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Antigenic and phylogenetic analyses of virus A/equi/Sisak/80 (H7N7) - last isolated equine influenza virus subtype H7N7 in Europe

Equine influenza is caused by two subtypes of influenza A viruses. First isolation of equine influenza viruses subtype H7N7 was in Czechoslovakia 1956, and subtype H3N8 1963 in USA. Those subtypes are antigenically different and between them there were no immunological cross protection. Furthermore, imunological protection was in correlation with strain homology within the same subtype of the virus. From 1963 both subtypes co-circulated worldwide till 1980 when subtype H7N7 disappeared from equine population. Concerning the disappearance of the subtype H7N7, conclusion of the 1992 meeting of World Health Organization (WHO) and Office International des Epizooties (OIE) Experts was the recommendation to exclude subtype H7N7 from the equine influenza vaccine. Despite that, some vaccine manufacturer’ s still use the subtype H7N7 with the explanation that subtype H7N7 could still be present in some closed equine populations. In our investigation last isolated strain of the subtype H7N7 in Europe, isolated in Sisak, Croatia 1980, was antigenic and phylogenetic characterized to investigate in order to establish relationship between it and other isolated equine influenza viruses subtype H7N7. Results of antigenic and phylogenetic analyses showed significant difference between all former isolated strains of equine influenza virus subtype H7N7 and Croatian 1980 isolate. Evolutionary distance with evidence of amino acid changes on antigenic sites indicate absence of immunological cross protection between it and suggested accelerated evolution of H7N7 subtype before disappearing from equine population. Following this conclusion, theoretically present strain of subtype H7N7 in some closed equine population would be significantly distinct from all strains isolated before 1980. In conclusion, vaccination with any known H7N7 subtype of the equine influenza virus couldn’ t produce sufficient immunological protection. This results supports recommendation of WHO and OIE experts to exclude subtype H7N7 in equine influenza vaccine.

equine influenza virus; antigenic characterization; phlogenetic analyses; vaccination

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