Background: Influenza viruses are classified into types A, B, C, and D. Types A and B are further classified into subtypes based on which single hemagglutinin (H) and neuraminidase (N) they have of the many possible ones. Only 2 subtypes of influenza A virus (H1N1 and H3N2) are currently circulating in humans; however, many subtypes of influenza A virus have been found in wild aquatic birds, wild waterfowl, and poultry (1). These subtypes are known as avian influenza viruses (AIVs).

Transmission of AIV from birds to humans can sometimes occur, but subsequent human-to-human transmission has been rare and has been ...