H3N2 influenza viruses from domestic chickens in Italy: an increasing role for chickens in the ecology of influenza?

In Italy, multiple H3N2 influenza viruses were isolated from chickens with mild respiratory disease and were shown to replicate in the respiratory tracts of experimentally infected chickens; this finding is the first to show that H3N2 influenza viruses can replicate and cause disease in chickens. H3N2 influenza viruses in pigs on nearby farms seemed a likely source of the virus; however, antigenic and molecular analyses revealed that the gene segments of the viruses in chickens were mainly of Eurasian avian origin and were distinguishable from those isolated from pigs and wild aquatic birds in Italy. Thus, several different H3 influenza viruses were circulating in Italy, but we failed to identify the source of the chicken H3N2 influenza viruses that have disappeared subsequently from Italian poultry. Until recently, the transmission of influenza viruses (other than the H5 and H7 subtypes) from their reservoir in aquatic birds to chickens was rarely detected and highly pathogenic and non-pathogenic viruses were considered to be restricted to poultry species. However, the recent reports of the transmission of H9N2 and H5N1 influenza viruses to chickens in Hong Kong and, subsequently, to humans and our findings of the transmission of H3N2 influenza viruses to domestic chickens in Italy suggest an increased role for chickens as an intermediate host in the ecology of influenza.