Antigenic and genetic evolution of equine H3N8 influenza A viruses

1 Centre for Preventive Medicine, Animal Health Trust, PO Box 5, Suffolk CB8 7DW, UK, 2 Department of Veterinary Science, University of Kentucky, Lexington, Kentucky 40546-0099, USA and 3 Instituto de Virologia, INTA, Buenos Aires, Argentina Evolution of equine influenza A H3N8 viruses was examined by antigenic and genetic analysis of a collection of isolates from around the world. It was noted that antigenic and genetic variants of equine H3N8 viruses cocirculate, and in particular that variants currently circulating in Europe and the USA are distinguishable from one another both in terms of antigenic reactivity and genetic structure of the HA1 portion of the haemagglutinin (HA) molecule. Whilst the divergent evolution of American and European isolates may be due to geographical isolation of the two gene pools, some mixing is believed to occur as ‘American-like’ viruses have been isolated during outbreaks of equine influenza in the UK. The cocirculation of two antigenically and genetically distinct lineages of equine influenza H3N8 viruses has serious implications for vaccine strain selection. * Author for correspondence. Fax +44 1638 665789. Received 7 June 1995; accepted 4 December 1995.