Determination of Antibody Levels Against Equine Influenza Virus A2 (H3N8) Using the Hemagglutination Inhibition Test Before and After Annual Vaccination

The humoral response of racehorses vaccinated against equine influenza (EI) was monitored using the hemagglutination inhibition test. One hundred eleven racehorses, 1-10 years of age and stabled at the Santa Ana and San Lazaro Racetracks, were tested before and 3-4 weeks after their annual booster against EI. Depending on their age, these horses have received previously either their initial and/or yearly booster vaccinations in compliance to the Philippine Racing Commission’s resolution on the EI vaccination program. Subsequent antibody (Ab) titers were monitored in 54, 36, and 33 of the original horses on the fourth, fifth and sixth months post-vaccination (PV), respectively. There was a decrease over time in number of horses tested primarily because of removal of some animals from the stables and prevention of blood collection by owner or caretakers just before or after a race. Despite previous vaccinations, 97 out of 111 or 87.4% of pre-booster Ab levels were below the protective titer of ³1:32. The Ab titers increased at 3-4 weeks PV with only 97 or 87.4% of the horses tested had protective titers. In the fourth, fifth, and sixth months PV, 100% (54/54), 83.3% (30/36) and 75.7% (25/33) of the horses tested, respectively had protective titers. The decreasing proportions of horses that had protective titers indicate that a single annual EI booster does not sustain protective Ab levels year round. Therefore, it is recommended that additional booster shots during the year be considered. Keywords: Equine Influenza virus, Hemagglutination Inhibition Test, racehorses, vaccination