Immunological relationships between the neuraminidases of human and animal influenza viruses.

The neuraminidases of 3 human A2 virus strains-A2/Singapore/1/57, A2/England/76/66 and A2/Hong Kong/1/68-were compared immunologically with those of a number of influenza A virus strains of animal origin (14 avian, 2 equine and 1 porcine). The methods used were enzyme-inhibition tests and immuno-double-diffusion. In enzyme-inhibition tests, the neuraminidases of the A2/57 and A2/66 viruses cross-reacted with those of Turkey/Wisconsin/66, Turkey/Massachusetts/65 and Duck/Italy/574/66 viruses. The enzyme of the Hong Kong variant had a wider spectrum of cross-reactions than did those of the A2/57 and A2/66 viruses. No cross-reaction was detected with Turkey/Massachusetts/65 but the A2/Hong Kong enzyme showed major antigenic similarities with the enzymes of Turkey/Wisconsin/66 and Duck/Italy/574/66 and Duck/Germany/1998/68 viruses. Minor cross-reactions were also detected between the A2/Hong Kong enzyme and those of fowl plague virus, virus "N", Duck/England/56 and Swine/Cambridge/39.The cross-reactions between the enzymes of A2/Hong Kong and Turkey/Wisconsin/66 were confirmed by immunoprecipitin tests. Those between A2/Hong Kong and fowl plague virus, virus "N", Duck/England/56 and Swine/Cambridge/39 could not be confirmed by immunoprecipitation.The finding of antigenic similarities between the enzymes of human and avian influenza A viruses may be of importance in considering the origin of the new antigenic variants of influenza A virus which appear in man.