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

Young Thoroughbred racehorses (222 yearlings entering training and 246 2-year-old horses already in training) from eight flat-training yards in Newmarket, UK were used to monitor serological responses to vaccination with an inactivated influenza virus vaccine. Blood samples taken prior to and after vaccination were tested by single radial haemolysis (SRH) to determine antibody titres (expressed as area of haemolysis in mm(2)). Prior to vaccination, yearlings had mean antibody titres (64+/-4 mm(2)) that were approximately half of those of 2-year-olds (115+/-3 mm(2)) and 89% of yearlings and 73% of 2-year-olds had SRH titres <140 mm(2). Extrapolation from experimental and field studies suggests that these levels would not protect against homologous influenza virus infection. Both age-groups showed anamnestic responses to vaccination resulting in similar peak mean titres ( approximately 160+/-2mm(2)) with 67% of yearlings and 73% of 2-year-olds achieving levels > or =140 mm(2). A second dose of vaccine administered a month after the first in yearlings did not increase the mean titre but 75% of horses had levels of antibody > or =140 mm(2). The vaccination history in the official passport of yearlings showed that 23% had no record of previous vaccination and were probably fully susceptible to infection. For yearlings entering training, the important predictors from multiple-regression analyses of SRH titres prior to vaccination were "Time since last vaccination," "Total number of previous vaccines" and "Age at first vaccination." In 2-year-olds and following two doses of vaccine in yearlings, there was no significant relationship between these factors and SRH titre.