Pathogenicity reactivation of nonpathogenic influenza virus recombinants under von Magnus conditions

The pathogenicity for the chicken of a number of nonpathogenic recombinants between fowl plague virus and various avian and mammalian influenza A viruses can be reactivated by passaging serially at high multiplicities (von Magnus conditions) at 41 degrees, which is the nonpermissive temperature of nonpathogenic recombinants. While the mechanism underlying this reactivation is unclear, it could be excluded that it was due to segregation of heterozygotes to the wild type homozygotes.