Antibody Responses Against Equine Influenza Virus Induced by Concurrent and by Consecutive Use of an Inactivated Equine Influenza Virus Vaccine and a Modified Live Equine Herpesvirus Type 1 Vaccine in Thoroughbred Racehorses.

This study evaluated the immune response of Thoroughbred racehorses in Japan to concurrent and sequential administration of an inactivated equine influenza virus (EIV) vaccine and a live equine herpesvirus type 1 (EHV-1) vaccine. The results suggested that sequential administration might prompt a stronger immune response.

Objective and Approach

• The research aimed to determine whether the concurrent (at the same time) or consecutive (one after another) usage of an inactivated EIV and live EHV-1 vaccine influences the antibody response against EIV in Thoroughbred racehorses.
• The study was conducted on two groups of horses: Group A (27 horses) that received both vaccines on the same day, and Group B (20 horses) that were first given the EIV vaccine followed by the EHV-1 vaccine 1-2 weeks later.

Methodology and Parameters Used

• After immunization, researchers examined the horses’ antigen-antibody reactions using tests for hemagglutination inhibition (HI) titers against two strains of the EIV.
• The main parameter of interest was the percentage of horses that demonstrated a twofold increase or more in HI titers, a measure of successful immune response, against a specific EIV variant.

Findings and Implications

• Both groups showed a significant increase in HI titers against the EIV strains after vaccination, thus indicating a considerable immune response.
• However, the proportion of horses that exhibited a twofold or greater increase in HI titers was notably higher in Group B (75%) compared to Group A (37%), suggesting a more potent immune response.
• The results imply that concurrent application of the inactivated EIV vaccine and the live EHV-1 vaccine may somewhat diminish the immune response against EIV.
• The research suggests easing the use of these vaccines consecutively, especially in horses that are either unexposed to these diseases (naïve) or have an incomplete vaccination history.