an influenza vaccine to be effective in protecting horses from respiratory diseases, a greater level of protection is necessary.
• The study concluded by emphasizing the need for objective field evaluations of commercial vaccines to properly document their efficacy in preventing equine influenza.