EIV is pleomorphic and has spherical or lamentous virions with a diameter of 80-120 nm. It is an enveloped virus with a

segmented, single-stranded, negative-sense RNA genome. The entire EIV RNA genome is approximately 13.5 kb and contains 8 RNA segments that encode for 11 viral proteins: PB2 (encoded by segment 1), PB1 and PB1-F2 (segment 2), PA (segment 3), HA (segment 4), NP (segment 5), NA (segment 6), M1 and M2 (segment 7), and NS1 and NEP (segment 8). The viral proteins include three surface glycoproteins (hemagglutinin [H], neuraminidase [N], and ion channel protein [M2]) embedded in the host-cell-derived lipid envelope, ve structural proteins (M1 matrix protein, nucleoprotein [NP], and polymerase complex proteins [PA, PB1, and PB2]), and three nonstructural proteins (NS1, NS2 [nuclear export protein-NEP], and PB1-F2). The RNA gene segments are closely associated with the NP and are surrounded by the M1 protein which is very closely associated with the lipid envelope containing the three surface glycoproteins. Both the HA and NA are major surface glycoproteins and horses mount a strong humoral antibody response to these two proteins following infection; the major neutralization epitopes of EIV are located in the HA protein. The HA protein has an important role in determining the binding to the host cell receptors that have α-2-6-linked or α-2-3-linked sialic acid moieties (α2-3 sialic acid in horses), as well as a cleavage site that must be cleaved by host cell proteases. HA protein also causes hemagglutination of red blood cells. The NA glycoprotein plays an important role in budding and release of progeny virus from the host cell surface by cleaving the glycosidic bonds of the mannosaccharide, neuraminic acid.