Printed in Great Britain Studies with Avian Influenza A Viruses: Cross Protection Experiments in Chickens

Previous work on the serological relationships of avian influenza A viruses provided an opportunity to assess the protective ffects of anti-neuraminidase anti-body in a single species system. Three viruses naturally lethal for the chicken (A/DUTCH, A/TURKEY/ENGLAND/63 and A/TERN/SOUTH AFRICA/6I) were selected so that two (A/DUTCH and A/TURKEY/ENGLAND/63) had an antigenically similar haemagglutinin, two (A/TURKEY/ENGLAND/63 and A/TERN/SOUTHAFRICA/6I) had a similar neuraminidase and two (A/DUTCH and A/TERN/SOUTH AFRICA/6I) 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. (I) Immunization with any of the viruses pro-tected the birds completely against he homologous virus. (2) Immunization pro-tected 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 (P: o-oi) than unimmunized controls when birds immunized with A/TURKEY/ENGLAND/63 virus were challenged with A/TERN/SOUTH AFRICA/6I. In birds immunized with A/TERN/SOUTH AFRICA/61 and challenged with A/TURKEY/ ENGLAND/63 the protection was approaching significance (P < o.I> o'o5). The results of testing the sera from the immunized birds confirmed the seriological relationships of the viruses