Duck and human pandemic influenza A viruses retain sialidase activity under low pH conditions

The majority of influenza A viruses isolated from wild birds, but not human s, can replicate in the duck intestinal tract. Here we demonstrate that all duck isolates tested universally retain sialidase activities under low pH conditions independent of their neuraminidase (NA) subtypes. In contrast, t he sialidase activities of most isolates from humans and pigs practically d isappear below pH 4.5, with the exception of four human pandemic viruses is olated in 1957 and 1968. Sequence comparisons among duck human, and swine N 2 NA subtypes indicate that amino acids at positions 153, 253, 307, 329, 34 4, 347, 356, 368, 390, and 431 may be associated with the low pH stability of duck and human pandemic N2 NAs. This finding suggests that the low pH st ability of duck influenza A virus NA may be a critical factor for replicati on in the intestinal tract through the digestive tract of ducks, and that t he properties of NAs are important for understanding the epidemiology of th e influenza virus.