Antigenic relationship between influenza A viruses of human and animal origin.

Reciprocal antigenic relationships between 17 influenza A viruses of human, porcine, equine and avian origin were investigated by haemagglutination-inhibition (HI) and strain-specific complement-fixation (CF). Cross-reactions were observed between the following strains: (a) Equi/1/Prague/1/56, Fowl plague (Dutch strain) and Turkey/England/1/63 (Langham strain); (b) Equi/2/Miami/1/63, Quail/Italy/1117/65, Pheasant/Italy/647/66, Duck/England/1/62 and Turkey/Canada/1/63; (c) A2/Singapore/1/57 and Turkey/Massachussets/65; (d) Swine/S15/30 and Chicken/Scotland/1/59. The results of HI tests performed with post-infection sera showed on the whole narrower specificity than those of HI with hyperimmune sera or those of strain-specific CF. There is clearly no sharp demarcation of antigenic subtypes of influenza A viruses, and studies over a yet wider range of strains are likely to disclose a continuous spectrum of antigenic variation for the whole group. The authors suggest that, in practice, host specificity rather than antigenic specificity may have to be used as the main criterion in classifying influenza A viruses.