GENETIC AND ANTIGENIC ANALYSIS OF AN EQUINE INFLUENZA H-3 ISOLATE FROM THE 1989 EPIDEMIC

The haemagglutinin (HA) gene from the equine influenza H3N8 isolate Su ffolk/89 has been cloned by reverse transcription and polymerase chain reaction amplification. The nucleotide sequence of the HA gene was de termined from two independently cloned copies of the gene and was foun d to be most closely related to recent American isolates supporting th e idea that most isolates of equine H3N8 are evolving as a single line age. When the predicted amino acid sequence of the Suffolk/89 HA was e xamined, changes had taken place in at least four of the major antigen ic sites, A, B, C, and D when compared to the sequences of the isolate s used in the current vaccines (Miami/63 and Fontainebleau/79). Surpri singly, when the Suffolk/89 isolate was tested in haemagglutination in hibition (HI) assays with a panel of six mouse monoclonal antibodies, no differences were observed between the Suffolk/89 and the Fontainebl eau/79 isolates, suggesting that this panel of monoclonal antibodies m ay recognise a limited subset of the major antigenic sites. Three anti -HA horse heterohybridoma monoclonals were able to distinguish between the Suffolk/89 and Fontainebleau/79 viruses, demonstrating that the h orse does recognise these isolates as being antigenically different. T he results of the work suggest that the isolates used in current equin e influenza vaccines may need updating.