Influenza virus: appearance of high mouse-neurovirulent recombinants

Recombinants from two influenza A strains that lacked mouse neurovirulence were tested, along with their parent strains, for mouse neurovirulence and for the ability to propagate in dissociated mouse embryo brain cells. The parents used were (i) strain A/Rostock/34 (FPV) (Hav1N1), with a high chicken neurovirulence, and (ii) the mouse-lung-adapted human strain Engl/1/61 (H2N2), lacking neurovirulence. In some of the recombinants high mouse neurovirulence could be detected after intracerebral inoculation of low virus doses. There was neither a correlation between surface antigen and neurovirulence nor between neurovirulence and mouse lung virulence in our system, although neurovirulence was only found in strains with Hav1 hemagglutinin. There was an association between replication in mouse embryo brain cells in culture and high mouse neurovirulence.