Four Swedish strains of equine H3N8 influenza virus
isolated from outbreaks during the last
4 years were characterized. Antigenic typing using monoclonal
antibodies raised against a
variety of H3N8 strains showed that the viruses are
heterogeneous, the 1993 isolate being
closely related to the 1991 Swedish isolate TAB/91 and
the other three isolates from 1994 and
1996 being more closely related to each other. This pattern
is reflected in the phylogenetic data
calculated from nucleotide sequencing of the haemagglutinin
genes. H3N8 equine influenza can
be seen to be evolving in two distinct lineages, one European
and one American. The 1993
isolate is closely related to the European lineage and is
the most recent Swedish strain of this
lineage to be isolated. The 1994 and 1996 isolates fit into
the American lineage, which contains
recent isolates from the United States and also Britain.
These results indicate that American-type H3N8 viruses have
become endemic in Sweden and, in
light of the antigenic differences
which can be observed between viruses belonging to the
two lineages, we believe that equine
influenza virus vaccines should be updated with an
American-type virus strain.