For example, in the horse, C4-O-acetyl modification of Neu5Ac (SA) occupies more than 50% of the total SA content. The C4-O-acetylated Neu5Ac, Neu4,5Ac2, inhibits the influenza A2 virus HA. De-acetylation reagents such as NaOH or NaIO4 treatment completely hemagglutinate Neu4,5Ac2 by elimination of the C4-O-acetyl group [33]. The C4-O-acetyl Neu5Ac species are found in various sources such as equine erythrocyte GM3, starfish A. rubens and fish [34,35,36,37,38]. C4-O-acetylated Neu5Ac facilitates the initial attachment of viruses to target cells. Like the influenza C virus, infectious salmon anemia virus (ISAV), a member of the Orthomyxoviridae family, contains HE and HEF proteins to mediate virus entry and exit. C4-O-Ac Neu5Ac is the major receptor determinant of ISAV in receptor binding and destruction [38], while the influenza C virus recognizes C9-O-Ac Neu5Ac. The acetylesterase RDE of ISAV cleaves C4-O-Ac via 4-SA-O-acetylesterase with a short turnover time, whereas C9-O-Ac Neu5Ac is cleaved by 9-SA-O-acetylesterase with a long turnover time [34].