Risk factors for equine influenza serum antibody titres in young thoroughbred racehorses given an inactivated vaccine.

This research investigates the antibody response to an inactivated influenza vaccine in young Thoroughbred racehorses, identifying various factors that influence the varied vaccine responses, including time elapsed since last vaccination, the total number of previous vaccines, and age at first vaccination.

Study Overview

• The study involved 222 yearlings (younger horses not yet in training) and 246 2-year-old horses already in training from eight flat-training yards in Newmarket, UK.
• The horses were given an inactivated influenza virus vaccine, and blood samples were taken before and after vaccination to measure their antibody responses, using a laboratory test called Single Radial Haemolysis (SRH).
• The findings suggest the current vaccination efforts might not be sufficient to protect all horses against influenza infection.

Antibody Titres Before Vaccination

• Prior to vaccination, yearlings’ average antibody titres were approximately half compared to the 2-year-old horses.
• A large proportion of both yearlings and 2-year-olds exhibited SRH antibody titres less than 140mm(2), which the researchers suggest may not be enough to protect against a homologous influenza virus infection.

Antibody Responses Post-Vaccination

• Both groups demonstrated an “anamnestic” response to vaccination, or a rapid and strong immune response, resulting in similar peak mean titres.
• A second vaccine dose administered a month after the first did not increase the average titre in yearlings, although 75% of horses achieved levels of antibody over the suggested protection threshold.

Vaccination History and Risk Factors Influencing the Responses

• For yearlings entering training, the significant predictors of SRH titres prior to vaccination were revealed as the “Time since last vaccination,” “Total number of previous vaccines,” and “Age at first vaccination.”
• In 2-year-olds and after two vaccine doses in yearlings, no significant relationship was found between these factors and SRH titre.
• The researchers observed that the vaccination history recorded in the official passport of yearlings showed that 23% had no prior vaccination record and were likely to be fully susceptible to infection.

Implications

• This research indicates the necessity of re-evaluation of the influenza vaccination strategies for racehorses, considering the key factors affecting vaccine responses such as the total number of vaccines and the age at the first vaccination.