Genetic typing of equine influenza virus isolated in Poland in 2005 and 2006

Two equine influenza virus strains were isolated from horses during the local respiratory disease outbreaks in Poland in 2005 and 2006. The H3 equine influenza viral RNA was amplified directly from the clinical specimens with RT-PCR and HA1 fragments were sequenced. The highest homology of HA1 nucleotide sequences of A/eq/Pulawy/05 with A/eq/Aboyne/1/05 and A/eq/Pulawy/06 with A/eq/Essex/2/05 was found. The phylogenetic analysis based on amino acid sequences of HA1 fragments of 84 equine influenza virus strains isolated in Europe during the period of 1976–2007 was conducted to determine the evolutionary relationship of the two Polish and the other European isolates. The resulting phylogenetic tree clearly clustered A/eq/Pulawy/05 with the strains belonging to the European lineage of the equine influenza virus. On the other hand A/eq/Pulawy/06 was placed in the Florida sub-lineage of the American type strains. The presence of the same amino acids: methionine, asparagine and threonine at the positions 48, 159 and 163 respectively, in both Polish isolates, despite the fact that the strains are grouped in two different lineages may indicate the existence of the common ancestor. It is possible that A/eq/Pulawy/06 evolved locally rather than was introduced.