Previous work on the serological relationships of avian influenza A viruses provided an opportunity to assess the protective effects of anti-neuraminidase anti-body in a single species system. Three viruses naturally lethal for the chicken (A/, A//63 and A//61) were selected so that two (A/ and A//63) had an antigenically similar haemagglutinin, two (A//63 and A//61) had a similar neuraminidase and two (A/ and A//61) were unrelated by surface antigens. Each virus was used to immunize a group of 7-week-old chickens using one dose of killed virus followed by one dose of live virus. Each group was then divided into three subgroups and each subgroup challenged with one of the viruses. The results of cross-challenge with live virus were assessed objectively on mortality. (1) Immunization with any of the viruses protected the birds completely against the homologous virus. (2) Immunization protected the birds completely against a heterologous virus possessing a similar haemagglutinin. (3) Immunization gave partial protection against a heterologous virus possessing a similar neuraminidase and this protection was significantly greater ( = 0.01) than unimmunized controls when birds immunized with A//63 virus were challenged with A//61. In birds immunized with A//61 and challenged with A//63 the protection was approaching significance ( < 0.1 > 0.05). The results of testing the sera from the immunized birds confirmed the seriological relationships of the viruses.


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