Characterisation of three equine influenza A H3N8 viruses from Germany (2000 and 2002): evidence for frozen evolution

Reported here are the results of antigenic and genetic characterisation of equine influenza strains causing local outbreaks reported to the Equine Diagnostic Centre in Berlin, Germany. In 2000, equine influenza virus was detected in a nasal swab from a non-vaccinated horse using a rapid diagnostic kit, but was not successfully isolated. Partial direct sequencing of the haemagglutinin (HA1) gene, indicated that the virus was a European lineage H3N8 subtype strain representative of strains isolated in several European countries during 2000. In 2002, two equine influenza viruses were isolated from nasal swabs both taken from unvaccinated horses with acute respiratory symptoms housed at the same stables. Antigenic characterisation using a panel of ferret antisera suggested that these isolates also belonged to the European lineage of H3N8 viruses. Analysis of deduced HA1 amino acid sequences confirmed that the HA1 of both isolates were identical and belonged to the European lineage. However, from phylogenetic analysis, both strains appeared to be more closely related to viruses isolated between 1989 and 1995 than to viruses isolated more recently in Europe. These results suggested that viruses with fewer changes than those on the main evolutionary lineage may continue to circulate. The importance of expanding current equine influenza surveillance efforts is emphasised.