Emergence of distinct avian-like influenza A H1N1 viruses in pigs in Ireland and their reassortment with cocirculating H3N2 viruses

Abstract

The influenza A H1N1 viruses, initially isolated from pigs in Ireland in 1991, were genetically distinct from previously identified human and swine H1N1 viruses, and appear to be the result of introduction of an avian virus distinct from those circulating in pigs in other European countries. The first Irish H3N2 swine isolates were genetically closely related to human H3N2 viruses. In contrast, H3N2 viruses isolated during the latter half of the 1990s were reassortants possessing internal genes similar to those of the H1N1 viruses, and both subtypes acquired amantadine resistance. These changes mimic those which occurred among swine influenza viruses in continental Europe during the 1980s, and emphasize the apparent advantage of properties associated with the avian internal genes and amantadine resistance for maintenance of the viruses in pigs.