Serological diagnosis of equine influenza using the hemagglutinin protein produced in a baculovirus expression system

The hemagglutinin (HA) protein of an equine influenza strain, A/equine/La P lata/1/93 (LP/93), was produced using a baculovirus expression system. Silk worm larvae inoculated with recombinant baculovirus expressed high quantiti es of the HA protein which was then purified to greater than 95%,, purity b y fetuin-affinity chromatography. Purified HA protein was used subsequently in an ELISA for detection of antibodies in horse sera. Two hundred serum s amples from vaccinated racehorses were reacted on ELISA plates coated with 40.0 ng/ml of purified HA protein. Subsequent optical density (OD) levels r evealed titers which correlated highly with respective hemagglutinin inhibi tion (HI) antibody titers which ranged from < 1:8 to 1:256 (correlation coe fficient among them was 0.850). ELISA OD levels and HI titers increased at 5 and 7 days post-inoculation, respectively, in a horse inoculated intranas ally with LP/93. Respective antibody levels were observed to change in an e ssentially parallel manner during a period of I month. Similarly, ELISA OD levels correlated with HI titers in horses during a period of 6 weeks follo wing intramuscular inoculation with inactivated single-strain vaccines cont aining LP/93, A/equine/Kentucky/1/81 (H3N8) or A/equine/Rome/5/91 (H3N8). A similar pattern was also observed in eight horses throughout a 10-week per iod following inoculation with a commercially available inactivated trivale nt vaccine containing A/equine/Newmarket/1/77(H7N7), A/equine/Kentucky/81 a nd LP/93. From these results, it is suggested that this ELISA system could be used for disease diagnosis and surveillance of HI antibody titers among vaccinated horses. (C) 2001 Elsevier Science B.V. All rights reserved.