Performs attachment to host receptor thereby inducing virus particle entry into target cell. Binds specifically to 5-N-acetyl-4-O-acetyl neuraminic acid on host cells, which plays a major role in cell tropism of the virus (PubMed:14990724, PubMed:22811536). ALso mediates de-O-acetylation of N-acetyl-4-O-acetylneuraminic acid (PubMed:14990724). This receptor-destroying activity is important for virus release as it probably helps preventing self-aggregation and ensures the efficient spread of the progeny virus from cell to cell (PubMed:14990724). The highly polymorphic region (HPR) modulates the virulence in host (PubMed:24486627). Catalyzes the removal of terminal sialic acid residues from viral and cellular glycoconjugates (PubMed:14990724). H2O + N-acetyl-9-O-acetylneuraminate = acetate + H(+) + N-acetylneuraminate H2O + N-acetyl-4-O-acetylneuraminate = acetate + H(+) + N-acetylneuraminate