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

OBJECTIVE

To evaluate efficacy of a commercial vaccine for prevention of infectious upper respiratory tract disease (IURD) caused by equine influenza virus.

DESIGN

Double-masked, randomized, controlled field trial.

ANIMALS

462 horses stabled at a Thoroughbred racetrack.

PROCEDURE

Vaccine or saline solution placebo was administered 4 times in the population at 6-week intervals. The vaccine contained 3 strains of inactivated influenza virus, and inactivated equine herpesvirus type 4. Horses received 1 or 2 doses of vaccine or placebo prior to onset of a natural influenza epidemic, and were examined 5 d/wk to identify and monitor horses with IURD. Serum antibody concentrations were determined, and virus isolation was performed.

RESULTS

Vaccination of horses prior to the influenza epidemic did not result in significant decrease in risk of developing respiratory tract disease. Severity of clinical disease was not different between affected vaccinated horses with IURD and controls with IURD, but median duration of clinical disease was 3 days shorter in vaccinated horses. Serum concentrations of antibodies to H3N8 influenza viruses were lower prior to initial vaccination in horses that were sick during the epidemic, and did not increase in these horses in response to vaccination. On arrival at the racetrack, young horses had lower antibody concentrations than older horses, and did not respond to vaccination as well.

CONCLUSIONS AND CLINICAL RELEVANCE

Vaccination was of questionable benefit. A greater degree of protection must be obtained for influenza vaccines to be effective in protecting horses from IURD. Objective field evaluations of commercial vaccines are needed to adequately document their efficacy.