The relationship between single radial hemolysis, hemagglutination inhibition, and virus neutralization assays used to detect antibodies specific for equine influenza viruses

Antibodies specific for equine influenza viruses are usually quantified using single radial hemolysis (SRH), hemagglutination inhibition (HI) or virus neutralization (VN). Neutralizing antibodies are thought to provide optimum protection to challenged animals. The purpose of this study was to determine the extent to which SRH and HI assays detect antibodies which neutralize equine influenza viruses. Acute and convalescent sera from 41 horses were analyzed using VN, SRH, and HI assays. These horses were present in a population of Thoroughbred racehorses during an epidemic of upper respiratory tract disease associated with influenza A/equine/Saskatoon/1/91 (H3N8), infections. Concentrations of antibodies binding to influenza A/equine/Kentucky/1/81 (H3N8), A/equine/Miami/1/63 (H3N8), and A/equine/Prague/1/56 (H7N7) were determined. Results of the VN assay were compared with results from the SRH and HI assays for acute antibody levels, changes in antibody concentrations between acute and convalescent sampling, and the occurrence of seroconversion. The correlation between assays for pre-exposure antibody levels ranged from 88% to 96%. The correlation between assays for change in antibody concentration ranged from 83% to 90% for the H3N8 viruses. This study shows that antibody concentrations specific for equine influenza virus, measured using SRH and HI assays, are highly correlated with concentrations detected using a VN assay.