Studies on the origin of pandemic influenza: III. Evidence implicating duck and equine influenza viruses as possible progenitors of the Hong Kong strain of human influenza

Two strains of influenza virus isolated from horses and ducks in 1963, A/equine/Miami/1/63 (Heq2 Neq2) and A/duck/Ukraine/1/63 (Hav7 Neq2) were found to possess hemagglutinin subunits which cross-reacted in hemagglutination-inhibition and immunodiffusion tests with those of the Hong Kong strain of human influenza A/Hong Kong/1/68 (H3 N2).Peptide maps of the heavy polypeptide chains from the hemagglutinin subunits of these three strains showed a number of differences, but maps of the light chains were almost identical, indicating that the light polypeptide chains from the hemagglutinin subunits of these animal, avian and human viruses had practically the same amino acid sequence.One explanation of these results is that the three viruses arose, by genetic recombination, from a common ancestor.