The impact of different equine influenza vaccine products and other factors on equine influenza antibody levels in Thoroughbred racehorses

Reasons for performing study: More knowledge of equine influenza (EI) vaccine usage in training yards and the factors that influence serological response to vaccination are required to determine evidence-based vaccination strategies. Objectives: The aim of this study was to ascertain the vaccination history of a population of Thoroughbred racehorses and identify factors that impacted on their antibody titres against EI. Study design: Observational field study. Methods: The study population consisted of 102 vaccinated Thoroughbred horses in training on a single premises. The vaccination histories recorded in their official passports were analysed. Blood samples for serological testing were collected by the veterinary surgeon one month after booster vaccination with ProteqFlu-Te. Antibodies against EI were measured by single radial haemolysis (SRH). Multivariate statistical analysis was undertaken to determine the predictors of SRH antibody titres. Results: There was a strong correlation between age and number of vaccine doses received. Over 70% of horses received their first vaccine dose between ages 6 and 12 months. On average, horses had received 6 vaccine doses and the mean interval between booster vaccinations was 7.7 months. The majority of horses (95%) received more than one influenza vaccine product while 32% had received 3 vaccine products. Significantly higher antibody levels were observed in females than males and there was a significant association between the number of vaccine products administered and antibody levels. In contrast, a negative association between number of vaccine doses and SRH antibody level was demonstrated. Conclusions: Important predictors of EI antibody titres in racehorses were sex, number of vaccine doses received and number of different vaccine products administered.