Studies of the evolution of the haemagglutinin protein of equine influenza virus H3N8Oxburgh, Leif;
Equine H3N8 influenza virus is an important respiratory pathogen of the horse, causing annual outbreaks of disease in many countries. The recurrent nature of influenza in the equine population has prompted numerous studies of the mechanism of recirculation of this virus amongst previously immunised individuals. This thesis summarises and discusses the results of five individual studies concerning the haemagglutinin (HA) of equine H3N8 influenza virus.
Phylogenetic analyses of both nucleotide and amino acid sequences of HA of equine influenza viruses revealed that viral variants isolated at any one time in European and American countries were to a large degree homologous, with changes accumulating in HA with time. This basically linear evolution was seen up to approximately 1990, at which time a divergence into two distinct lineages, an American and a European, took place. Antigenic analyses demonstrated a significant variability in the HA protein of viruses isolated at different times, supporting previous evidence of antigenic drift of H3N8 influenza virus, and also showed a substantial difference between isolates of the American and European lineages. Virus strains belonging to both lineages were shown to cocirculate in Sweden, suggesting that the antigenic differences between the two could be sufficient to compromise cross-protection between them. Differences at the putative receptor binding site (RBS) of HA were found in recent representatives of the American lineage. Variability of this type has in previous studies been associated with immune evasion by human influenza virus.
A PCR-based system for the amplification of viral genes directly from clinical material was developed. HA genes were amplified from clinical samples and compared with HA genes from virus which had been cultured in embryonated hens' eggs. Nucleotide sequencing revealed that growth in eggs leads to amino acid substitutions at the RBS of HA ofrecent American lineage strains. Since HA is the most important target of the humoral immune response against influenza virus, this finding questions whether the embryonated hen's egg is a suitable culture system for the isolation and propagation ofrecent virus strains.
equine influenza; antigenic drift; epidemiology; phylogeny; adaptation
Published inActa Universitatis Agriculturae Sueciae. Veterinaria 1998, number: 41
Publisher: Swedish University of Agricultural Sciences
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